How to calculate concrete volume
Concrete is ordered by volume. For a slab or footing, multiply length by width by thickness, with every dimension in the same unit. Then add a waste allowance of 5 to 10% for spillage, over-excavation and uneven subgrade.
For a circular column use V = π × D² ÷ 4 × H. For stairs, add the volume of the triangular steps to the inclined waist slab under them.
Worked example
A slab 5 m long, 4 m wide and 150 mm thick needs 5 × 4 × 0.15 = 3.00 m³ of concrete. With 5% waste, order 3.15 m³ (about 4.12 yd³).
From volume to cement, sand and gravel
Dry materials take up more space than the finished concrete, so the wet volume is multiplied by 1.54 to get the dry volume. That dry volume is split by the mix ratio. For M20 (1 : 1.5 : 3), cement is 1/5.5 of the dry volume, and one cubic metre of cement weighs about 1440 kg.
| Grade | Cement : Sand : Gravel | Typical use |
|---|---|---|
| M10 | 1 : 3 : 6 | Mass concrete, levelling |
| M15 | 1 : 2 : 4 | Blinding, pathways |
| M20 | 1 : 1.5 : 3 | Slabs, beams, columns |
| M25 | 1 : 1 : 2 | Structural members |
Frequently asked questions
Why multiply by 1.54 for dry volume?
When water is added, the voids between sand and gravel particles fill and the mix shrinks. About 54% more dry material is needed than the final wet volume.
How many cubic yards is one cubic meter?
One cubic meter equals about 1.308 cubic yards, and one cubic yard equals about 0.765 cubic meters.
How much waste should I allow?
Use around 5% for formed elements like columns and beams, and up to 10% for slabs on uneven ground.
Is this calculator suitable for ready-mix orders?
Yes. Use the total volume including waste when ordering ready-mix. The cement, sand and gravel breakdown is for site-mixed concrete.
Results are estimates for planning. Confirm quantities with your engineer or concrete supplier.