Enter your slab or column dimensions and get the concrete to order — in cubic yards for ready-mix and bags if you're mixing it yourself, with a waste factor built in.
Concrete volume is length × width × thickness, with the thickness converted from inches to feet. A handy shortcut for slabs is length(ft) × width(ft) × thickness(in) ÷ 324, which folds in both the inch-to-foot and the cubic-feet-to-cubic-yards conversions. A 10 × 10 ft slab at 4 inches works out to about 1.23 cubic yards before waste. Round columns use π × radius² × height instead. This calculator handles both and adds your waste allowance.
Bagged concrete works for small jobs — an 80 lb bag yields 0.6 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30. But bags add up fast: a single cubic yard is 45 eighty-pound bags. Once you're past about 1–2 cubic yards, ready-mix delivery is usually cheaper and far faster, though suppliers often have a short-load minimum (around 1–1.5 yards) and a surcharge below it.
Always order a little extra — 10% is the practical standard, more for uneven ground or a sloped subgrade. Running short mid-pour is a real problem: concrete sets on its own schedule, and a second batch that arrives late won't bond cleanly to the first. A small amount left over is cheap insurance.
A 10 × 10 ft slab at 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet) before waste — roughly 1.36 yards with a 10% allowance. That's about 56 eighty-pound bags, or a ready-mix short load.
About 45 bags of 80 lb (each yields 0.6 cu ft), 60 bags of 60 lb, or 90 bags of 40 lb. A cubic yard is 27 cubic feet. Past 1–2 yards, ready-mix is usually cheaper than bags.
Add about 10% for waste — spillage, uneven subgrade, and measurement variation. Use 15% for sloped or rough ground. Running short mid-pour is far worse than having a little left over.