How to calculate concrete
Concrete is ordered by volume, in cubic yards. The formula is:
Cubic yards = length × width × thickness ÷ 27 (all measurements in feet)
Because thickness is usually given in inches, divide it by 12 to convert to feet first. This calculator does the conversion for you, adds your waste allowance, and also shows how many bags it would take if you are mixing by hand.
Common slab thicknesses
| Project | Typical thickness |
|---|---|
| Walkways and patios | 4 in |
| Driveways (cars) | 4–5 in |
| Driveways (heavy vehicles) | 6 in |
| Shed and garage floors | 4–6 in |
Bags vs. ready-mix
- Small jobs. For posts and small pads, bagged mix is convenient — but it adds up fast, as it takes about 45 80-lb bags per cubic yard.
- Larger pours. At roughly a cubic yard or more, ready-mix delivery is usually cheaper and dramatically less work.
- Order extra. Add 5–10% — you cannot pause a pour to run to the store.
Planning a slab? Read our guide on how much concrete you need for a slab.