Concrete volume formulas
- Slab, footing or wall: length × width × thickness
- Round column or post hole: π × (diameter ÷ 2)² × height
Convert thickness to feet before multiplying (4 inches = 0.333 ft), then divide cubic feet by 27 to get cubic yards. A 10 × 10 ft slab 4 inches thick needs 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd, or 1.36 cu yd with 10% waste.
Bags or ready-mix?
An 80 lb bag makes about 0.6 cu ft of concrete, so one cubic yard takes 45 bags. For anything larger than about one cubic yard, ready-mix delivery is usually cheaper and far less work.
Typical thickness
- Paths and patios: 4 inches (10 cm)
- Driveways: 5–6 inches (12–15 cm)
- Garage floors: 6 inches (15 cm)