12×20 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 70.00 | 2.59 | 117 | 156 |
| 4″ | 80.00 | 2.96 | 134 | 178 |
| 5″ | 100.00 | 3.70 | 167 | 223 |
| 6″ | 120.00 | 4.44 | 200 | 267 |
No waste included in the table.
Worked example
12 ft × 20 ft = 240 sq ft. At 4 inches: 240 × 4 ÷ 12 = 80.00 cu ft, or 2.96 cubic yards. With 10% extra that is 3.26 yd³, or 147 bags of 80 lb mix.
At over 1 cubic yard, compare the cost and labor of bags against a ready-mix delivery.
Assumes standard concrete mix bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft. Check the yield on your bag. 1 cubic yard = 27 cubic feet.
Frequently asked questions
How many bags of concrete for a 12×20 slab?
134 × 80 lb or 178 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 12×20 slab?
2.96 cubic yards at 4 inches; 4.44 at 6 inches.