How much concrete do you need?

Work out cubic yards or cubic metres for slabs, patios, and post holes — and how many bags that means.

Concrete is sold by volume: cubic yards in the United States, cubic metres in most other places. The math is length × width × depth, but the units trip people up — especially slab thickness, which is usually given in inches. Slabs, patios, and driveways Volume = length × width × thickness, with all three in feet. Turn the thickness into feet by dividing the inches by 12: a 4-inch slab is 0.333 ft. Then divide the cubic feet by 27 to get cubic yards, because one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Round post holes and columns Volume = π × radius² × depth. A post hole 10 inches across has a radius of 5 inches (0.417 ft). At 3 ft deep it holds 3.1416 × 0.417² × 3 ≈ 1.64 cubic feet. Subtract the post itself if it is large. Footings and steps A footing is a long, narrow strip of concrete under a wall or deck, so it is just another box: length × width × depth. A trench 20 ft long, 1.5 ft wide, and 1 ft deep holds 30 cubic feet, or about 1.11 cubic yards. For solid concrete steps, treat each step as its own box: the step’s width × its tread depth × its height measured from the ground. Add the boxes together. Bags or a ready-mix truck? For small jobs, premixed bags are easies…