Concrete slab calculator
Enter the length, width and thickness of your slab. See how much concrete to order.
The slab formula
Cubic feet = length (ft) x width (ft) x thickness (in) / 12. Divide that by 27 for cubic yards. For a 12 x 12 ft slab at 4 inches, that is 48.00 cubic feet, or 1.78 cubic yards.
| Slab size | Volume | Cubic yards | Order with 10% extra | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 x 4 ft | 5.33 cu ft | 0.20 | 0.22 cu yd | 10 | 14 |
| 6 x 6 ft | 12.00 cu ft | 0.44 | 0.49 cu yd | 22 | 30 |
| 8 x 8 ft | 21.33 cu ft | 0.79 | 0.87 cu yd | 40 | 53 |
| 10 x 10 ft | 33.33 cu ft | 1.23 | 1.36 cu yd | 62 | 82 |
| 10 x 12 ft | 40.00 cu ft | 1.48 | 1.63 cu yd | 74 | 98 |
| 12 x 12 ft | 48.00 cu ft | 1.78 | 1.96 cu yd | 88 | 118 |
| 12 x 20 ft | 80.00 cu ft | 2.96 | 3.26 cu yd | 147 | 196 |
| 20 x 20 ft | 133.33 cu ft | 4.94 | 5.43 cu yd | 245 | 326 |
Thickness changes the total a lot
Every extra inch adds volume across the whole slab. Going from 4 to 6 inches adds half again as much concrete.
| Thickness | Cubic yards | Order with 10% extra | 80 lb bags |
|---|---|---|---|
| 3 inches | 0.93 | 1.02 cu yd | 46 |
| 4 inches | 1.23 | 1.36 cu yd | 62 |
| 5 inches | 1.54 | 1.70 cu yd | 77 |
| 6 inches | 1.85 | 2.04 cu yd | 92 |
| 8 inches | 2.47 | 2.72 cu yd | 123 |
Before you order
- Measure the length and width at several points, and use the larger numbers if the edges are not square.
- If the ground is uneven or the base is soft, you will use more concrete than the flat math. That is what the extra percentage is for.
- Many suppliers have a minimum delivery, so ask what size loads they send.
The numbers here are estimates for planning, not engineering advice. For driveways, garage floors and anything that carries heavy loads, follow local requirements and talk to a contractor. To see the same slab as bags, use the bags of concrete calculator.
Questions people ask
How many bags of concrete for a 10 x 10 slab?
At 4 inches thick with 10% extra, about 62 bags of 80 lb mix or 82 bags of 60 lb mix.
How many cubic yards is a 12 x 12 slab?
At 4 inches thick it is 1.78 cubic yards, or 1.96 with 10% extra.
How much does a yard of concrete cover?
One cubic yard covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches.
Do I need rebar or gravel under the slab?
That depends on the use and local rules. A base and reinforcement are common for driveways and larger slabs. Ask a contractor or your building department.