30×30 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 262.50 | 9.72 | 438 | 584 |
| 4″ | 300.00 | 11.11 | 500 | 667 |
| 5″ | 375.00 | 13.89 | 625 | 834 |
| 6″ | 450.00 | 16.67 | 750 | 1000 |
No waste included in the table.
Worked example
30 ft × 30 ft = 900 sq ft. At 4 inches: 900 × 4 ÷ 12 = 300.00 cu ft, or 11.11 cubic yards. With 10% extra that is 12.22 yd³, or 550 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 30×30 slab?
500 × 80 lb or 667 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 30×30 slab?
11.11 cubic yards at 4 inches; 16.67 at 6 inches.