Concrete calculator
Work out the cubic metres of concrete for a slab, base, footing, post holes or steps, and how many bags that is.
Uses only the sizes you enter.
Show the workings
| Step | Sum | Answer |
|---|---|---|
| Volume | 3 m × 3 m × 0.1 m | 0.9 m³ |
| In litres | 0.9 × 1,000 | 900 litres |
Where the figures come from
- Litres in a cubic metre: 1,000 litres per m3. Source: Weights and Measures Act 1985, Schedule 1. Checked 13 November 2026.
How much concrete do I need?
Concrete is ordered by volume. Multiply the length and width of the slab, in metres, by its depth, also in metres. A base 3 m by 3 m and 100 mm (0.1 m) deep needs 3 × 3 × 0.1 = 0.9 cubic metres.
For post holes, work out one hole as a cylinder and multiply by the number of holes. The post itself takes up some room, so you will have a little left over.
How many bags?
Bags of ready-to-use concrete or post mix say how much concrete one bag makes, called its yield. It is not the same as the bag's weight. Enter the yield in litres and the calculator divides it into the volume. A cubic metre is 1,000 litres.
For bigger jobs, ready-mixed concrete is delivered by the cubic metre: give the supplier the volume above.
Mix ratios and depths
How strong the concrete needs to be, and how deep, depends on what it will carry and the ground under it. The mix ratio and depth for structural work, such as foundations, should come from your building control body or a structural engineer, not from a calculator.