How we estimate

This page explains how the BIY app works out what you need to buy. It has two sections, because the two tools work from different sources: the Estimator, for building a house, and the Workshop, for things you build yourself.

Section 1 · Estimator
Section 2 · Workshop

Section 1 · Estimator

The Estimator in the BIY app and the numbers in this section come from the same source. Every table, every formula and every worked example below is read directly from our store data, so what you see here is exactly what the app uses. If a number is ever corrected, this page and the app change together.

Data version 1 · updated 2026-10-17
Estimates are for planning only. Quantities are not rounded inside a calculation, so round up what you buy. Real needs vary with site conditions, workmanship and waste. Have a licensed professional confirm your plans before ordering or building.

Build It Yourself maintains these tables and formulas. Values marked † are calculated from the 40 kg figure using the method described on this page.

Cement bag size

Cement comes in 40 kg and 50 kg bags. Each table lists both. In the formulas the chosen size is the word bag, so a column named cement_40kg or cement_50kg is picked automatically.

† Marked values are calculated, not looked up: the 40 kg figure multiplied by the ratio between 50 kg and 40 kg cement for the same mixture class (concrete or mortar table above). The same method reproduces every 50 kg figure we have measured directly.

Concrete mixtures

Per cubic meter of concrete (concrete mixture table).

Class	Proportion	Cement 40 kg bags	Cement 50 kg bags	Sand (cu. m.)	Gravel (cu. m.)
AA	1 : 1.5 : 3	12.0	9.5	0.5	1.0
A	1 : 2 : 4	9.0	7.0	0.5	1.0
B	1 : 2.5 : 5	7.5	6.0	0.5	1.0
C	1 : 3 : 6	6.0	5.0	0.5	1.0

Mortar mixtures

Per cubic meter of mortar (mortar mixture table).

Class	Proportion	Cement 40 kg bags	Cement 50 kg bags	Sand (cu. m.)
A	1 : 2	18.0	14.5	1.0
B	1 : 3	12.0	9.5	1.0
C	1 : 4	9.0	7.0	1.0
D	1 : 5	7.5	6.0	1.0

Hollow-block walls

Per square meter of wall (block wall table). Cement depends on the mortar class (A to D) and the bag size.

Block (cm)	Size	Blocks / sq. m.	Sand (cu. m.)	A · 40 kg	A · 50 kg	B · 40 kg	B · 50 kg	C · 40 kg	C · 50 kg	D · 40 kg	D · 50 kg
10 x 20 x 40	4 inch	12.5	0.0435	0.792	0.638†	0.522	0.41325†	0.394	0.306444†	0.328	0.2624†
12.5 x 20 x 40	5 inch	12.5	0.06395	1.159	0.933639†	0.77	0.609583†	0.5785	0.449944†	0.4805	0.3844†
15 x 20 x 40	6 inch	12.5	0.0844	1.526	1.229278†	1.018	0.805917†	0.763	0.593444†	0.633	0.5064†
20 x 20 x 40	8 inch	12.5	0.125	2.26	1.820556†	1.5	1.1875†	1.125	0.875†	0.938	0.7504†

Cement plaster

Per square meter, one side (plaster table).

Thickness	Sand (cu. m.)	A · 40 kg	A · 50 kg	B · 40 kg	B · 50 kg	C · 40 kg	C · 50 kg	D · 40 kg	D · 50 kg
8 mm	0.008	0.144	0.116†	0.096	0.076†	0.072	0.056†	0.06	0.048†
12 mm	0.012	0.216	0.174†	0.144	0.114†	0.108	0.084†	0.09	0.072†
16 mm	0.016	0.288	0.232†	0.192	0.152†	0.144	0.112†	0.12	0.096†
20 mm	0.02	0.36	0.29†	0.24	0.19†	0.18	0.14†	0.15	0.12†
25 mm	0.025	0.45	0.3625†	0.3	0.2375†	0.225	0.175†	0.188	0.1504†

Tile adhesive

Area covered by one bag of regular tile adhesive, by adhesive thickness.

Thickness	Sq. m. per bag
3 mm	5.0
6 mm	2.6
10 mm	1.5

Constants

Constant	Value	Unit	Source
Tile adhesive pieces per bag of cement in the tile bed `tile_adhesive_per_cement_bag`	2.0	pieces	Tile adhesive calculation, tile-bed formula
Cement per cu. m. of sand-cement tile bed (40 kg bags) `tile_bed_cement_bags_per_cubic_m_40kg`	12.0	bags per cu. m.	Tile adhesive calculation, cement formula
Cement per cu. m. of sand-cement tile bed (50 kg bags) `tile_bed_cement_bags_per_cubic_m_50kg`	9.5 †	bags per cu. m.	derived

Reference tables

Lookup tables that the calculations read row by row.

Quantity of cement, sand and gravel for slab and walls per sq. m. of area

Table 1-3

Row	Thickness (cm)	Cement A 40 kg	Cement A 50 kg	Cement B 40 kg	Cement B 50 kg	Cement C 40 kg	Cement C 50 kg	Sand (cu. m.)	Gravel (cu. m.)
5.0 cm	5.0	0.45	0.35	0.375	0.3	0.3	0.25	0.025	0.05
7.5 cm	7.5	0.675	0.525	0.563	0.45	0.45	0.375	0.0375	0.075
10.0 cm	10.0	0.9	0.7	0.75	0.6	0.6	0.5	0.05	0.1
12.5 cm	12.5	1.125	0.875	0.938	0.75	0.75	0.625	0.0625	0.125
15.0 cm	15.0	1.35	1.05	1.125	0.9	0.9	0.75	0.075	0.15
17.5 cm	17.5	1.575	1.225	1.313	1.05	1.05	0.875	0.0875	0.175
20.0 cm	20.0	1.8	1.4	1.5	1.2	1.2	1.0	0.1	0.2
22.5 cm	22.5	2.025	1.575	1.688	1.35	1.35	1.125	0.1125	0.225
25.0 cm	25.0	2.25	1.75	1.875	1.5	1.5	1.25	0.125	0.25
27.5 cm	27.5	2.475	1.925	2.063	1.65	1.65	1.375	0.1375	0.275
30.0 cm	30.0	2.7	2.1	2.25	1.8	1.8	1.5	0.15	0.3

Quantity of cement, sand and gravel per meter length of circular column

Table 1-5

Row	Diameter (cm)	Cement A 40 kg	Cement B 40 kg	Cement A 50 kg	Cement B 50 kg	Sand (cu. m.)	Gravel (cu. m.)
25 cm	25.0	0.442	0.368	0.344	0.294	0.025	0.05	† Calculated, not looked up. 50 kg class A: 0.344 (= 0.442 x 0.7778); 50 kg class B: 0.294 (= 0.368 x 0.8000).
30 cm	30.0	0.636	0.53	0.495	0.424	0.035	0.07	† Calculated, not looked up. 50 kg class A: 0.495 (= 0.636 x 0.7778); class B: 0.424 (= 0.53 x 0.8000); gravel: 0.07 (= 2 x sand 0.035, as in every other row).
35 cm	35.0	0.866	0.722	0.673	0.577	0.048	0.096
40 cm	40.0	1.131	0.942	0.88	0.754	0.063	0.126
50 cm	50.0	1.767	1.473	1.374	1.178	0.098	0.196
55 cm	55.0	2.138	1.782	1.663	1.425	0.119	0.238
60 cm	60.0	2.545	2.121	1.979	1.696	0.141	0.282
65 cm	65.0	2.986	2.488	2.323	1.991	0.166	0.332
70 cm	70.0	3.464	2.886	2.694	2.309	0.192	0.384
75 cm	75.0	3.976	3.313	3.093	2.651	0.221	0.442
80 cm	80.0	4.524	3.77	3.519	3.016	0.251	0.502
85 cm	85.0	5.107	4.256	3.972	3.405	0.284	0.568
90 cm	90.0	5.726	4.771	4.453	3.817	0.318	0.636
100 cm	100.0	7.069	5.89	5.498	4.712	0.393	0.786

Quantity of cement, sand and gravel per concrete pipe

Table 1-6

Row	Outer label	Inner label	Outer (cm)	Inner (cm)	Cement A 40 kg	Cement B 40 kg	Sand (cu. m.)	Gravel (cu. m.)
21 cm	21 cm	15 cm	21.0	15.0	0.152	0.127	0.009	0.018
26 cm	26 cm	20 cm	26.0	20.0	0.195	0.163	0.011	0.022
33 cm	33 cm	25 cm	33.0	25.0	0.328	0.274	0.018	0.037
39 cm	39 cm	30 cm	39.0	30.0	0.436	0.363	0.025	0.05
50 cm	50 cm	40 cm	50.0	40.0	0.635	0.53	0.036	0.072
60 cm	60 cm	50 cm	60.0	50.0	0.779	0.65	0.044	0.088
78 cm	78 cm	60 cm	78.0	60.0	1.755	1.463	0.098	0.196
90 cm	90 cm	70 cm	90.0	70.0	2.26	1.883	0.126	0.252
100 cm	100 cm	80 cm	100.0	80.0	2.538	2.115	0.141	0.282
110 cm	110 cm	90 cm	110.0	90.0	2.826	2.355	0.157	0.314
120 cm	120 cm	100 cm	120.0	100.0	3.11	2.595	0.173	0.346
144 cm	144 cm	120 cm	144.0	120.0	4.482	3.735	0.249	0.498
174 cm	174 cm	150 cm	174.0	150.0	5.5	4.583	0.306	0.612

Quantity of cement and sand per one hundred blocks

Table 2-3

Row	Cement A 40 kg	Cement B 40 kg	Cement C 40 kg	Cement D 40 kg	Sand (cu. m.)
10 x 20 x 40	6.336	4.176	3.152	2.624	0.348
15 x 20 x 40	12.15	8.104	6.072	5.064	0.675
20 x 20 x 40	18.072	12.0	9.0	7.504	1.0

Quantity of cement, sand and gravel for CHB footing per meter length

Table 2-5

Row	Cement A 40 kg	Cement B 40 kg	Cement C 40 kg	Sand (cu. m.)	Gravel (cu. m.)
10 x 30	0.27	0.225	0.18	0.015	0.03
10 x 35	0.315	0.263	0.21	0.018	0.035
10 x 40	0.36	0.3	0.24	0.02	0.04
10 x 50	0.45	0.375	0.3	0.025	0.05
15 x 40	0.54	0.45	0.36	0.03	0.06
15 x 45	0.612	0.51	0.48	0.034	0.068
15 x 50	0.675	0.563	0.55	0.038	0.076
15 x 60	0.81	0.675	0.54	0.045	0.09

Quantity of reinforcing steel bars in a one-way RC slab (bars per sq. m.)

Table 3-10

Row	Spacing (cm)	Bars 6 00m	Bars 7 50m	Bars 9 00m	Bars 12 00m
10 cm	10.0	3.764	2.937	2.421	1.769
12.5 cm	12.5	3.062	2.381	1.934	1.427
15 cm	15.0	2.584	2.004	1.636	1.197
17.5 cm	17.5	2.232	1.726	1.407	1.033
20 cm	20.0	1.98	1.528	1.258	0.914
22.5 cm	22.5	1.786	1.369	1.109	0.81
25 cm	25.0	1.627	1.25	1.014	0.736

Quantity of tie wire on a one-way reinforced concrete slab (kg per sq. m.)

Table 3-11

Row	Spacing (cm)	kg 6 00m at 0 3	kg 6 00m at 0 4	kg 7 50m at 0 3	kg 7 50m at 0 4	kg 9 00m at 0 3	kg 9 00m at 0 4	kg 12 00m at 0 3
10 cm	10.0	0.474	0.632	0.462	0.616	0.46	0.614	0.448
12.5 cm	12.5	0.316	0.422	0.307	0.409	0.295	0.394	0.293
15 cm	15.0	0.228	0.303	0.217	0.289	0.214	0.285	0.207
17.5 cm	17.5	0.168	0.224	0.162	0.216	0.158	0.21	0.153
20 cm	20.0	0.132	0.175	0.126	0.168	0.125	0.167	0.12
22.5 cm	22.5	0.11	0.147	0.102	0.135	0.099	0.132	0.095
25 cm	25.0	0.089	0.118	0.086	0.114	0.081	0.108	0.077

Commercial lengths of rebar (m)

Table REBAR

Row	Length m
6 m	6.0
7.5 m	7.5
9 m	9.0
10.5 m	10.5
12 m	12.0

Calculations and formulas

Each calculation lists what you enter, then the steps in order. A step marked step is an intermediate figure; result steps are the quantities you get. Quantities are not rounded inside a calculation.

Concrete

Footing — Square or rectangular footings.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`length`	Footing length	number	m
`width`	Footing width	number	m
`thickness`	Footing thickness	number	m
`qty`	Quantity of footings	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, thickness, qty)`	Length x width x thickness x number of footings.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Wall footing — Continuous footing under a wall.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`length`	Length or perimeter	number	m
`width`	Width	number	m
`thickness`	Thickness	number	m
`qty`	Quantity of wall footings	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, thickness, qty)`	Length x width x thickness x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Footing tie beam — Beam tying the footings together.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`length`	Beam length	number	m
`width`	Beam width	number	m
`height`	Beam height	number	m
`qty`	Quantity of beams	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, height, qty)`	Length x width x height x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Girder / beam — Main beams.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`length`	Beam length	number	m
`width`	Beam width	number	m
`height`	Beam height	number	m
`qty`	Quantity of beams	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, height, qty)`	Length x width x height x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Lintel beam — Beam over doors and windows.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`length`	Lintel beam length	number	m
`width`	Lintel beam width	number	m
`height`	Lintel beam height	number	m
`qty`	Quantity of lintel beams	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, height, qty)`	Length x width x height x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Column — Rectangular columns.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`length`	Column length	number	m
`width`	Column width	number	m
`height`	Column height	number	m
`qty`	Quantity of columns	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, height, qty)`	Length x width x height x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Stiffener column — Small columns that stiffen a wall.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`length`	Column length	number	m
`width`	Column width	number	m
`height`	Column height	number	m
`qty`	Quantity of columns	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(length, width, height, qty)`	Length x width x height x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Slab — Floor or roof slab, by thickness and area.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`thickness_cm`	Slab thickness	number	cm	12.5
`area`	Slab area	number	sq. m.
`qty`	Quantity of slabs	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`thickness_cm / 100 * area * qty`	Thickness in meters x area x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Reinforced concrete wall — Concrete wall, by thickness and area.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`thickness_cm`	Wall thickness	number	cm	30
`area`	Wall area	number	sq. m.
`qty`	Quantity of walls	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`thickness_cm / 100 * area * qty`	Thickness in meters x area x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Stair landing — Landing slab of a staircase.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		B	Concrete mix proportion (concrete mixture table).
`thickness_cm`	Landing thickness	number	cm	10
`area`	Landing area	number	sq. m.
`qty`	Quantity of landings	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`thickness_cm / 100 * area * qty`	Thickness in meters x area x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Stairs — Stair flight with stringer (slab under the steps).

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`steps`	Number of steps	number	steps
`riser`	Riser height	number	m
`tread`	Tread run	number	m
`width`	Stair width	number	m
`stringer`	Stringer thickness	number	m
`qty`	Quantity of stairs	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`(SQRT(riser^2 + tread^2) * steps * stringer * width + 0.5 * riser * tread * width * steps) * qty`	Slanted stringer: sloping length of one step x steps x stringer thickness x width. Plus the triangular steps: half x riser x tread x width x steps. All multiplied by the quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Septic tank — Walls plus top and bottom slabs, as volumes you already have.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`walls_volume`	Walls volume	number	cu. m.
`top_volume`	Top slab volume	number	cu. m.
`bottom_volume`	Bottom slab volume	number	cu. m.
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`walls_volume + top_volume + bottom_volume`	Walls + top slab + bottom slab.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Concrete gutter — Gutter, from its cross-section and length.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		C	Concrete mix proportion (concrete mixture table).
`area`	Cross-sectional area	number	sq. m.
`length`	Length	number	m
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(area, length)`	Cross-sectional area x length.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Concrete saddle — Saddle, from its cross-section and thickness.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		AA	Concrete mix proportion (concrete mixture table).
`area`	Cross-sectional area	number	sq. m.
`thickness`	Saddle thickness	number	m
`qty`	Quantity	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`PRODUCT(area, thickness, qty)`	Cross-sectional area x thickness x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Concrete by volume — You already know the volume; get the materials.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`volume`	Volume	number	cu. m.
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Slab or wall (area method) — Materials per sq. m. from the slab and wall table.

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick one: A, B, C		A	The slab and wall table covers classes A, B and C.
`thickness`	Thickness	pick from table `1-3`		7.5 cm
`area`	Area	number	sq. m.
`qty`	Quantity	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`LOOKUP("1-3", thickness, "thickness_cm") / 100 * area * qty`	Thickness in meters (from the slab and wall table) x area x quantity.
Cement `cement`	bags · result	`LOOKUP("1-3", thickness, "cement_" & mix & "_" & bag & "kg") * area * qty`	Bags per sq. m. for the thickness, class and bag size (slab and wall table) x area x quantity.
Sand `sand`	cu. m. · result	`LOOKUP("1-3", thickness, "sand_cum") * area * qty`	Sand per sq. m. (slab and wall table) x area x quantity.
Gravel `gravel`	cu. m. · result	`LOOKUP("1-3", thickness, "gravel_cum") * area * qty`	Gravel per sq. m. (slab and wall table) x area x quantity.

Circular column — Round column, per meter length (circular column table).

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick one: A, B		A	The circular column table covers classes A and B.
`diameter`	Column diameter	pick from table `1-5`		60 cm
`height`	Column height	number	m
`qty`	Quantity of columns	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`0.7854 * (LOOKUP("1-5", diameter, "diameter_cm") / 100) ^ 2 * height * qty`	0.7854 x diameter squared (in meters) x height x quantity.
Cement `cement`	bags · result	`LOOKUP("1-5", diameter, "cement_" & mix & "_" & bag & "kg") * height * qty`	Bags per meter of column (circular column table) x height x quantity.
Sand `sand`	cu. m. · result	`LOOKUP("1-5", diameter, "sand_cum") * height * qty`	Sand per meter (circular column table) x height x quantity.
Gravel `gravel`	cu. m. · result	`LOOKUP("1-5", diameter, "gravel_cum") * height * qty`	Gravel per meter (circular column table) x height x quantity.

Concrete pipe — Pipe wall volume from outer and inner diameter (pipe table).

You enter

Name	Label	Kind	Unit	Default	Notes
`mix`	Mixture class	pick from table `est_concrete_mix`		A	Concrete mix proportion (concrete mixture table).
`outer`	Outer diameter	pick from table `1-6`		110 cm
`inner`	Inner diameter	pick from table `1-6`		90 cm
`qty`	Quantity of pipes	number	pcs
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total volume `volume`	cu. m. · step	`(0.7854 * (LOOKUP("1-6", outer, "outer_cm") / 100) ^ 2 - 0.7854 * (LOOKUPBY("1-6", "inner_label", inner, "inner_cm") / 100) ^ 2) * qty`	Area of the outer circle minus the inner circle (0.7854 x diameter squared, in meters) x quantity.
Cement `cement`	bags · result	`volume * LOOKUP("est_concrete_mix", mix, "cement_" & bag & "kg")`	Volume multiplied by the bags of cement per cu. m. for the chosen mix class and bag size.
Sand `sand`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "sand_cum")`	Volume multiplied by the sand per cu. m. for the mix class.
Gravel `gravel`	cu. m. · result	`volume * LOOKUP("est_concrete_mix", mix, "gravel_cum")`	Volume multiplied by the gravel per cu. m. for the mix class.

Masonry

Hollow-block wall — Blocks and mortar for a wall (block wall table).

You enter

Name	Label	Kind	Unit	Default	Notes
`mortar`	Mortar mixture class	pick one: A, B, C, D		C	Mortar proportion (mortar mixture table).
`chb`	Block size (cm)	pick from table `est_chb_size`		10 x 20 x 40
`length`	Wall length	number	m
`height`	Wall height	number	m
`deductions`	Deductions to wall area	number	sq. m.	0	Doors and windows.
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total wall area `area`	sq. m. · step	`length * height - deductions`	Length x height, minus the area of doors and windows.
Cement `cement`	bags · result	`area * LOOKUP("est_chb_size", chb, "cement_" & mortar & "_" & bag & "kg")`	Wall area x bags of cement per sq. m. for the block size, mortar class and bag size (block wall table).
Sand `sand`	cu. m. · result	`area * LOOKUP("est_chb_size", chb, "sand_cum_per_sqm")`	Wall area x sand per sq. m. (block wall table).
Hollow blocks `blocks`	pcs · result	`area * LOOKUP("est_chb_size", chb, "pieces_per_sqm")`	Wall area x blocks per sq. m. (12.5). Not rounded; round up when buying.

Cement plaster — Plaster over a wall (plaster table).

You enter

Name	Label	Kind	Unit	Default	Notes
`mortar`	Mortar mixture class	pick one: A, B, C, D		C	Mortar proportion (mortar mixture table).
`thickness`	Plaster thickness	pick from table `est_plaster_thickness`		12 mm
`sides`	Sides to plaster	number	side/s	1	1 or 2.
`length`	Surface length	number	m
`height`	Surface height	number	m
`deductions`	Deductions to surface area	number	sq. m.	0
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total surface area `area`	sq. m. · step	`sides * length * height - deductions`	Sides x length x height, minus deductions.
Cement `cement`	bags · result	`area * LOOKUP("est_plaster_thickness", thickness, "cement_" & mortar & "_" & bag & "kg")`	Area x bags of cement per sq. m. for the thickness, mortar class and bag size (plaster table).
Sand `sand`	cu. m. · result	`area * LOOKUP("est_plaster_thickness", thickness, "sand_cum_per_sqm")`	Area x sand per sq. m. (plaster table).

Tiling

Tile adhesive — Adhesive, or cement and sand, for floor and wall tiles.

You enter

Name	Label	Kind	Unit	Default	Notes
`floor_area`	Floor area	number	sq. m.
`wall_area`	Wall area	number	sq. m.	0
`tile_cm`	Tile size (one side)	pick one: 30, 40, 50, 60	cm	60	Square tiles.
`thickness`	Adhesive thickness	pick from table `est_tile_coverage`		10 mm
`bag`	Cement bag size	pick one: 40, 50	kg	40	Chosen in the app.

Steps

Step	Gives	Formula	In plain words
Total area `total_area`	sq. m. · step	`floor_area + wall_area`	Floor area + wall area.
Number of tiles `tiles`	pcs · result	`ROUNDUP(total_area / ((tile_cm / 100) * (tile_cm / 100)), 0)`	Total area divided by the area of one tile, rounded up.
Tile adhesive (bagged) `adhesive`	bags · result	`total_area / LOOKUP("est_tile_coverage", thickness, "sqm_per_bag")`	Total area divided by the area one bag covers at the chosen thickness.
Cement (sand-cement alternative) `bed_cement`	bags · result	`total_area * (LOOKUP("est_tile_coverage", thickness, "thickness_mm") / 1000) * CONSTANT("tile_bed_cement_bags_per_cubic_m_" & bag & "kg")`	Total area x thickness in meters x bags of cement per cu. m. of tile bed.
Sand (sand-cement alternative) `bed_sand`	cu. m. · result	`total_area * (LOOKUP("est_tile_coverage", thickness, "thickness_mm") / 1000)`	Total area x thickness in meters.
Tile adhesive (with the cement alternative) `adhesive_pcs`	pcs · result	`bed_cement * CONSTANT("tile_adhesive_per_cement_bag")`	Bags of cement x tile adhesive pieces per bag of cement.

Steel

Rebar cutting list — Which commercial rebar length wastes the least for a cut length.

You enter

Name	Label	Kind	Unit	Default	Notes
`cut_length`	Cut length	number	m	1.4

Steps

The app tries every commercial rebar length, counts how many pieces of the cut length fit in each, and picks the length that leaves the least waste.

Formula language

Numbers, text in "quotes", names of inputs and earlier steps.
Operators: + - * / ^, & joins text, comparisons = <> < > <= >=.
PRODUCT, SUM, MIN, MAX, ABS, SQRT, COUNT, IF, ROUNDUP, ROUNDDOWN.
LOOKUP(table, row, column) reads a value from a table row. LOOKUPBY(table, column, value, result) finds the row by a column value instead.
CONSTANT(name) reads a constant. bag is the chosen cement bag size (40 or 50).

Worked examples

Each example has a known-good answer. The BIY app checks every one of them against the current numbers; if any disagree, the app flags the estimate instead of showing it.

Calculation	Example	Inputs	Expected
concrete_volume	Class A, 3.768 cu. m. (40 kg bags)	mix=A, volume=3.768	cement=33.912, sand=1.884, gravel=3.768
concrete_volume	Class A, 3.768 cu. m. (50 kg bags)	mix=A, volume=3.768	cement=26.376, sand=1.884, gravel=3.768
footing	Class A, 1.5 x 1.2 x 0.4 m, 12 pcs (40 kg bags)	mix=A, length=1.5, width=1.2, thickness=0.4, qty=12	volume=8.64, cement=77.76, sand=4.32, gravel=8.64
footing	Class A, 1.5 x 1.2 x 0.4 m, 12 pcs (50 kg bags)	mix=A, length=1.5, width=1.2, thickness=0.4, qty=12	volume=8.64, cement=60.48, sand=4.32, gravel=8.64
slab_wall_by_area	Class B, 7.5 cm, 23 sq. m. (40 kg bags)	mix=B, thickness=7.5 cm, area=23, qty=1	volume=1.725, cement=12.949, sand=0.8625, gravel=1.725
slab_wall_by_area	Class B, 7.5 cm, 23 sq. m. (50 kg bags)	mix=B, thickness=7.5 cm, area=23, qty=1	volume=1.725, cement=10.35, sand=0.8625, gravel=1.725
concrete_stairs	Class A, 7 steps, 0.2 / 0.3 / 1.2 / 0.15 m (40 kg bags)	mix=A, steps=7, riser=0.2, tread=0.3, width=1.2, stringer=0.15, qty=1	volume=0.7063, cement=6.3567, sand=0.3531, gravel=0.7063
concrete_stairs	Class A, 7 steps, 0.2 / 0.3 / 1.2 / 0.15 m (50 kg bags)	mix=A, steps=7, riser=0.2, tread=0.3, width=1.2, stringer=0.15, qty=1	volume=0.7063, cement=4.9441, sand=0.3531, gravel=0.7063
circular_column	Class A, 60 cm, 2 m high, 3 pcs (40 kg bags)	mix=A, diameter=60 cm, height=2, qty=3	volume=1.6965, cement=15.27, sand=0.846, gravel=1.692
circular_column	Class A, 60 cm, 2 m high, 3 pcs (50 kg bags)	mix=A, diameter=60 cm, height=2, qty=3	volume=1.6965, cement=11.874, sand=0.846, gravel=1.692
concrete_pipe	Class A, 110 / 90 cm, 1 pc (40 kg bags)	mix=A, outer=110 cm, inner=90 cm, qty=1	volume=0.3142, cement=2.8274, sand=0.1571, gravel=0.3142
concrete_pipe	Class A, 110 / 90 cm, 1 pc (50 kg bags)	mix=A, outer=110 cm, inner=90 cm, qty=1	volume=0.3142, cement=2.1991, sand=0.1571, gravel=0.3142
tile_adhesive	32 sq. m. floor + 9 sq. m. wall, 60 x 60, 10 mm (40 kg bags)	floor_area=32, wall_area=9, tile_cm=60, thickness=10 mm	total_area=41, tiles=114, adhesive=27.3333, bed_cement=4.92, bed_sand=0.41, adhesive_pcs=9.84
wall_footing	Class AA, 24 x 0.4 x 0.2 m, 1 pc (40 kg)	mix=AA, length=24, width=0.4, thickness=0.2, qty=1	volume=1.92, cement=23.04, sand=0.96, gravel=1.92
footing_tie_beam	Class A, 3 x 0.2 x 0.3 m, 8 pcs (40 kg)	mix=A, length=3, width=0.2, height=0.3, qty=8	volume=1.44, cement=12.96, sand=0.72, gravel=1.44
girder_beam	Class AA, 4 x 0.25 x 0.4 m, 6 pcs (40 kg)	mix=AA, length=4, width=0.25, height=0.4, qty=6	volume=2.4, cement=28.8, sand=1.2, gravel=2.4
lintel_beam	Class AA, 2 x 0.15 x 0.2 m, 10 pcs (40 kg)	mix=AA, length=2, width=0.15, height=0.2, qty=10	volume=0.6, cement=7.2, sand=0.3, gravel=0.6
concrete_column	Class A, 0.3 x 0.3 x 3 m, 8 pcs (40 kg)	mix=A, length=0.3, width=0.3, height=3, qty=8	volume=2.16, cement=19.44, sand=1.08, gravel=2.16
stiffener_column	Class A, 0.15 x 0.15 x 2.8 m, 12 pcs (40 kg)	mix=A, length=0.15, width=0.15, height=2.8, qty=12	volume=0.756, cement=6.804, sand=0.378, gravel=0.756
concrete_slab	Class AA, 12.5 cm, 30 sq. m. (40 kg)	mix=AA, thickness_cm=12.5, area=30, qty=1	volume=3.75, cement=45.0, sand=1.875, gravel=3.75
concrete_wall	Class A, 30 cm, 12 sq. m. (40 kg)	mix=A, thickness_cm=30, area=12, qty=1	volume=3.6, cement=32.4, sand=1.8, gravel=3.6
stair_landing	Class B, 10 cm, 4 sq. m. (40 kg)	mix=B, thickness_cm=10, area=4, qty=2	volume=0.8, cement=6.0, sand=0.4, gravel=0.8
septic_tank	Class AA, 6 + 2 + 2 cu. m. (40 kg)	mix=AA, walls_volume=6, top_volume=2, bottom_volume=2	volume=10, cement=120.0, sand=5.0, gravel=10
concrete_gutter	Class C, 0.04 sq. m. x 12 m (40 kg)	mix=C, area=0.04, length=12	volume=0.48, cement=2.88, sand=0.24, gravel=0.48
concrete_saddle	Class AA, 0.5 sq. m. x 0.1 m, 3 pcs (40 kg)	mix=AA, area=0.5, thickness=0.1, qty=3	volume=0.15, cement=1.8, sand=0.075, gravel=0.15
wall_footing	Class AA, 24 x 0.4 x 0.2 m, 1 pc (50 kg)	mix=AA, length=24, width=0.4, thickness=0.2, qty=1	volume=1.92, cement=18.24, sand=0.96, gravel=1.92
footing_tie_beam	Class A, 3 x 0.2 x 0.3 m, 8 pcs (50 kg)	mix=A, length=3, width=0.2, height=0.3, qty=8	volume=1.44, cement=10.08, sand=0.72, gravel=1.44
girder_beam	Class AA, 4 x 0.25 x 0.4 m, 6 pcs (50 kg)	mix=AA, length=4, width=0.25, height=0.4, qty=6	volume=2.4, cement=22.8, sand=1.2, gravel=2.4
lintel_beam	Class AA, 2 x 0.15 x 0.2 m, 10 pcs (50 kg)	mix=AA, length=2, width=0.15, height=0.2, qty=10	volume=0.6, cement=5.7, sand=0.3, gravel=0.6
concrete_column	Class A, 0.3 x 0.3 x 3 m, 8 pcs (50 kg)	mix=A, length=0.3, width=0.3, height=3, qty=8	volume=2.16, cement=15.12, sand=1.08, gravel=2.16
stiffener_column	Class A, 0.15 x 0.15 x 2.8 m, 12 pcs (50 kg)	mix=A, length=0.15, width=0.15, height=2.8, qty=12	volume=0.756, cement=5.292, sand=0.378, gravel=0.756
concrete_slab	Class AA, 12.5 cm, 30 sq. m. (50 kg)	mix=AA, thickness_cm=12.5, area=30, qty=1	volume=3.75, cement=35.625, sand=1.875, gravel=3.75
concrete_wall	Class A, 30 cm, 12 sq. m. (50 kg)	mix=A, thickness_cm=30, area=12, qty=1	volume=3.6, cement=25.2, sand=1.8, gravel=3.6
stair_landing	Class B, 10 cm, 4 sq. m. (50 kg)	mix=B, thickness_cm=10, area=4, qty=2	volume=0.8, cement=4.8, sand=0.4, gravel=0.8
septic_tank	Class AA, 6 + 2 + 2 cu. m. (50 kg)	mix=AA, walls_volume=6, top_volume=2, bottom_volume=2	volume=10, cement=95.0, sand=5.0, gravel=10
concrete_gutter	Class C, 0.04 sq. m. x 12 m (50 kg)	mix=C, area=0.04, length=12	volume=0.48, cement=2.4, sand=0.24, gravel=0.48
concrete_saddle	Class AA, 0.5 sq. m. x 0.1 m, 3 pcs (50 kg)	mix=AA, area=0.5, thickness=0.1, qty=3	volume=0.15, cement=1.425, sand=0.075, gravel=0.15
chb_wall	4 inch block, class C, 10 x 2.4 m, 3 sq. m. deducted (40 kg)	mortar=C, chb=10 x 20 x 40, length=10, height=2.4, deductions=3	area=21.0, cement=8.274, sand=0.9135, blocks=262.5
chb_wall	6 inch block, class B, 8 x 3 m (40 kg)	mortar=B, chb=15 x 20 x 40, length=8, height=3, deductions=0	area=24, cement=24.432, sand=2.0256, blocks=300.0
chb_wall	4 inch block, class C, 10 x 2.4 m, 3 sq. m. deducted (50 kg)	mortar=C, chb=10 x 20 x 40, length=10, height=2.4, deductions=3	area=21.0, cement=6.4353, sand=0.9135, blocks=262.5
chb_wall	6 inch block, class B, 8 x 3 m (50 kg)	mortar=B, chb=15 x 20 x 40, length=8, height=3, deductions=0	area=24, cement=19.342, sand=2.0256, blocks=300.0
cement_plaster	12 mm, class C, 2 sides, 10 x 2.4 m (40 kg)	mortar=C, thickness=12 mm, sides=2, length=10, height=2.4, deductions=3	area=45.0, cement=4.86, sand=0.54
cement_plaster	25 mm, class A, 1 side, 5 x 3 m (40 kg)	mortar=A, thickness=25 mm, sides=1, length=5, height=3, deductions=0	area=15, cement=6.75, sand=0.375
cement_plaster	12 mm, class C, 2 sides, 10 x 2.4 m (50 kg)	mortar=C, thickness=12 mm, sides=2, length=10, height=2.4, deductions=3	area=45.0, cement=3.78, sand=0.54
cement_plaster	25 mm, class A, 1 side, 5 x 3 m (50 kg)	mortar=A, thickness=25 mm, sides=1, length=5, height=3, deductions=0	area=15, cement=5.4375, sand=0.375
tile_adhesive	32 + 9 sq. m., 60 x 60, 10 mm (50 kg bags)	floor_area=32, wall_area=9, tile_cm=60, thickness=10 mm	total_area=41, tiles=114, adhesive=27.3333, bed_cement=3.895, bed_sand=0.41, adhesive_pcs=7.79
rebar_cut	Cut length 1.4 m	cut_length=1.4	best_length_m=6.0, best_waste_m=0.4, best_pieces=4

Materials

The quantities above are in these units. Pricing and product links are not part of the calculation.

Material	Unit
Cement	bags
Concrete hollow block 4"	pcs
Concrete hollow block 5"	pcs
Concrete hollow block 6"	pcs
Concrete hollow block 8"	pcs
Gravel	cu. m.
Reinforcing bar	pcs
Sand	cu. m.
Tile adhesive (bagged)	bags
Tile adhesive (tile-bed mix)	pcs
Tiles	pcs

Section 2 · Workshop

The Workshop is for things you build yourself: a shelf, a cabinet, a table, a gate. It works out the parts, how to cut them from the plywood or boards we sell, and the other supplies the job needs. These formulas are separate from the Estimator's formulas for the house, which come from BIY's own workbook. Everything here is an estimate for budgeting and shopping. Check the final quantities against your own plan before you buy.

Supply formulas

Every number below is read directly from our store data, the same data the app uses, so this table and the app cannot disagree. For hinges, handles, glue, finish, wall fixings and nails we use the rules of the open-source project WoodworkingShop by Ryair (MIT licence), rewritten for the app. Where a rule is ours, the source column says "BIY rule". A supply can use the result of the one before it by its key, and a supply that does not apply counts as 0.

Supply	Unit	Applies when	How many	In plain words	Searched in our store as	Source
Doors and hinges
Cabinet hinge `hinge`	pcs	`doors>0`	`doors*IF(door_height<=600,2,IF(door_height<=1200,3,IF(door_height<=1800,4,IF(door_height<=2200,5,6))))`	Hinges per door by the height of the door: 2 up to 600 mm, 3 up to 1,200, 4 up to 1,800, 5 up to 2,200, otherwise 6.	concealed hinge 35mm or: cabinet hinge, hinge	WoodworkingShop src/engine/geometry/dimensions.ts (computeHingesPerDoor); WoodworkingShop src/engine/hardware/hardware.ts
Hinge mounting plate `hinge_plate`	pcs	`doors>0`	`hinge`	One plate for each hinge.	hinge mounting plate or: hinge plate	WoodworkingShop src/engine/hardware/hardware.ts
Door handle `door_handle`	pcs	`doors>0`	`doors`	One handle for each door.	cabinet handle or: pull handle, cabinet knob	WoodworkingShop src/engine/hardware/hardware.ts
Door bumper pad `door_bumper`	pcs	`doors>0`	`doors*CONSTANT("bumper_pads_per_door")`	Two pads for each door.	bumper pad or: rubber bumper, furniture glide	WoodworkingShop src/engine/hardware/hardware.ts
Drawers
Drawer slide, pair `drawer_slide`	pairs	`drawers>0`	`drawers`	One pair for each drawer; the longest standard slide that fits the depth minus 50 mm.	drawer slide or: drawer guide	WoodworkingShop src/engine/hardware/hardware.ts
Drawer handle `drawer_handle`	pcs	`drawers>0`	`drawers`	One handle for each drawer.	cabinet handle or: pull handle, cabinet knob	WoodworkingShop src/engine/hardware/hardware.ts
Screws and nails
Wood screw `wood_screw`	pcs	`1`	`joints*(CONSTANT("assembly_screws_per_join")+CONSTANT("camlock_screws_per_join"))+CONSTANT("base_screws")+IF(height>CONSTANT("tall_height_mm"),CONSTANT("tall_extra_screws"),0)`	The repo's counts: 12 assembly screws and 4 in place of cam locks for each panel join, 8 base screws, and 4 more for a unit taller than 1,200 mm. Screw length = thickness of the piece fixed + 25 mm (BIY rule).	wood screw or: screw	WoodworkingShop src/engine/hardware/hardware.ts (assembly screws, confirmat screws and cam locks, converted to wood screws)
Back panel nail `back_nail`	pcs	`has_back>0`	`ROUNDUP(back_perimeter_mm/CONSTANT("back_nail_spacing_mm"),0)`	One nail every 150 mm around the edge of the back panel.	common nail 1 in or: wire nail	WoodworkingShop src/engine/hardware/hardware.ts
Wall fixing
Wall bracket `wall_bracket`	pcs	`height>CONSTANT("tall_height_mm")`	`IF(width>=CONSTANT("wall_bracket_wide_width_mm"),4,2)`	4 brackets for a unit 800 mm wide or more, otherwise 2. Used when the unit is taller than 1,200 mm.	angle bracket or: L bracket, corner brace, shelf bracket	WoodworkingShop src/engine/hardware/hardware.ts (L-brackets); BIY rule for when it applies
Wall plug `wall_plug`	pcs	`IF(height>CONSTANT("tall_height_mm"),1,IF(wall_run_mm>0,1,0))`	`IF(height>CONSTANT("tall_height_mm"),2*wall_bracket,0)+IF(wall_run_mm>0,ROUNDUP(wall_run_mm/CONSTANT("wall_fixing_spacing_mm"),0)+1,0)`	2 plugs for each bracket; for a cleat or rack, one about every 300 mm along the wall plus one.	wall plug 8mm or: wall plug, plastic anchor, tox	WoodworkingShop src/engine/hardware/hardware.ts (wall screw + dowel); BIY rule for cleats and racks
Wall screw `wall_screw`	pcs	`IF(wall_plug>0,1,0)`	`wall_plug`	One wall screw for each wall plug.	wood screw 3 in or: screw	WoodworkingShop src/engine/hardware/hardware.ts (wall screw + dowel)
Cabinet parts
Shelf pin `shelf_pin`	pcs	`adjustable_shelves>0`	`adjustable_shelves*CONSTANT("shelf_pins_per_shelf")`	4 pins for each adjustable shelf.	shelf support pin 5mm or: shelf pin	WoodworkingShop src/engine/hardware/hardware.ts
Leveller foot `leveller_foot`	pcs	`floor_standing>0`	`CONSTANT("leveller_feet")`	Four feet for a cabinet standing on the floor.	adjustable furniture leg or: leveller, plinth foot	WoodworkingShop src/engine/hardware/hardware.ts
Corner brace `corner_brace`	pcs	`is_carcass>0`	`CONSTANT("corner_braces")`	Four braces for a box of panels.	corner brace or: flat corner brace	WoodworkingShop src/engine/hardware/hardware.ts
Edge banding `edge_banding`	m	`edge_run_m>0`	`edge_run_m`	The visible edge: twice the width plus the height.	PVC edging or: edge banding	WoodworkingShop src/engine/hardware/hardware.ts
Hanging rail `hanging_rail`	pcs	`has_hanging_rail>0`	`has_hanging_rail`	A wardrobe has one hanging rail.	closet rod or: hanging rail, clothes rod	BIY rule
Gate
Gate hinge `gate_hinge`	pcs	`is_gate>0`	`3`	Three hinges on the hinge side of a gate.	bearing hinge 4 in or: gate hinge, heavy hinge	BIY rule
Gate latch `gate_latch`	pcs	`is_gate>0`	`1`	One latch.	barrel bolt or: gate latch, tower bolt	BIY rule
Glue, finish and sanding
Wood glue `wood_glue`	bottles	`1`	`MAX(1,ROUNDUP(glue_area_mm2/1000000/CONSTANT("glue_pva_spread_m2_per_l")1000CONSTANT("glue_waste_factor")/CONSTANT("glue_bottle_ml"),0))`	Glued area / 180 m2 per litre, plus 15% for waste, in 237 mL bottles; at least one bottle, as in the repo's list.	wood glue or: carpenter glue, pva glue	WoodworkingShop src/engine/calculators/glue-coverage.ts; WoodworkingShop src/engine/hardware/hardware.ts
Paint `finish_paint`	litres	`1`	`finish_area_m2*CONSTANT("finish_coats_paint")/CONSTANT("finish_coverage_paint_m2_per_l")`	Area x 2 coats / 12 m2 per litre.	enamel paint or: paint, boysen, enamel	WoodworkingShop src/engine/calculators/finish-calculator.ts
Sandpaper `sandpaper`	sheets	`1`	`CONSTANT("sandpaper_sheets")`	The repo lists one assorted pack; BIY uses 3 sheets, one of each grit.	sandpaper or: sanding paper	WoodworkingShop src/engine/hardware/hardware.ts; BIY rule for the count
Boards
Sheet board `sheet_board`	sheets	`sheets>0`	`sheets`	One 4 x 8 ft sheet each, in the thickness picked on the Measure tab. Ordinary plywood comes first; marine plywood, plyboard and laminated MDF of the same thickness are offered under Swap.	plywood ordinary or: plywood marine, plyboard, mdf laminated, laminated mdf	BIY rule
Lumber
Lumber 1" x 2" x 8 ft `lumber_1x2_8`	pcs	`boards_1x2_8>0`	`boards_1x2_8`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 1x2x8 or: coco lumber 1x2x8, pine wood 1x2x8	BIY rule
Lumber 1" x 2" x 10 ft `lumber_1x2_10`	pcs	`boards_1x2_10>0`	`boards_1x2_10`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 1x2x10 or: coco lumber 1x2x10, pine wood 1x2x10	BIY rule
Lumber 1" x 2" x 12 ft `lumber_1x2_12`	pcs	`boards_1x2_12>0`	`boards_1x2_12`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 1x2x12 or: coco lumber 1x2x12, pine wood 1x2x12	BIY rule
Lumber 2" x 2" x 8 ft `lumber_2x2_8`	pcs	`boards_2x2_8>0`	`boards_2x2_8`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x2x8 or: coco lumber 2x2x8, pine wood 2x2x8	BIY rule
Lumber 2" x 2" x 10 ft `lumber_2x2_10`	pcs	`boards_2x2_10>0`	`boards_2x2_10`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x2x10 or: coco lumber 2x2x10, pine wood 2x2x10	BIY rule
Lumber 2" x 2" x 12 ft `lumber_2x2_12`	pcs	`boards_2x2_12>0`	`boards_2x2_12`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x2x12 or: coco lumber 2x2x12, pine wood 2x2x12	BIY rule
Lumber 2" x 3" x 8 ft `lumber_2x3_8`	pcs	`boards_2x3_8>0`	`boards_2x3_8`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x3x8 or: coco lumber 2x3x8, pine wood 2x3x8	BIY rule
Lumber 2" x 3" x 10 ft `lumber_2x3_10`	pcs	`boards_2x3_10>0`	`boards_2x3_10`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x3x10 or: coco lumber 2x3x10, pine wood 2x3x10	BIY rule
Lumber 2" x 3" x 12 ft `lumber_2x3_12`	pcs	`boards_2x3_12>0`	`boards_2x3_12`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x3x12 or: coco lumber 2x3x12, pine wood 2x3x12	BIY rule
Lumber 2" x 4" x 8 ft `lumber_2x4_8`	pcs	`boards_2x4_8>0`	`boards_2x4_8`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x4x8 or: coco lumber 2x4x8, pine wood 2x4x8	BIY rule
Lumber 2" x 4" x 10 ft `lumber_2x4_10`	pcs	`boards_2x4_10>0`	`boards_2x4_10`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x4x10 or: coco lumber 2x4x10, pine wood 2x4x10	BIY rule
Lumber 2" x 4" x 12 ft `lumber_2x4_12`	pcs	`boards_2x4_12>0`	`boards_2x4_12`	Boards for the frame, from the cutting plan: pine wood or coco lumber, whichever the store has at the best price.	lumber 2x4x12 or: coco lumber 2x4x12, pine wood 2x4x12	BIY rule

Fixed numbers

Number	Value	Unit	Source
Assembly screws per panel join `assembly_screws_per_join`	12.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Back panel nail spacing `back_nail_spacing_mm`	150.0	mm	WoodworkingShop src/engine/hardware/hardware.ts
Base screws for a carcass `base_screws`	8.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Door bumper pads per door `bumper_pads_per_door`	2.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Screws per panel join in place of cam locks `camlock_screws_per_join`	4.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Corner braces per cabinet `corner_braces`	4.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Edge banding roll length `edge_roll_m`	50.0	m	WoodworkingShop src/engine/hardware/hardware.ts
Oil coats `finish_coats_oil`	2.0	coats	WoodworkingShop src/engine/calculators/finish-calculator.ts
Paint coats `finish_coats_paint`	2.0	coats	WoodworkingShop src/engine/calculators/finish-calculator.ts
Primer coats `finish_coats_primer`	1.0	coats	WoodworkingShop src/engine/calculators/finish-calculator.ts
Stain coats `finish_coats_stain`	2.0	coats	WoodworkingShop src/engine/calculators/finish-calculator.ts
Varnish coats `finish_coats_varnish`	3.0	coats	WoodworkingShop src/engine/calculators/finish-calculator.ts
Oil coverage `finish_coverage_oil_m2_per_l`	12.0	m2 per litre per coat	WoodworkingShop src/engine/calculators/finish-calculator.ts
Paint coverage `finish_coverage_paint_m2_per_l`	12.0	m2 per litre per coat	WoodworkingShop src/engine/calculators/finish-calculator.ts
Primer coverage `finish_coverage_primer_m2_per_l`	8.0	m2 per litre per coat	WoodworkingShop src/engine/calculators/finish-calculator.ts
Stain coverage `finish_coverage_stain_m2_per_l`	10.0	m2 per litre per coat	WoodworkingShop src/engine/calculators/finish-calculator.ts
Varnish coverage `finish_coverage_varnish_m2_per_l`	6.0	m2 per litre per coat	WoodworkingShop src/engine/calculators/finish-calculator.ts
Wood glue bottle `glue_bottle_ml`	237.0	mL	BIY rule
PVA glue spread rate `glue_pva_spread_m2_per_l`	180.0	m2 per litre	WoodworkingShop src/engine/calculators/glue-coverage.ts
Glue waste factor `glue_waste_factor`	1.15	x	WoodworkingShop src/engine/calculators/glue-coverage.ts
Leveller feet per cabinet `leveller_feet`	4.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Sandpaper sheets per project `sandpaper_sheets`	3.0	sheets	BIY rule
A screw must go this far into the second piece `screw_penetration_mm`	25.0	mm	BIY rule
Shelf pins per adjustable shelf `shelf_pins_per_shelf`	4.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
Extra screws for a tall unit `tall_extra_screws`	4.0	pcs	WoodworkingShop src/engine/hardware/hardware.ts
A unit taller than this is fixed to the wall `tall_height_mm`	1200.0	mm	WoodworkingShop src/engine/hardware/hardware.ts
A unit this wide or wider takes 4 wall brackets `wall_bracket_wide_width_mm`	800.0	mm	WoodworkingShop src/engine/hardware/hardware.ts
A wall plug about every `wall_fixing_spacing_mm`	300.0	mm	BIY rule

Values the app gives to the formulas

The app works these out from the sizes you type and the design you pick, then runs the formulas above on them.

Value	Unit	What it is
Adjustable shelves `adjustable_shelves`	pcs	0: the designs use fixed shelves.
Back panel edge length `back_perimeter_mm`	mm	Twice height plus width; 0 without a back.
Boards 1x2 x 10 ft `boards_1x2_10`	pcs	The boards of this section and length the cutting plan needs.
Boards 1x2 x 12 ft `boards_1x2_12`	pcs	The boards of this section and length the cutting plan needs.
Boards 1x2 x 8 ft `boards_1x2_8`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x2 x 10 ft `boards_2x2_10`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x2 x 12 ft `boards_2x2_12`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x2 x 8 ft `boards_2x2_8`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x3 x 10 ft `boards_2x3_10`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x3 x 12 ft `boards_2x3_12`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x3 x 8 ft `boards_2x3_8`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x4 x 10 ft `boards_2x4_10`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x4 x 12 ft `boards_2x4_12`	pcs	The boards of this section and length the cutting plan needs.
Boards 2x4 x 8 ft `boards_2x4_8`	pcs	The boards of this section and length the cutting plan needs.
Depth `depth`	mm	The depth typed on the Measure tab.
Door height `door_height`	mm	0 when there are no doors.
Doors `doors`	pcs	One door under 600 mm wide, otherwise two; none for open designs.
Drawers `drawers`	pcs	None of the current designs has drawers.
Visible edge length `edge_run_m`	m	The repo's visible carcass edge: 2 x (width + height).
Area to finish `finish_area_m2`	m2	Both faces and the edges of every piece (the repo's method).
Thickness of the piece fixed `fix_thickness`	mm	The plywood thickness, or 50 for lumber.
Stands on the floor `floor_standing`	0 or 1
Glued area `glue_area_mm2`	mm2	The faces that meet at every joint.
Has a back panel `has_back`	0 or 1
Has a hanging rail `has_hanging_rail`	0 or 1
Height `height`	mm	The height typed on the Measure tab.
Is a box of panels `is_carcass`	0 or 1
Is a gate `is_gate`	0 or 1
Panel joins `joints`	pcs	The repo's count: 4 + shelves for a cabinet; the number of frame joints for lumber designs.
Sheets `sheets`	pcs	The sheets of plywood, plyboard or MDF the cutting plan needs.
Shelves `shelves`	pcs	The shelves (or the count the design asks for).
Plywood thickness `thickness`	mm	The plywood thickness picked.
Length fixed along a wall `wall_run_mm`	mm	The width of a cleat or rack; 0 for other designs.
Width `width`	mm	The width typed on the Measure tab.

What BIY adds

Cutting plans. Plywood parts are placed on 1,220 × 2,440 mm sheets with a 4 mm saw cut between them. "Straight cuts" runs every cut from edge to edge; "Save more material" packs the pieces tighter and is used only when it saves a sheet or leaves a clearly bigger offcut. Lumber parts are cut from the 8, 10 and 12 ft boards we sell, longest first, choosing the board length that wastes least.
Shelf and table-top sag. The standard beam formula, 5 × load × span⁴ ÷ (384 × stiffness × section stiffness), checked against span ÷ 360. We use a plywood stiffness of 6,500 N/mm², a little lower than the 8,300 N/mm² WoodworkingShop uses, so the warnings lean cautious. Loads: 15 kg for every metre of a book shelf, 25 kg for every metre of a desk top. These are still set in the app, not in the table above.
Sizes of parts. The sizes of the sides, tops, shelves, doors, aprons, rails and pickets are our own design for each project.

How a supply is matched to what we sell

The formulas give an amount and, for screws, plugs, hinges and slides, a size. The app then searches our store with the words in the "Searched in our store as" column, the same search you use in the app. Every variant that comes back is scored: how many of the words its title, type and tags have, whether the size written in its title fits (a screw must be long enough, a plug the right width), and whether it is sold by the piece or in a pack with a count. A product with a word it must not have, such as a metal screw for wood, is left out. The best match is shown first, with the others one tap away under "Swap". A supply nothing in our store fits is listed as "Not in our store yet", so you still see how many you need. The sheets and boards are found the same way: the cutting plan says how many, and the search finds them by kind and thickness (ordinary or marine plywood, plyboard or laminated MDF; a nominal 3/4 in sheet is sold as 18 mm) or by section and length (pine wood or coco lumber, whichever costs less comes first).

Materials and units

Material	Unit
Door handle	pcs
Gate latch	pcs
Cabinet hinge	pcs
Pine wood 1" × 2" × 10 ft	pcs
Pine wood 1" × 2" × 12 ft	pcs
Pine wood 1" × 2" × 8 ft	pcs
Coco lumber 2" × 2" × 10 ft	pcs
Coco lumber 2" × 2" × 12 ft	pcs
Coco lumber 2" × 2" × 8 ft	pcs
Coco lumber 2" × 3" × 10 ft	pcs
Coco lumber 2" × 3" × 12 ft	pcs
Coco lumber 2" × 3" × 8 ft	pcs
Coco lumber 2" × 4" × 10 ft	pcs
Coco lumber 2" × 4" × 8 ft	pcs
Plywood 1/2"	sheets
Plywood 1/4"	sheets
Plywood 3/4"	sheets
Wall bracket	pcs
Wall plug	pcs
Hanging rail	pcs
Wood screw 2"	pcs
Wood screw 1½"	pcs
Wood screw 3"	pcs
Paint for wood	litres
Wood glue (PVA)	bottles
Hinge mounting plate	pcs

Quantities are rounded up to whole items. If a supply is not sold yet, the app still tells you how many you need so you can find it elsewhere.

How we estimate

Contents

Section 1 · Estimator

Cement bag size

Concrete mixtures

Mortar mixtures

Hollow-block walls

Cement plaster

Tile adhesive

Constants

Reference tables

Quantity of cement, sand and gravel for slab and walls per sq. m. of area

Quantity of cement, sand and gravel per meter length of circular column

Quantity of cement, sand and gravel per concrete pipe

Quantity of cement and sand per one hundred blocks

Quantity of cement, sand and gravel for CHB footing per meter length

Quantity of reinforcing steel bars in a one-way RC slab (bars per sq. m.)

Quantity of tie wire on a one-way reinforced concrete slab (kg per sq. m.)

Commercial lengths of rebar (m)

Calculations and formulas

Concrete

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Masonry

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Tiling

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Steel

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Formula language

Worked examples

Materials

Section 2 · Workshop

Supply formulas

Fixed numbers

Values the app gives to the formulas

What BIY adds

How a supply is matched to what we sell

Materials and units

Slideshow