How the concrete math works
Every shape becomes cubic feet, the waste allowance is added, and the total is divided by the yield printed on standard bags.
Footing: length × (width ÷ 12) × (depth ÷ 12)
Column: π × (diameter ÷ 24)² × height × count
Post hole: (hole area − post area) × depth ÷ 1,728 × count
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Worked examples
Patio slab: a 10 × 10 ft slab 4 inches thick is 33.3 cubic feet. With 10% extra that’s 36.7 cubic feet (1.36 cubic yards): 62 bags of 80 lb, 82 of 60 lb or 123 of 40 lb.
Fence post: a 10-inch hole 36 inches deep around a 4×4 post holds about 1.38 cubic feet of concrete. With 10% extra, that’s 2.5 bags of 80 lb mix, so buy 3 for one post. For several posts, total the job first; partial bags carry over.
Common questions
How many 80 lb bags of concrete make a yard?
45. Each 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet. For 60 lb bags it’s 60, and for 40 lb bags it’s 90.
How thick should a concrete slab be?
4 inches is typical for patios, walkways and shed floors. Driveways and slabs that carry heavier loads are often 5 to 6 inches. Local code may set a minimum.
How much concrete per fence post?
It depends on hole size. A common rule is a hole about three times the post width, with depth set by post height and frost line. A 10-inch hole 36 inches deep around a 4×4 takes about 2.3 bags of 80 lb mix before waste.
When should I order ready-mix instead of bags?
Above about one cubic yard, hand-mixing gets slow: that’s 45 bags weighing 3,600 lb. Many people switch to ready-mix delivery, a short-load supplier or a towable mixer at that size.
Related calculators
See how we calculate for every formula and constant used on this site.