Concrete Slab Calculator
Calculate concrete volume and bag requirements for rectangular slabs, patios, and floor pours.
Formula & Explanation
A rectangular concrete slab volume is calculated using the formula: Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft) The thickness must be converted from inches to feet by dividing by 12. For example, a 4-inch thickness equals 4/12 = 0.333 feet. To convert cubic feet to cubic yards, divide by 27 (since 1 yard = 3 feet, and 3³ = 27). The number of bags required depends on the yield of the concrete product. The bag yield varies by bag type: - 60 lb bag yields approximately 0.45 cubic feet - 80 lb bag yields approximately 0.60 cubic feet A waste percentage (typically 5-10%) is added to account for spillage, uneven surfaces, and material loss. Final bags needed = ceil(Volume × (1 + Waste%) ÷ Bag Yield)
Assumptions
- •The slab is rectangular with uniform thickness
- •Thickness is entered in inches; length and width in feet
- •Bag yield values (0.45 and 0.60 cu ft) are industry standards for standard pre-mixed concrete bags-confirm with your product
- •A waste percentage of 5-10% is typical; adjust based on site conditions and experience
- •The calculator assumes concrete is ordered in whole bags (fractional bags are rounded up)
Example Calculation
A 20 ft × 15 ft patio poured 4 inches thick requires 100 cubic feet or 3.7 cubic yards of concrete. With a 10% waste allowance and 0.45 cu ft per 60 lb bag, you need approximately 271 bags.
Inputs
Results
Common Slab Sizes & Concrete Volume
| size | volume_cuft | volume_cuyd | bags_60lb |
|---|---|---|---|
| 10 × 10 ft (4") | 33.3 | 1.2 | 82 |
| 10 × 10 ft (6") | 50.0 | 1.9 | 123 |
| 12 × 12 ft (4") | 48.0 | 1.8 | 118 |
| 20 × 15 ft (4") | 100.0 | 3.7 | 248 |
| 20 × 15 ft (6") | 150.0 | 5.6 | 372 |
Frequently Asked Questions
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