Concrete Calculator
Calculate concrete volume and the cement, sand and aggregate needed for a slab using an M20 mix.
Formula last reviewed 4 August 2026 · How we verify our calculators
Concrete volume
0.90 m³
- Cement
- 8 bags
- Sand
- 0.38 m³
- Aggregate
- 0.76 m³
Updates live as you type
Frequently asked questions
Multiply length × width × thickness, all in metres. A 3 × 2 m slab that is 0.15 m thick is 0.9 cubic metres of concrete.
M20 is a common structural grade with a 1:1.5:3 ratio of cement, sand and aggregate, used for slabs, beams and columns in residential work.
Dry materials have voids that fill when mixed with water, so the dry quantity is about 1.54 times the finished wet volume. This factor is built into the estimate.
Roughly 8 bags (about 400 kg) of cement per cubic metre of M20 concrete, varying slightly with the exact mix and assumptions.
Double the slab thickness, double every material on the order
A slab 3 m long, 2 m wide and 0.15 m thick works out to a wet volume of 3 × 2 × 0.15 = 0.9 m³, which an M20 (1:1.5:3) mix turns into 8 bags of cement, roughly 0.38 m³ of sand and 0.76 m³ of aggregate. Double the thickness to 0.3 m for a heavier-duty slab, and every one of those figures doubles in step — the whole calculation scales linearly with volume, a useful mental shortcut whenever a design changes mid-planning without needing to re-derive the whole estimate from scratch.
Where the 1.54 factor comes from, and why it's not optional
Multiplying length × width × thickness gives the wet, finished volume of concrete — but the dry cement, sand and aggregate needed to produce that volume is larger, because dry materials carry voids between particles that only close once water is added and the mix compacts. The standard 1.54 dry-volume factor accounts for that gap; skip it and the material estimate comes in short of what the pour actually needs.
Why M20 is the default here, and when it isn't the right one
M20 is the workhorse grade for much of residential construction — slabs, beams, columns — which is why it's the default mix ratio in this calculator. If a specific design calls for a different grade, the ratio changes, and so does every downstream quantity; always confirm the specified grade with the engineer before ordering rather than assuming M20 fits every job.
This is an estimate for planning purposes. Always add a wastage margin and confirm exact quantities with your contractor or engineer before ordering materials. Site conditions, compaction technique, formwork leakage and general spillage during mixing and pouring all push real-world consumption above this theoretical figure — pair this with the cement and steel calculators to build a fuller bill of materials for the same slab, footing or beam rather than pricing each material in isolation.