Concrete for a 30×40 slab at different thicknesses
The slab covers 1,200 square feet. Every extra inch of thickness adds about 3.7 cubic yards. Quantities below include 10% for waste.
| Thickness | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 3.5 in | 14.26 | 642 | 856 | 1,284 |
| 4 in | 16.3 | 734 | 978 | 1,467 |
| 5 in | 20.37 | 917 | 1,223 | 1,834 |
| 6 in | 24.44 | 1,100 | 1,467 | 2,200 |
| 8 in | 32.59 | 1,467 | 1,956 | 2,934 |
Bags or ready-mix for this size?
At 16.3 cubic yards, bags mean mixing 734 bags weighing roughly 29.4 tons. Ready-mix delivery is usually the practical choice at this size, and it gives a more consistent slab.
How heavy is a 30×40 slab?
Normal-weight concrete weighs about 150 lb per cubic foot, so a 4-inch 30×40 slab weighs roughly 60,000 lb (30 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 30x40 slab?
734 bags for a 4-inch slab including 10% extra (667 with no extra). At 6 inches thick you need 1,100 bags.
How many yards of concrete for a 30x40 slab 6 inches thick?
24.44 cubic yards including 10% extra, or 22.22 yards exactly.
How many 60 lb bags for a 30x40 slab?
978 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.