Concrete calculator
Size a patio, shed pad or walkway and see whether to buy bags or order ready-mix by the yard.
Estimates round up. Check product labels and supplier figures before ordering.
How thick should a slab be?
| Project | Thickness |
|---|---|
| Walkway or patio | 4 in |
| Shed or hot tub pad | 4–6 in |
| Driveway | 5–6 in |
| Garage floor | 4–6 in |
How it's calculated
The calculator multiplies area by thickness for the volume, then divides by the yield of each bag size: an 80 lb bag makes about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
Forms flex and ground is never perfectly flat, so the extra field defaults to 10%. Above about one cubic yard, ready-mix delivery is usually less work and often cheaper than bags.
What you'll need
- Concrete mixer rental
- Wood stakes and form boards
- Bull float
- Rebar or wire mesh
Common questions
How many 80 lb bags of concrete make a yard?
About 45 bags of 80 lb, or 60 bags of 60 lb.
How many bags for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 slab is about 1.23 cubic yards: roughly 56 bags of 80 lb before adding extra.
Should I buy bags or ready-mix?
Bags suit small jobs under about one cubic yard. Larger pours are easier with a truck, which also gives a more even mix.
Does this include gravel base?
No. Use the gravel calculator for the 4 inch compacted base most slabs sit on.