Supply MathHow the calculation works
Volume (ft³) = length (ft) × width (ft) × thickness (ft). Convert inches to feet by dividing by 12, so a 4-inch slab is 0.333 ft thick. Divide cubic feet by 27 to get cubic yards. For round post holes and sonotubes, volume = π × radius² × depth.
Bag counts use the manufacturer’s yields: an 80 lb bag makes about 0.60 ft³, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. We round up, because you can’t buy part of a bag.
Concrete for common slab sizes (4 inches thick, +10%)
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 ft | 0.22 | 10 | 14 |
| 8 × 8 ft | 0.87 | 40 | 53 |
| 10 × 10 ft | 1.36 | 62 | 82 |
| 10 × 12 ft | 1.63 | 74 | 98 |
| 12 × 12 ft | 1.96 | 88 | 118 |
| 10 × 20 ft | 2.72 | 123 | 163 |
| 20 × 20 ft | 5.43 | 245 | 326 |
| 24 × 24 ft | 7.82 | 352 | 470 |
Estimates only. Ground prep, forms and spillage change the real amount, so order a little extra.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet (divide inches by 12). That gives cubic feet. Divide by 27 to get cubic yards, then add 5–10% for waste and uneven ground. The calculator above does all of this for you.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb, based on Quikrete’s published yields (0.60, 0.45 and 0.30 cubic feet per bag).
Should I use bags or ready-mix?
Bags work well up to about 1 cubic yard. Above that, ready-mix delivery is usually cheaper per yard and far less work, though many suppliers charge a short-load fee for small orders.
How thick should a concrete slab be?
4 inches is typical for patios, walkways and shed floors. Driveways are usually 4–6 inches, and 6 inches or more for heavy vehicles. Follow local codes for structural work.