Bags or a truck?
At this size a ready-mix truck is the sensible choice. Mixing 134 bags by hand is a long, heavy day, and the first batches start to set before the last ones are placed.
Concrete slab charts · 12 × 20 ft
How much concrete for a 12×20 slab?
A 12 × 20 ft slab 4 inches thick needs 2.96 cubic yards of concrete (80.0 cubic feet), or about 134 bags of 80 lb premix. Add 5–10% for waste. Other thicknesses are in the table, or adjust the calculator below.
Units
12 × 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 | 2.22 yd³ | 60.0 ft³ | 100 | 134 | 1.70 m³ |
| 4 in | 2.96 yd³ | 80.0 ft³ | 134 | 178 | 2.27 m³ |
| 5 in | 3.70 yd³ | 100.0 ft³ | 167 | 223 | 2.83 m³ |
| 6 in | 4.44 yd³ | 120.0 ft³ | 200 | 267 | 3.40 m³ |