Concrete slab calculator
A slab is a rectangular prism. The arithmetic is trivial; the waste allowance is where the judgement is.
%
m3 per bag
A 25 kg bag makes about 0.011 m3; an 80 lb bag about 0.017 m3.
Concrete to order
2.640m³
- Net volume
- 2.400
- In cubic yards
- 3.453
- Bags needed
- 240
- Weight of the concrete
- 6336
- Slab area
- 24.00
- Volume in litres
- 2640
Show the working
- Length = 6
- Width = 4
- Thickness = 100
- Waste allowance = 10
- Bag yield = 0.011
- Net volume = 6 * 4 * 100 / 1000 = 2.4
- Concrete to order = 2.4 * (1 + 10/100) = 2.64
- Bags needed = ceil(2.64 / 0.011) = 240
- Weight of the concrete = 2.64 * 2400 = 6336
- In cubic yards = 2.64 / 0.764554857984 = 3.452 99
- Slab area = 6 * 4 = 24
- Volume in litres = 2.64 * 1000 = 2640
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- volume = length x width x thickness
- order = volume x (1 + waste)
Ready-mix is sold in fixed increments, usually a quarter cubic metre or a quarter cubic yard. Round up to the next one.
Running out mid-pour leaves a cold joint. Ten percent waste is cheap insurance.