How many bags of concrete for a 12x20 slab, 6 inches thick?
A 12x20 slab 6 inches thick needs 120.0 cubic feet (4.44 cubic yards) of concrete: about 220 bags of 80 lb mix with 10% extra.
That is over a cubic yard. A ready-mix truck delivery (about 4.89 yards) is usually cheaper and easier than mixing 220 bags by hand.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 200 | 220 |
| 60 lb | 267 | 294 |
| 50 lb | 320 | 352 |
| 40 lb | 400 | 440 |
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.