How the concrete calculator works
Concrete is a volume calculation followed by a conversion to whatever you can buy: bags of ready mix, or cement, sand and aggregate for mixing on site.
- Volume. Length × width × depth for each slab or footing, and π ÷ 4 × diameter² × depth for each post hole or column, times the count, added up.
- Waste. A default 10 % for uneven ground and spillage.
- Bags. Volume with waste ÷ yield per bag, rounded up. Enter the yield from your bag: 25 kg gives about 11.7 litres.
- Site mix. Volume × 1.54 gives the dry material. It is split by the mix ratio; cement volume × 1440 kg/m³ gives kilograms and whole bags, while sand and aggregate are reported in cubic metres and approximate kilograms.
Measuring slabs and footings
For a slab, measure the length and width of the formwork and the thickness of the slab, not the excavation. For a strip footing, use the length of the trench, its width and the depth of concrete. For post holes and round columns, enter the diameter of the hole or column and its depth or height in the second list; the post itself displaces very little, so do not subtract it. Enter a count for repeated elements.
Bags or ready mixed?
Bagged concrete is convenient up to about 0.5 m³, which is around 43 bags of 25 kg. Beyond that, mixing on site from cement, sand and aggregate is cheaper, and beyond 2 to 3 m³ a ready-mixed delivery is usually cheaper still and far quicker. Use the site-mix option to size a delivery of sand and aggregate; the cubic metres and approximate weights are what the merchant will ask for.
Imperial units
Switch to imperial to enter feet and inches, with bag yields in cubic feet and the result in cubic yards. The calculation is done in cubic metres behind the scenes.
Worked example
A 4 × 3 m patio slab 100 mm thick is 1.2 m³. With the default 10 % waste that is 1.32 m³. In 25 kg bags yielding 11.7 litres each, that is 112.8 bags, so 113 bags, which is a clear sign to mix on site or order a delivery instead. As a 1:2:4 site mix, the 1.32 m³ needs 2.03 m³ of dry material: 0.29 m³ of cement, which is 418 kg or 17 bags of 25 kg, plus 0.58 m³ of sand and 1.16 m³ of aggregate. Those two volumes are what you order from the builders’ merchant.