30×40 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 350.00 | 12.96 | 584 | 778 |
| 4″ | 400.00 | 14.81 | 667 | 889 |
| 5″ | 500.00 | 18.52 | 834 | 1112 |
| 6″ | 600.00 | 22.22 | 1000 | 1334 |
No waste included in the table.
Worked example
30 ft × 40 ft = 1200 sq ft. At 4 inches: 1200 × 4 ÷ 12 = 400.00 cu ft, or 14.81 cubic yards. With 10% extra that is 16.30 yd³, or 734 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 30×40 slab?
667 × 80 lb or 889 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 30×40 slab?
14.81 cubic yards at 4 inches; 22.22 at 6 inches.