Concrete is ordered by volume, in cubic metres, so the key calculation is length times width times thickness, all in metres. The same idea extends to footings and columns, and a small waste allowance covers spillage and uneven ground.
How it works
Volume = length x width x thickness, with all three in metres. For a rectangular slab, that is the whole job. For footings, use the footing length, width, and depth. For a round column, use pi x radius squared x height. Add waste: 5 percent for a flat, well-formed slab, 10 percent for uneven ground or complex shapes. Round up to the nearest quarter or half cubic metre as your supplier requires, and never round down.
Step by step
- Measure length and width of the slab in metres.
- Measure the thickness, or depth, in metres, converting centimetres by dividing by 100.
- Multiply the three numbers for the net volume.
- For footings and columns, compute each shape separately and add them.
- Multiply by 1.05 or 1.10 for waste.
- Round up to your supplier order increment and confirm the mix and slump.
Worked example
A slab 6 m long, 4 m wide, and 0.15 m thick: net volume = 6 x 4 x 0.15 = 3.6 cubic metres. With 10 percent waste: 3.6 x 1.10 = 3.96, so order 4.0 cubic metres. A footing 10 m long, 0.5 m wide, and 0.6 m deep adds 10 x 0.5 x 0.6 = 3.0 cubic metres before waste. A round column of radius 0.25 m and height 3 m adds 3.1416 x 0.0625 x 3 = 0.59 cubic metres.
Common mistakes
- Mixing centimetres and metres in one multiplication, which throws the volume off by a factor of 100.
- Forgetting waste and running short on the last section of the pour.
- Measuring the outside of the formwork instead of the inside, where concrete actually goes.
- Rounding down to save money and ending up with an unfinished slab.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
Measure thickness at several points, because ground rarely sits perfectly level. If the sub-base is uneven, use the larger waste allowance. Order a little extra rather than a second small load, which costs more per cubic metre.