Concrete calculator
Work out how much premix or how many bags you need for a slab, a strip footing or a set of post holes, plus mesh, bar chairs and formwork.
Your order list
How much concrete do I need?
Multiply length × width × depth, all in metres, to get cubic metres. A 6 × 3 m slab at 100 mm thick is 6 × 3 × 0.1 = 1.8 m³. Add around 10% for uneven ground and spillage, because running short mid-pour is far worse than a little left over.
Premix is ordered by the cubic metre, usually in 0.2 m³ steps. Under about half a cubic metre, bags are often easier. For small pours, a mini-mix or small-load truck saves paying for a full truck.
Before the truck arrives
- Excavate, compact the base and set your formwork level with the right fall for drainage.
- Lay the plastic barrier, then sit the mesh on bar chairs so it ends up in the middle of the slab, not on the ground.
- Plan access. A full truck needs a firm path, or book a pump or line up barrows and helpers.
- Keep the slab damp for the first week so it cures properly.
These results are estimates to help you plan and order. Slab thickness, mesh and footing sizes depend on what the concrete is carrying and on your soil, so check with your supplier or an engineer. Driveways, house slabs and structural footings usually need an engineered design.