Key takeaways
- Cubic yards = length × width × thickness, all in feet, divided by 27.
- For a slab, square feet × thickness in inches ÷ 324 gives the same answer.
- Add an allowance before you order, because running short mid-pour is a real problem.
Multiply length × width × thickness, all in feet, then divide by 27. There are 27 cubic feet in a cubic yard, because a yard is 3 feet and 3 × 3 × 3 = 27.
Most mistakes come from thickness, which is usually in inches. Divide inches by 12 before you multiply.
Calculator
Worked example
A driveway section 20 feet long, 10 feet wide and 4 inches thick:
- Thickness in feet: 4 ÷ 12 = 0.333
- Cubic feet: 20 × 10 × 0.333 = 66.7
- Cubic yards: 66.7 ÷ 27 = about 2.47
A shortcut for slabs
Cubic yards = square feet × thickness in inches ÷ 324
The 324 comes from 27 × 12. For the example above: 200 × 4 ÷ 324 = 2.47.
Other shapes
Find the shape's area in square feet, multiply by thickness in feet, then divide by 27.
| Shape | Area formula | Example, 4 inches thick | Cubic yards |
|---|---|---|---|
| Rectangle | length × width | 10 × 12 ft | 1.48 |
| Circle | 3.14 × radius² | 10 ft across (5 ft radius) | 0.97 |
| Triangle | ½ × base × height | 10 ft base, 8 ft height | 0.49 |
For an unusual shape, split it into rectangles, circles and triangles, then add the results. For footings, columns and posts, use these tools instead:
Cubic yards to bags
Using QUIKRETE's published yields, one cubic yard takes about 45 bags of 80 lb, 60 of 60 lb, or 90 of 40 lb. See how many bags of concrete you need.
The mistake that matters most
Entering inches as feet. A 4-inch slab entered as "4 feet" comes out 12 times too large. If your answer looks far off, check this first.
Here's a quick way to check a slab:
| Area at 4 inches thick | About |
|---|---|
| 81 sq ft | 1 cubic yard |
| 162 sq ft | 2 cubic yards |
| 405 sq ft | 5 cubic yards |
Frequently asked questions
How many cubic feet are in a cubic yard?
How do I convert inches of thickness to feet?
Divide by 12. So 4 inches is 0.333 feet, 5 inches is 0.417 feet, and 6 inches is 0.5 feet.
How much extra should I order?
There's no single rule we can cite, because it depends on how even your base is and how accurate your forms are. We default to 5% as a planning figure. Ask your supplier what they suggest for your job.
How much area does a cubic yard cover?
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches, it covers 54 square feet.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- QUIKRETE: Concrete Mix data sheet, product 1101 (bag yields)