Concrete Slab Calculator (Cubic Yards & Bags)
Estimate ready-mix cubic yards or the number of 80 lb bags for a rectangular slab, patio or driveway.
Formula
Cubic yards = Length(ft) × Width(ft) × Thickness(in)/12 ÷ 27 × (1 + waste%). Bags = cubic feet ÷ bag yield.
Worked example
Inputs: Length = 20 ft; Width = 10 ft; Thickness = 4 in; Waste allowance = 10 %; Bag yield = 0.6 cu ft/bag.
| Cubic yards | 2.72 |
|---|---|
| Cubic meters | 2.08 |
| Bags | 123 |
FAQ
How many 80 lb bags are in a cubic yard?
At 0.60 cu ft per bag, 27 ÷ 0.60 = 45 bags per cubic yard before waste.
Why add waste?
Uneven subgrade, form bowing and spillage mean you almost always need more than the exact geometric volume; 5–10% is a common allowance you can change.
Sources
- Geometry (volume/area formulas); no external constant needed
- QUIKRETE Concrete Mix (No. 1101) data sheet: 80 lb bag yields approx. 0.60 cu ft
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Last reviewed 2026-10-11.