Volume = length × width × depth
Measure all three in the same unit before multiplying.
- Metric: a slab 4 m × 3 m and 10 cm deep is 4 × 3 × 0.1 = 1.2 m³.
- Imperial: a slab 10 ft × 12 ft and 4 in deep is 10 × 12 × (4 ÷ 12) = 40 ft³. Divide by 27 for cubic yards: 1.48 yd³.
For a round column or post hole, the volume is π × radius² × depth.
Add extra
Ground is never perfectly level and some concrete is always lost, so add 5 to 10%. Running short in the middle of a pour leaves a weak joint, while a little left over is cheap.
Bags or ready-mix
An 80 lb bag of premix makes about 0.6 ft³ of concrete and a 60 lb bag about 0.45 ft³. The 40 ft³ slab above needs about 67 bags of 80 lb before the extra. At around a cubic metre or more, ready-mix from a truck is usually easier and more consistent than mixing bags by hand.
Reinforcement
Slabs are usually reinforced with mesh or a grid of rebar. The Rebar Calculator gives the number of bars each way at your spacing, their total length and weight.
Calculator
The Concrete Calculator works out slabs, footings and columns in metres or feet, with the extra you choose and the number of bags.