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20×24 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 140.00 | 5.19 | 234 | 312 |
| 4″ | 160.00 | 5.93 | 267 | 356 |
| 5″ | 200.00 | 7.41 | 334 | 445 |
| 6″ | 240.00 | 8.89 | 400 | 534 |
No waste included in the table.
Worked example
20 ft × 24 ft = 480 sq ft. At 4 inches: 480 × 4 ÷ 12 = 160.00 cu ft, or 5.93 cubic yards. With 10% extra that is 6.52 yd³, or 294 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 20×24 slab?
267 × 80 lb or 356 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 20×24 slab?
5.93 cubic yards at 4 inches; 8.89 at 6 inches.