How many bags of concrete for a 20x20 slab, 6 inches thick?
A 20x20 slab 6 inches thick needs 200.0 cubic feet (7.41 cubic yards) of concrete: about 367 bags of 80 lb mix with 10% extra.
That is over a cubic yard. A ready-mix truck delivery (about 8.15 yards) is usually cheaper and easier than mixing 367 bags by hand.
| Bag size | Exact | With 10% extra |
|---|---|---|
| 80 lb | 334 | 367 |
| 60 lb | 445 | 489 |
| 50 lb | 534 | 587 |
| 40 lb | 667 | 734 |
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.