How many bags of concrete for a 4x8 slab, 4 inches thick?
A 4x8 slab 4 inches thick needs 10.7 cubic feet (0.40 cubic yards) of concrete: about 20 bags of 80 lb mix with 10% extra.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 18 | 20 |
| 60 lb | 24 | 27 |
| 50 lb | 29 | 32 |
| 40 lb | 36 | 40 |
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.