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.
Decision guide
Break the project into rectangles, round holes, footings, walls, or other measurable shapes. Calculate each section in consistent units and keep a worksheet showing dimensions, base volume, known field additions, and final allowance. That record is easier to verify than one unexplained cubic-yard total.
Six holes that are 12 inches in diameter and 3 feet deep have a combined gross volume of about 14.14 cubic feet, or 0.52 cubic yard, before subtracting embedded posts or adding an allowance. Use actual hole diameters because an auger can produce wider or irregular holes.
A volume calculator cannot select the mixture. NRMCA recommends communicating strength, exposure, aggregate size, slump, air content, quantity, delivery schedule, and placement method as applicable. Share project specifications with the producer and confirm local design requirements.