How to calculate concrete
Cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27
A 10 × 10 ft patio, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, ÷ 27 = 1.23 cubic yards, before waste. That's about 56 bags of 80 lb mix.
For a round column or post hole, the volume is π × radius² × height. Keep inches and feet straight: a 12-inch diameter has a radius of 0.5 ft.
Bags or ready-mix?
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 ft³ | 45 |
| 60 lb | about 0.45 ft³ | 60 |
| 40 lb | about 0.30 ft³ | 90 |
Bags make sense for small jobs like setting posts or a small pad. Above about a cubic yard, ready-mix delivered by truck is usually cheaper and much less work. Many suppliers sell in quarter-yard steps and charge a fee for small loads.
Typical thickness
- Sidewalks and patios: 4 inches.
- Driveways: 4 to 6 inches (6 for heavy vehicles).
- Garage floors: 4 to 6 inches.
- Footings: follow your local building code and plans.
Add 5 to 10% for spillage and uneven ground; uneven subgrade can eat more than you expect.