Concrete slab volume by size
This table converts common rectangular slab dimensions into cubic feet and cubic yards. Use the calculator for different thicknesses or bag yields.
Published August 30, 2026 · Reviewed September 3, 2026 · Method: BetterHomeHabits calculator model
Concrete volume is length × width × thickness. Convert inches of thickness to feet, then divide cubic feet by 27 to get cubic yards. A purchase allowance can be added for spillage and site variation.
Reference table
| Slab size | Thickness | Volume incl. 10% | Cubic yards |
|---|---|---|---|
| 8 × 10 ft | 4 in | 29.3 cu ft | 1.09 yd³ |
| 10 × 10 ft | 4 in | 36.7 cu ft | 1.36 yd³ |
| 12 × 12 ft | 4 in | 52.8 cu ft | 1.96 yd³ |
| 15 × 15 ft | 4 in | 82.5 cu ft | 3.06 yd³ |
| 20 × 20 ft | 4 in | 146.7 cu ft | 5.43 yd³ |
Assumptions used
- 4 in slab thickness
- 10% planning allowance
- Rectangular slab
- No footings, thickened edges or grade beams
Why your result may differ
Uneven subgrade, thickness variation, footings, thickened edges and the exact bag yield are outside this rectangular slab table. Structural dimensions require a qualified plan.
Methodology
Each row converts a 4 in thickness to feet, multiplies by slab area, applies a 10% planning allowance and divides cubic feet by 27.