Concrete Calculator
Enter the size of your slab to get cubic yards of concrete and the number of bags to buy.
These are estimates. Real jobs vary with surface, product and waste, so round up and check your product's label before you buy.
How much concrete is required for a 10×10 slab?
A 10×10 foot slab that is 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet), or roughly 56 bags of 80-lb mix. With the usual 10% extra for uneven ground and spillage, order about 1.36 cubic yards. Other common slab sizes at 4 inches:
| Slab size (4 in thick) | Cubic yards | 80-lb bags |
|---|---|---|
| 10×10 ft | 1.23 | 56 |
| 10×12 ft | 1.48 | 67 |
| 12×12 ft | 1.78 | 80 |
| 12×20 ft | 2.96 | 134 |
| 20×20 ft | 4.94 | 223 |
How the concrete calculator works
Volume is length × width × thickness. We convert the thickness from inches to feet, divide cubic feet by 27 to get cubic yards, then add your waste allowance. An 80-lb bag makes about 0.6 cubic feet and a 60-lb bag about 0.45, so bag counts are cubic yards × 45 or × 60. Check your bag's label for its exact yield.
Planning tips
Patios and walkways are commonly poured 4 inches thick. Driveways and garage slabs are often thicker and may need a gravel base or reinforcement, so check your local building code. Bags make sense for small jobs, while ready-mix delivery is usually easier for anything over a couple of cubic yards. For large or structural pours, talk to a licensed contractor.
Frequently asked questions
How many 80-lb bags of concrete make a cubic yard?
About 45 bags, since each 80-lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
How thick should a concrete slab be?
Four inches is common for patios and walkways. Driveways and heavy loads need more, so follow local code and professional advice.
Should I order extra concrete?
Yes. Ten percent extra is a common allowance because ground is rarely perfectly level and some mix is always lost.