Concrete for a 12×20 slab at different thicknesses
The slab covers 240 square feet. Every extra inch of thickness adds about 0.74 cubic yards. Quantities below include 10% for waste.
| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 3.5 in | 2.85 | 129 | 172 | 257 |
| 4 in | 3.26 | 147 | 196 | 294 |
| 5 in | 4.07 | 184 | 245 | 367 |
| 6 in | 4.89 | 220 | 294 | 440 |
| 8 in | 6.52 | 294 | 392 | 587 |
Bags or ready-mix for this size?
At 3.26 cubic yards, bags mean mixing 147 bags weighing roughly 5.9 tons. Ready-mix delivery is usually the practical choice at this size, and it gives a more consistent slab.
How heavy is a 12×20 slab?
Normal-weight concrete weighs about 150 lb per cubic foot, so a 4-inch 12×20 slab weighs roughly 12,000 lb (6 tons) once cured. Make sure your base is compacted gravel on firm, undisturbed soil.
The math
Common questions
How many 80 lb bags of concrete for a 12x20 slab?
147 bags for a 4-inch slab including 10% extra (134 with no extra). At 6 inches thick you need 220 bags.
How many yards of concrete for a 12x20 slab 6 inches thick?
4.89 cubic yards including 10% extra, or 4.44 yards exactly.
How many 60 lb bags for a 12x20 slab?
196 bags of 60 lb concrete for a 4-inch slab with 10% extra.
Want the full method, tips and other sizes? See the concrete calculator guide.