How to calculate concrete
Multiply length × width × thickness to get the volume, keeping every measurement in the same unit. For a slab measured in feet with thickness in inches, divide the inches by 12 first. Divide cubic feet by 27 to get cubic yards, which is how ready-mix concrete is sold. For a round column or post hole, the volume is π × radius² × depth.
Typical thicknesses
| Project | Thickness |
|---|---|
| Sidewalk, patio, shed floor | 4 in |
| Driveway for cars | 4–5 in |
| Driveway for trucks, RVs | 5–6 in |
| House floor slab (IRC minimum) | 3 1/2 in |
Bags per slab
For a 4 in slab with 10% extra, using 80 lb bags (0.60 cu ft each).
| Slab | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft | 0.22 | 10 |
| 8 × 8 ft | 0.87 | 40 |
| 10 × 10 ft | 1.36 | 62 |
| 12 × 12 ft | 1.96 | 88 |
Questions people ask
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. At the usual yields of 0.60 cu ft per 80 lb bag, 0.45 per 60 lb bag and 0.30 per 40 lb bag, that is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab 4 in thick is 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra, order about 1.36 yards (1 1/2 yards from a ready-mix plant).
How much concrete does a fence post hole take?
A 10 in diameter hole 3 ft deep holds about 1.6 cubic feet. With a 4×4 post in it, that drops to about 1.4 cubic feet: three 80 lb bags per hole. Choose the post size and the calculator subtracts it for you.
How much extra concrete should I order?
Add 5–10% for uneven ground, spillage and forms that bow. Running short mid-pour is much worse than having a little left over.
When should I order ready-mix instead of bags?
Above about 1 cubic yard (45 bags of 80 lb), a ready-mix truck is usually less work. Many plants charge a short-load fee below a few yards.