Concrete slab calculator
Enter the slab dimensions to get the volume in cubic yards, the equivalent in bags, and the approximate delivered weight. A 10% waste factor is applied by default because forms flex, subgrade is never perfectly level, and running short mid-pour creates a cold joint.
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This calculator needs JavaScript. The reference table below covers the common cases.
| Thickness | Coverage per yd³ | yd³ per 100 ft² |
|---|---|---|
| 4 in | 81 ft² | 1.24 |
| 5 in | 65 ft² | 1.54 |
| 6 in | 54 ft² | 1.85 |
| 8 in | 41 ft² | 2.47 |
The formula
length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards
Worked example
A 20 ft × 24 ft patio at 4 in: 20 × 24 × 0.333 = 160 ft³. 160 ÷ 27 = 5.93 yd³. With 10% waste, order 6.5 yd³.
Reference table
| Thickness | Coverage per yd³ | yd³ per 100 ft² |
|---|---|---|
| 4 in | 81 ft² | 1.24 |
| 5 in | 65 ft² | 1.54 |
| 6 in | 54 ft² | 1.85 |
| 8 in | 41 ft² | 2.47 |
Assumptions
- 27 cubic feet = 1 cubic yard, exactly.
- Default 10% waste factor. Raise it for uneven subgrade or hand-dug forms.
- Weight assumes normal-weight concrete at roughly 4,000 lb per cubic yard.
- Bag counts use 80 lb bags at 0.60 ft³, a manufacturer TDS figure that varies by formulation.