How Much Concrete for a 12x20 Slab?
3.26 cu yd
A 12 × 20 ft slab poured 4 inches thick needs about 3.26 cu yd of concrete, including 10% extra for spillage and uneven ground. That is 88 cu ft, or 147 bags of 80 lb premix concrete. Best option: order ready-mix (truck).
Concrete for a 12 × 20 ft slab by thickness (10% extra included)
| Thickness | Cubic yards | Cubic feet | 80 lb bags |
|---|---|---|---|
| 3 in | 2.44 cu yd | 66 cu ft | 110 bags |
| 4 in | 3.26 cu yd | 88 cu ft | 147 bags |
| 5 in | 4.07 cu yd | 110 cu ft | 184 bags |
| 6 in | 4.89 cu yd | 132 cu ft | 220 bags |
Adjust the numbers
The calculator below is pre-filled with the numbers above. Change any value to match your project.
Concrete needed3.26 cu yd
- In cubic meters
- 2.49 m³
- In cubic feet
- 88 cu ft
- 80 lb bags
- 147 bags
- Best option
- Order ready-mix (truck)
How it is calculated
Concrete is sold by volume, so the calculator multiplies length × width × thickness after converting every measurement to feet. Thickness and depth are usually measured in inches, so they are divided by 12 first.
For round columns and post holes the volume is π × radius² × depth.
Open the full concrete calculator for other shapes, a worked example and FAQs.