Concrete calculator
Enter the size of a slab, footing or floor to estimate the volume of concrete and the cement, sand and stone to order.
Planning figures from standard rules of thumb. Order quantities can differ with the moisture in the sand, the shape of the aggregate and site wastage.
How this is worked out
- Volume is length x width x thickness, with 5% added for wastage and spillage.
- Dry volume of materials is taken as 1.54 times the wet volume of concrete.
- The dry volume is shared between cement, sand and stone in the ratio of the mix.
- One 50 kg bag of cement is taken as 0.0347 m³.
- These are nominal mixes; a structural engineer specifies the grade for structural members.
What it works out
The calculator turns the size of a slab, footing or floor into a volume of concrete, then into the cement bags, sand and stone aggregate needed for the mix you choose. It is meant for small jobs such as a floor, a slab over a porch or a footing, and for checking a supplier's quantities.
How to use it
- Enter the length and width in feet and the thickness in inches.
- Choose the mix: M15 (1 : 2 : 4), M20 (1 : 1.5 : 3) or M25 (1 : 1 : 2).
- Read the cement in 50 kg bags, and the sand and stone in cubic metres and cubic feet.
A worked example
A slab 10 feet by 10 feet and 4 inches thick is about 0.94 m³ of concrete. At M20 it needs roughly 8 bags of cement, 0.42 m³ of sand and 0.83 m³ of stone, including 5% wastage.
Structural work
The grade and reinforcement for columns, beams and roof slabs are set by your structural engineer. Ready-mixed concrete is often specified for these, and it is ordered by volume.
Questions about this tool
Cement, sand and stone shrink when they are mixed with water, so the dry materials take up about 1.54 times the volume of the finished concrete.
M15 suits levelling and plain floors, M20 is common for slabs and footings in small houses and M25 for stronger work. Your structural engineer decides the grade for structural members.
No. Reinforcement is designed and quantified separately by the structural engineer.
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