4×8 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 9.33 | 0.35 | 16 | 21 |
| 4″ | 10.67 | 0.40 | 18 | 24 |
| 5″ | 13.33 | 0.49 | 23 | 30 |
| 6″ | 16.00 | 0.59 | 27 | 36 |
No waste included in the table.
Worked example
4 ft × 8 ft = 32 sq ft. At 4 inches: 32 × 4 ÷ 12 = 10.67 cu ft, or 0.40 cubic yards. With 10% extra that is 0.43 yd³, or 20 bags of 80 lb mix.
At this size, bagged mix is manageable by hand.
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 4×8 slab?
18 × 80 lb or 24 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 4×8 slab?
0.40 cubic yards at 4 inches; 0.59 at 6 inches.