Free tool
Concrete Slab Calculator
Enter the length, width and thickness of your slab and get the cubic yards for a ready-mix order or the number of bags to mix yourself.
How we calculate it
- Slab = length × width × thickness in inches ÷ 12
- Footing = length × width in inches ÷ 12 × depth in inches ÷ 12
- Post holes = holes × (3.1416 × (hole diameter ÷ 24)² − the post's cross-section) × depth in inches ÷ 12
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet × (1 + waste %) ÷ the yield printed on your bag, rounded up
We do not assume a yield: bag yields differ by product, so the bag count appears only after you enter the yield printed on your bag. This is a material estimate, not an engineering calculation.
How to use this tool
- Pick the jobChoose a slab, a footing or post holes and enter the sizes.
- Enter your bagPick the bag size and type the yield printed on the bag.
- Read the countSee the volume, the bags to buy rounded up and the weight you will haul.
Frequently asked questions
How many yards of concrete for a 12 by 20 slab?
At 4 inches thick, a 12 by 20 foot slab is 12 times 20 times 4 divided by 12, which is 80 cubic feet or about 2.96 cubic yards. At 5 inches it is 100 cubic feet and at 6 inches 120 cubic feet, before any waste allowance. Check local code for the thickness you need.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 by 3 by 3 feet. A 10 by 10 foot slab 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards. Ready-mix concrete is sold by the cubic yard, so for larger pours compare the bag count with a delivery.
How many bags of concrete do I need?
Work out the volume in cubic feet (length times width times thickness, all in feet), add an allowance for waste and spillage, then divide by the yield printed on the bag. Yields differ by product and bag size, so always use the number on the bag you are buying rather than a general rule.
Is this an engineering calculation?
No. It estimates material volume only. Slab thickness, footing size, reinforcement and mix strength depend on the load, the soil, frost depth and local building code. Read the product instructions and check with your local building department, or ask a contractor or engineer for structural work.