Concrete Calculator
Work out how much concrete you need for a slab, patio or footing — in m³, yd³ and premix bags.
Bag counts use typical premix yields and are estimates. For volumes above about 1 m³ (1.5 yd³), ready-mix delivery is usually cheaper than bags.
About this tool
Enter the length, width and thickness of a slab, patio, pad or footing to get the concrete volume, the amount to order with a waste allowance, and how many premix bags it takes. Works in metric (m, cm) and imperial (ft, in).
Frequently asked questions
Multiply length × width × thickness, using the same unit for all three. A slab 4 m long, 3 m wide and 10 cm (0.10 m) thick needs 4 × 3 × 0.10 = 1.2 m³. In imperial, divide cubic feet by 27 to get cubic yards.
Most builders add 5–10% for spillage, uneven ground and formwork that bows slightly. Use 10% for small DIY jobs and uneven sub-bases, and 5% for well-prepared, level forms.
Typical thicknesses are 10 cm (4 in) for patios and paths, 10–15 cm (4–6 in) for shed and garage bases, and 15 cm (6 in) or more for driveways that carry vehicles. Always follow local building rules for structural work.
A 25 kg bag of premixed concrete yields roughly 0.0125 m³ (12.5 litres), so a cubic metre takes about 80 bags. In imperial units, an 80 lb bag yields about 0.6 ft³, so a cubic yard takes about 45 bags.
Bags are practical for small jobs such as post holes, steps and small pads. Above roughly 1 m³ (about 1.5 yd³), ready-mix delivered by truck is usually cheaper and gives a more even pour.
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