Work out how much concrete you need for a slab, footing or round column, in cubic meters or yards and in bags.
Concrete needed
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| Bag size | Bags needed |
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Slab: length × width × thickness
Round column: π × (diameter ÷ 2)² × height
1 cubic yard = 27 cubic feet = 0.765 m³. Typical thicknesses: 4 in (10 cm) for patios and paths, 5–6 in (12–15 cm) for driveways.
Covering the slab with tiles afterwards? Use the tile calculator.
Multiply length × width × thickness, using the same unit for all three. A 10 ft × 10 ft slab that is 4 inches (0.333 ft) thick needs 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards.
An 80 lb bag of premixed concrete makes about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. A 25 kg bag makes about 0.012 m³. The calculator divides your volume by the bag yield and rounds up.
Add 5–10% for spillage, uneven ground and forms that bow slightly. Running short in the middle of a pour is far worse than having a little left over.
Above about 1 cubic yard (0.75 m³), ready-mixed concrete from a truck is usually cheaper and much less work than mixing 45 or more bags by hand.