How much concrete do I need?

How it works

Concrete is ordered by volume. A slab or a strip footing is a box, length × width × thickness; a round post hole is a cylinder, π × r² × depth, where r is half the diameter. Multiply by how many you pour, then add a little extra, because forms bulge, the ground is never perfectly flat and some is always spilled.

Example: a 3 m × 2 m slab 10 cm thick is 3 × 2 × 0.1 = 0.6 m³. With 10 % extra, order 0.66 m³, or 660 litres. If a 25 kg bag of pre-mix makes about 12 litres, that is 660 / 12 = 55 bags. A post hole 30 cm across and 1 m deep holds π × 0.15² × 1 ≈ 0.071 m³, about six bags.

The yield of a bag depends on the mix, so use the figure printed on the bag. For more than a cubic metre or so, ready-mixed concrete from a truck is usually easier and cheaper than bags. The volume counts the hole or form as you measured it, not gravel, reinforcement or a post set into the hole; the post takes some room, so a little concrete is usually left over.