How to calculate concrete
Concrete is sold by volume. Multiply length × width × thickness, with all three in the same unit. In feet, divide the result by 27 to get cubic yards. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards.
How thick should a slab be?
- Walkways and patios: 4 in (10 cm)
- Driveways for cars: 5–6 in (12–15 cm)
- Garage floors and heavier loads: 6 in (15 cm) or more, with reinforcement
Bags or ready-mix?
An 80 lb bag of premix makes about 0.6 cubic feet, so one cubic yard takes 45 bags. That is a lot of mixing. For anything over about one yard, a ready-mix truck is usually cheaper and gives a more even slab.
Why add extra?
Ground is never perfectly flat, forms bow a little and some concrete always stays in the mixer. 5–10% extra is normal. Running short in the middle of a pour leaves a weak cold joint.