How to calculate concrete
cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324
column (yd³) = 3.1416 × (diameter in ÷ 24)² × height (ft) ÷ 27
Then add 5–10% for spillage, uneven subgrade and form bulges. To convert to bags, divide cubic feet by the bag's yield: an 80 lb bag makes 0.60 ft³, a 60 lb bag 0.45 ft³, a 50 lb bag 0.375 ft³ and a 40 lb bag 0.30 ft³.
How many bags of concrete in a yard?
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 sq ft at 4" |
|---|---|---|---|
| 80 lb | 0.6 ft³ | 45 | 5.6 |
| 60 lb | 0.45 ft³ | 60 | 7.4 |
| 50 lb | 0.375 ft³ | 72 | 8.9 |
| 40 lb | 0.3 ft³ | 90 | 11.1 |
Concrete for common slabs
| Slab | Thickness | Cubic yards | With 10% extra | 80 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 4" | 0.2 | 0.22 | 10 |
| 10 × 10 ft | 4" | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 4" | 1.78 | 1.96 | 89 |
| 20 × 20 ft | 4" | 4.94 | 5.43 | 245 |
| 24 × 24 ft | 5" | 8.89 | 9.78 | 440 |
How many bags of concrete per fence post?
Hole volume minus the post, filled to the top:
| Hole | Post | Concrete | 80 lb bags | 50 lb bags |
|---|---|---|---|---|
| 8" × 24" deep | 4×4 | 0.53 ft³ | 0.9 | 1.4 |
| 10" × 30" deep | 4×4 | 1.15 ft³ | 1.9 | 3.1 |
| 12" × 36" deep | 4×4 | 2.1 ft³ | 3.5 | 5.6 |
| 12" × 36" deep | 6×6 | 1.73 ft³ | 2.9 | 4.6 |
| 16" × 42" deep | 6×6 | 4.15 ft³ | 6.9 | 11.1 |
Rule of thumb: dig holes 3× the post width and one-third of the post's above-ground height deep — and always below your frost line.
Bags or ready-mix?
Up to about 1 cubic yard (45 × 80 lb bags), bags are simpler: no minimum order, mix as you go. Above 1–2 yards, ready-mix is cheaper and far less work. Ready-mix runs about $130–$200 per yard in 2026, but most plants charge a short-load fee for orders under 4–5 yards.
FAQ
How thick should a concrete slab be?
4 inches for patios, walkways and shed floors; 5–6 inches for driveways and garage floors that carry vehicles.
How much does a yard of concrete weigh?
About 4,000 lb (2 tons) — normal concrete weighs roughly 150 lb per cubic foot.
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag yields 0.6 cubic feet and a cubic yard is 27 cubic feet.