Concrete calculator
Volume is length × width × depth, or πr² × height for a column. The two figures that matter on site are the number of bags and the allowance for spillage and uneven ground — a job calculated exactly always runs short.
- Bags of dry mix
- 127
- Volume with no allowance, m³
- 1.440
- Volume of one, m³
- 1.440
- Weight once placed, kg
- 3,802
- Weight of the bags, kg
- 3,175
- Volume in litres
- 1,584
- Bags or ready-mix
- Over 1 m³: ready-mix by truck is usually cheaper
A quantity estimate, not a structural design. Foundations, slabs that carry a building and anything reinforced should be specified by a qualified engineer.
The formula
Slab or footing: V = length × width × thickness. Column: V = π × (diameter ÷ 2)² × height. Order V × (1 + waste). A 25 kg bag of dry mix yields about 0.0125 m³, so roughly 80 bags make a cubic metre.
Worked example
A 4 × 3 m slab, 12 cm thick, is 1.44 m³; with 10% allowance, 1.584 m³. In 25 kg bags that is 127 bags — enough that ready-mixed concrete is the better choice.
How to use it
- 1Enter the thickness in centimetres and the other dimensions in metres.
- 2Use 5% on a well-prepared, level base and 10% on rough ground or for a first job.
- 3For a hollow column or a slab with a void, calculate the void separately and subtract it.
Questions people ask
How thick should a slab be?
Around 10 cm for a path or a shed base, 12 to 15 cm for a garage or driveway. Structural slabs carrying a building are specified by an engineer, not by rule of thumb.
Why is the wet weight so much more than the bags?
Because water is added on site, and placed concrete weighs about 2,400 kg per cubic metre. It matters for barrows, formwork and anything the concrete rests on before it cures.
Last reviewed: 2026-10-20