How this calculator works
Concrete is sold by volume, so the calculator finds the volume of your pour, adds your waste allowance, and converts the result into the units you can buy.
- Slab or pad: length × width × thickness.
- Round hole or footing: 3.14159 × (diameter ÷ 2)² × depth. If you enter a post width, the post's square footprint × depth is subtracted.
- Wall or footing strip: length × height × width.
Every measurement is converted to feet first, then multiplied by the number of pieces. Dividing cubic feet by 27 gives cubic yards. Bags are rounded up, since stores don't sell part of a bag.
How much concrete is in a bag?
These are the typical yields printed on standard concrete mix bags. Check your bag, since some mixes differ slightly.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Example: a 10 × 10 ft patio, 4 in thick
- 10 ft × 10 ft × 0.333 ft = 33.3 cubic feet
- Add 10% waste: 33.3 × 1.10 = 36.7 cubic feet
- 36.7 ÷ 27 = 1.36 cubic yards
- 36.7 ÷ 0.60 = 61.1, so buy 62 bags of 80 lb concrete
Common questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. That takes about 90 bags of 40 lb concrete, 60 bags of 60 lb concrete, or 45 bags of 80 lb concrete.
Should I buy bags or order ready-mix concrete?
Bags are practical for small jobs such as post holes, small pads and repairs. For larger pours, often around one cubic yard or more, ready-mix delivered by truck is usually easier and can cost less. Many suppliers have a minimum order, so call ahead.
How much extra concrete should I buy?
Most people add 5 to 10 percent for spills, uneven ground and forms that bow slightly. This calculator adds 10 percent by default, and you can change it.
How thick should a concrete slab be?
Patios, walkways and shed floors are commonly 4 inches thick. Driveways and slabs that carry vehicles are often 5 to 6 inches. Always check your local building code for your project.