Quick answer
To calculate yards of concrete: cu yd = length × width × thickness (ft) ÷ 27. Shortcut: sq ft × inches ÷ 324. Use the free concrete calculator.
How to calculate concrete (step-by-step)
- Measure length and width in feet (or start from square feet).
- Choose thickness in inches; convert to feet (÷12).
- Divide cubic feet by 27 for cubic yards.
- Add 5–10% waste.
- Convert to bags if the pour is small, or order ready-mix.
Cubic yards = Square feet × Thickness (inches) ÷ 324
Worked examples
| Project | Size | Thickness | Cu yd (no waste) |
|---|---|---|---|
| Patio slab | 12 × 12 | 4 in | 1.78 |
| Shed pad | 8 × 10 | 4 in | 0.99 |
| Driveway section | 12 × 40 | 5 in | 7.41 |
| Sidewalk | 4 × 50 | 4 in | 2.47 |
Bags vs ready-mix
About 45 of the 80 lb bags make one cubic yard. For anything over roughly 1–1.5 cubic yards, ready-mix is usually faster and cheaper once you include labor and short-load fees.
Cost and slab depth: concrete slab cost guide. Bags overview: concrete calculator guide.