How many bags of concrete for a 12x16 slab, 4 inches thick?
A 12x16 slab 4 inches thick needs 64.0 cubic feet (2.37 cubic yards) of concrete: about 118 bags of 80 lb mix with 10% extra.
That is over a cubic yard. A ready-mix truck delivery (about 2.61 yards) is usually cheaper and easier than mixing 118 bags by hand.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 107 | 118 |
| 60 lb | 143 | 157 |
| 50 lb | 171 | 188 |
| 40 lb | 214 | 235 |
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.