How many bags of concrete for a 4x4 slab, 4 inches thick?
A 4x4 slab 4 inches thick needs 5.33 cubic feet (0.20 cubic yards) of concrete: about 10 bags of 80 lb mix with 10% extra.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 9 | 10 |
| 60 lb | 12 | 14 |
| 50 lb | 15 | 16 |
| 40 lb | 18 | 20 |
Typical yields: 80 lb bag 0.60 cu ft, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30. Check your bag; brands vary slightly. The 10% covers spills, uneven ground and form flex.