About this calculator
The Concrete Calculator works out how much concrete a slab, patio, walkway, footing or round pier needs. It shows cubic yards for a ready-mix order and the number of 40, 60 or 80 pound bags for mixing it yourself.
How to calculate concrete
- Choose the shape. Use Rectangle for slabs and footings, or Circle for a round pier or tube form (enter the diameter).
- Enter the thickness in the Depth box. A slab is commonly 4 inches thick.
- Keep a 10 percent allowance for uneven ground and spills.
- Pick your bag size and optionally add a price.
The concrete formula
Cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27. Divide thickness in inches by 12 first. For a round pier, use the area of the circle, which is π × radius squared.
Example: a 10 ft × 10 ft slab, 4 inches thick, is 100 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. Adding 10 percent gives 1.36 cubic yards, which is about 62 bags of 80 lb concrete.
Bags needed per cubic yard
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 cubic feet | 45 |
| 60 lb | about 0.45 cubic feet | 60 |
| 40 lb | about 0.30 cubic feet | 90 |
Things to watch out for
- Yields vary by brand. The numbers above are typical. Check the bag label for the exact yield.
- Bags or truck. For larger pours, ready-mix delivery is usually more practical than mixing dozens of bags. Suppliers often have minimum loads or short-load fees, so ask.
- Thickness matters. Driveways and areas with heavy loads often need more than a 4 inch slab. Check local code and your supplier's guidance.
- Order a little extra. Running short mid-pour can leave a weak seam.
Frequently asked questions
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. Yields vary a little by brand.
About 1.23 cubic yards, or around 1.36 cubic yards with a 10 percent allowance. That is roughly 62 bags of 80 lb concrete.
Most people add 5 to 10 percent for uneven ground, spills and over-excavation.
Choose Circle, enter the diameter and the depth. The calculator uses the area of the circle times the depth.
A cubic yard is 27 cubic feet, which is 3 feet by 3 feet by 3 feet.