Bags or a truck?
At this size a ready-mix truck is the sensible choice. Mixing 223 bags by hand is a long, heavy day, and the first batches start to set before the last ones are placed.
Concrete slab charts · 20 × 20 ft
How much concrete for a 20×20 slab?
A 20 × 20 ft slab 4 inches thick needs 4.94 cubic yards of concrete (133.3 cubic feet), or about 223 bags of 80 lb premix. Add 5–10% for waste. Other thicknesses are in the table, or adjust the calculator below.
Units
20 × 20 ft slab by thickness
Exact volumes without waste. Bag counts are rounded up.
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | Metric |
|---|---|---|---|---|---|
| 3 in | 3.70 yd³ | 100.0 ft³ | 167 | 223 | 2.83 m³ |
| 4 in | 4.94 yd³ | 133.3 ft³ | 223 | 297 | 3.78 m³ |
| 5 in | 6.17 yd³ | 166.7 ft³ | 278 | 371 | 4.72 m³ |
| 6 in | 7.41 yd³ | 200.0 ft³ | 334 | 445 | 5.66 m³ |