20×20 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 116.67 | 4.32 | 195 | 260 |
| 4″ | 133.33 | 4.94 | 223 | 297 |
| 5″ | 166.67 | 6.17 | 278 | 371 |
| 6″ | 200.00 | 7.41 | 334 | 445 |
No waste included in the table.
Worked example
20 ft × 20 ft = 400 sq ft. At 4 inches: 400 × 4 ÷ 12 = 133.33 cu ft, or 4.94 cubic yards. With 10% extra that is 5.43 yd³, or 245 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 20×20 slab?
223 × 80 lb or 297 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 20×20 slab?
4.94 cubic yards at 4 inches; 7.41 at 6 inches.