Concrete calculator
Enter your measurements. See cubic yards for a truck order and bags for a small job.
How to figure out how much concrete you need
Concrete is sold by volume. Multiply length, width and thickness, all in feet, to get cubic feet. Divide by 27 to get cubic yards, because one cubic yard is 27 cubic feet. A 10 x 10 ft slab that is 4 inches thick is 10 x 10 x (4 / 12) = 33.33 cubic feet, or 1.23 cubic yards.
Then add extra. Uneven ground, spilled concrete and a slightly thicker pour all use more than the math says. Adding 10% is a common starting point, which makes that slab 1.36 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 |
Slabs, footings and posts
The calculator has three shapes. A slab uses length, width and thickness. A footing uses the length in feet and the width and depth in inches. A round column or post hole uses the diameter and depth in inches and how many you are pouring.
| Job | Volume | Order with 10% extra | 80 lb bags |
|---|---|---|---|
| Fence post hole, 10 in wide, 30 in deep | 1.36 cu ft | 0.06 cu yd | 3 |
| Deck footing, 12 in wide, 42 in deep | 2.75 cu ft | 0.11 cu yd | 6 |
| 4 x 4 ft pad, 4 in thick | 5.33 cu ft | 0.22 cu yd | 10 |
| Footing 40 ft long, 16 in wide, 8 in deep | 35.56 cu ft | 1.45 cu yd | 66 |
Bags or a truck?
Bags of mix suit small jobs, like post holes and small pads. For larger pours, ready-mix delivered by truck is sold by the cubic yard, and many suppliers set a minimum order or add a fee for small loads, so ask before you decide. For the bag count on its own, use the bags of concrete calculator.
Bag yields come from the manufacturer's data: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag 0.45, a 50 lb bag 0.375 and a 40 lb bag 0.3 (Quikrete data sheet).
More calculators
- Concrete slab calculator: patios, walkways and driveways
- Bags of concrete calculator: how many 40, 50, 60 or 80 lb bags
- Bags of concrete in a yard: bags per cubic yard, by bag size
- Gravel calculator: tons and cubic yards
- Driveway gravel calculator: tons for a gravel driveway
- Pea gravel calculator: tons for paths and patios
- Mulch calculator: cubic yards and bags
Questions people ask
How do I calculate how much concrete I need?
Multiply length x width x thickness in feet to get cubic feet, then divide by 27 for cubic yards. Add about 10% extra.
How much concrete do I need for a 10 x 10 slab?
At 4 inches thick it is 1.23 cubic yards, or 1.36 with 10% extra. That is about 62 bags of 80 lb mix.
How many cubic feet are in a cubic yard?
27 cubic feet. A cubic yard is 3 ft x 3 ft x 3 ft.
How thick should a concrete slab be?
Patios and walkways are commonly 4 inches thick, and driveways are often thicker. Check local building rules and your contractor for your project.
Should I order extra concrete?
Yes. Most people add about 10% for waste and uneven ground. You can change the percentage in the calculator.