How many bags of concrete for a 20x20 slab, 4 inches thick?
A 20x20 slab 4 inches thick needs 133.3 cubic feet (4.94 cubic yards) of concrete: about 245 bags of 80 lb mix with 10% extra.
That is over a cubic yard. A ready-mix truck delivery (about 5.43 yards) is usually cheaper and easier than mixing 245 bags by hand.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 223 | 245 |
| 60 lb | 297 | 326 |
| 50 lb | 356 | 392 |
| 40 lb | 445 | 489 |
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.