How many bags of concrete for a 10x20 slab, 4 inches thick?
A 10x20 slab 4 inches thick needs 66.7 cubic feet (2.47 cubic yards) of concrete: about 123 bags of 80 lb mix with 10% extra.
That is over a cubic yard. A ready-mix truck delivery (about 2.72 yards) is usually cheaper and easier than mixing 123 bags by hand.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 112 | 123 |
| 60 lb | 149 | 163 |
| 50 lb | 178 | 196 |
| 40 lb | 223 | 245 |
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.