Enter the length, width and thickness of your slab, patio or footing to estimate the concrete to order. Results are shown in cubic yards (how ready-mix is sold in the US), cubic metres, and the number of pre-mixed bags for smaller jobs.
Formula
Volume = Length × Width × Thickness × (1 + Waste %)
Convert everything to the same unit first. In imperial, divide cubic feet by 27 to get cubic yards. Bag counts are rounded up and use typical yields; check the label on the bags you buy.
Worked example
A 10 ft × 10 ft slab, 4 in thick: 10 × 10 × 0.333 = 33.3 ft³ ≈ 1.23 yd³. Adding 10% for waste gives about 1.36 yd³, or 62 bags of 80 lb mix.
How thick should a slab be?
4 inches (10 cm) is typical for patios, sidewalks and shed bases. Driveways usually need 5–6 inches (12–15 cm). Follow local building codes for structural work.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet and divide by the bag yield: about 0.6 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag. Round up.
Should I order extra concrete?
Yes. 5–10% extra covers uneven ground, spillage and form movement. Running short mid-pour is far worse than a little left over.
Last updated · Calculations checked by automated tests· Editorial policy