How to calculate concrete for a slab
Concrete is ordered by volume. Multiply length × width × thickness, keeping every measurement in feet (divide inches by 12). That gives cubic feet; divide by 27 to get cubic yards, the unit ready-mix suppliers use.
Example: a 10 × 10 ft patio, 4 in thick: 10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cu yd. Adding 10% waste gives about 1.36 cu yd.
Bags vs. ready-mix truck
Typical pre-mixed bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft (check your bag label—yields vary slightly by brand). For jobs over about 1 cubic yard (45 × 80 lb bags), a ready-mix delivery is usually cheaper and gives a more uniform pour.
Recommended thickness
- Walkways and patios: 4 in
- Driveways for passenger cars: 4–5 in; heavier vehicles: 6 in
- Shed bases: 4 in, thickened at the edges
Why add waste?
Uneven subgrade, form flex and spillage commonly consume 5–10% extra. Running short mid-pour creates a cold joint, so it is cheaper to order a little more.
✓ Formula reviewed and tested · Updated Oct 2026