Concrete calculator
How much concrete you need for a slab, patio, footing or post holes, in cubic yards and in 40, 50, 60 or 80 lb bags.
Concrete needed
- 80 lb bags
- 60 lb bags
- 50 lb bags
- 40 lb bags
How to use it
- Choose a slab (anything rectangular: patios, pads, footings, walkways) or a round post hole or column.
- Enter the measurements. Length and width are in feet; thickness, diameter and depth are in inches.
- For several identical pieces, like a row of fence posts, enter how many.
- Leave the 10% extra in unless you're sure of your measurements. Uneven ground and spills eat into it fast.
You get the total in cubic yards, which is how ready-mix is sold, and the number of bags for each common bag size.
How it's calculated
| Shape | Volume |
|---|---|
| Slab | length × width × (thickness ÷ 12) cubic feet |
| Round hole or column | 3.1416 × (diameter ÷ 24)² × (depth ÷ 12) cubic feet |
Divide cubic feet by 27 to get cubic yards. Then divide by each bag's yield and round up:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
Yields are from Quikrete's published data sheet for its standard concrete mix. Other brands are very close, but check the bag.
Common slab thicknesses
| Project | Typical thickness |
|---|---|
| Walkway or patio | 4 in |
| Driveway for cars | 4 in |
| Driveway for heavier vehicles | 5 to 6 in |
Local codes and soil conditions can call for more. Check with your building department for anything structural.
Tips
- A 10 × 10 ft patio at 4 in thick takes about 1.36 cubic yards with 10% extra: roughly 62 bags of 80 lb concrete. That's a lot of mixing, which is why larger pours are usually ordered as ready-mix.
- Measure thickness in a few places. Ground is rarely flat, and an extra half inch across a slab adds up to a lot of concrete.
- Order slightly more ready-mix than you need. Running short mid-pour leaves a weak joint where the second load meets the first.