Figuring yards of concrete
A cubic yard is a cube 3 feet on each side, which is 27 cubic feet. So every yard calculation ends with ÷ 27.
yd³ = L(ft) × W(ft) × D(ft) ÷ 27
yd³ = sq ft × D(in) ÷ 324
4 in slab: yd³ ≈ sq ft ÷ 81
The ÷ 81 shortcut comes from 324 ÷ 4 = 81. It is exact at 4 inches and wrong at any other depth. At 6 inches the divisor is 54.
Worked example: 12 × 20 ft driveway section, 5 in thick
- Area: 12 × 20 = 240 sq ft.
- Cubic yards: 240 × 5 ÷ 324 = 3.70 yd³ net.
- With 5% waste: 3.70 × 1.05 = 3.89 yd³.
- Rounded up to the next ¼ yard: order 4.0 yd³.
How much does a yard of concrete cover?
Coverage depends only on thickness. Coverage (sq ft) = 324 ÷ thickness (in).
| Thickness | Sq ft per cubic yard |
|---|---|
| 3.5 in | 92.6 |
| 4 in | 81.0 |
| 5 in | 64.8 |
| 6 in | 54.0 |
| 8 in | 40.5 |
| 12 in | 27.0 |
This is coverage, not price. For cost per yard with your own quote, use the concrete cost calculator.
How to estimate concrete yards for an order
- Measure each part of the pour separately: slab, thickened edges, footings, piers.
- Add them as sections above. The total updates as you go.
- Decide on waste. 0% shows the true geometry. 5–10% covers uneven subgrade and spillage .
- Ask your supplier how they sell partial yards (¼ or ½ yd³ steps are common), and what the truck minimum is .
- Order a little over, not under. A short load costs more than a few spare cubic feet.
Yards, bags, or both?
Under about 1 yard, bags may be cheaper and simpler. One yard is about 45 bags of 80 lb mix. The concrete bag calculator shows exact counts, and the main concrete calculator handles stairs, curbs, and other shapes.