The formula
Volume = length × width × thickness. For a 10 × 10 ft patio 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 cubic yards (divide cubic feet by 27). Round columns and post holes use π × (diameter ÷ 2)² × depth.
How many bags?
An 80 lb bag of premix makes about 0.60 ft³, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. That 1.23 yd³ patio needs about 61 bags of 80 lb with 10% extra — at that size, a ready-mix truck is usually cheaper and much less work.
Always add extra
Ground is never perfectly flat and forms bow a little. Add 5–10% so you don't run short in the middle of a pour.
Frequently asked questions
How thick should a slab be?
4 inches is typical for patios and walkways; driveways usually need 5–6 inches. Follow local codes and your contractor's advice.
How much does a yard of concrete weigh?
About 4,000 lb (roughly 1.8 tonnes).
Can I mix my own concrete?
Yes — switch to "Mix on site" to see cement, sand and gravel by volume ratio.