How it is calculated
Volume is the product of the three measurements, and waste is added. Cement is estimated at 280, 320, or 380 kg per cubic metre, as a guide for grades M200, M250, and M300. The whole mix is taken as 2 400 kg/m³.
Volume = length × width × thickness
Where the numbers come from
- Length — unit in the field: m
- Width — unit in the field: m
- Thickness — unit in the field: m
- A guide for cement
- Waste — unit in the field: %
Example
A slab 4 × 3 m and 0,15 m thick. Volume 1,8 m³, with 5% waste — 1,89 m³.
Questions
Does this replace the factory recipe?
No. Bought-in concrete needs the volume first. Kilograms of cement are a rough figure for mixing on the spot.
Thickness in meters?
Yes. 15 cm is entered as 0,15. A strip footing is easier the same way: the full length, the trench width, and the depth of the pour.
Is the rebar subtracted?
No. Its volume is small beside the waste allowance, so a concrete estimate usually leaves it in.