20×30 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 175.00 | 6.48 | 292 | 389 |
| 4″ | 200.00 | 7.41 | 334 | 445 |
| 5″ | 250.00 | 9.26 | 417 | 556 |
| 6″ | 300.00 | 11.11 | 500 | 667 |
No waste included in the table.
Worked example
20 ft × 30 ft = 600 sq ft. At 4 inches: 600 × 4 ÷ 12 = 200.00 cu ft, or 7.41 cubic yards. With 10% extra that is 8.15 yd³, or 367 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 20×30 slab?
334 × 80 lb or 445 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 20×30 slab?
7.41 cubic yards at 4 inches; 11.11 at 6 inches.