Concrete Volume Calculator

The formula this calculator uses

V = L x W x T

L

Length of the pour, in metres

W

Width of the pour, in metres

T

Thickness or depth, in metres once converted

w

Waste allowance as a decimal, so ten percent is 0.1

y

Volume one bag of mix actually yields, in cubic metres

How to check the result by hand

1. 1

   Measure length and width in metres

   Run a tape along the longest side and then across it, and record both in metres rather than mixing feet into one dimension and metres into the other. For a footing, use the run of the trench as the length and its width as the width.

2. 2

   Convert the thickness to metres

   Divide a millimetre figure by one thousand. A 100 millimetre slab becomes 0.1 metres. Skipping this conversion multiplies the answer by a thousand, which is the mistake that turns a garden path into a commercial pour.

3. 3

   Multiply the three dimensions

   4 metres times 3 metres times 0.1 metres is 1.2 cubic metres. This is the geometric volume, the space the concrete will occupy once it is placed and struck off level.

4. 4

   Add the waste allowance

   Multiply by one plus the allowance as a decimal. Ten percent means multiplying by 1.1, which lifts 1.2 cubic metres to 1.32 cubic metres. Use a larger allowance where the sub-base is rough or the formwork is flimsy.

5. 5

   Convert to bags or order ready-mix

   Divide by the yield of one bag and round up: 1.32 divided by 0.0125 is 105.6, so buy 106 bags. For a delivery, order the volume in cubic metres and round up to the nearest half metre the supplier will sell.