Yardage formula
Concrete is usually ordered by the cubic yard. Measure the shape in feet, calculate cubic feet, then divide by 27 and add a practical waste buffer.
For a rectangular slab, patio, or footing, use length x width x depth in feet. A 12 ft x 20 ft slab at 4 in thick is 12 x 20 x 0.333 = 80 cubic feet. Divide by 27 to get 2.96 cubic yards before waste.
Add 10% waste and the same patio becomes about 3.26 cubic yards to order.
For fence posts or tubes, use pi x radius x radius x depth x count. Convert diameter from inches to feet first, then divide by two for radius.
Round post holes often need more buffer if the auger wanders, the soil crumbles, or the bottom bells out.
| Project | Dimensions | Base yards | With 10% waste |
|---|---|---|---|
| Small patio | 10 ft x 12 ft x 4 in | 1.48 yd3 | 1.63 yd3 |
| Driveway section | 12 ft x 24 ft x 5 in | 4.44 yd3 | 4.89 yd3 |
| Sidewalk | 4 ft x 40 ft x 4 in | 1.98 yd3 | 2.17 yd3 |
| Continuous footing | 40 ft x 16 in x 8 in | 1.32 yd3 | 1.45 yd3 |
Use the live calculator for exact dimensions, post holes, bag counts, and cost.