Drain Slope & Grade Calculator
Calculate drain slope, grade percentage, fall distance, and run length for proper drainage design. Enter any two known values to solve for the remaining unknowns.
Fill in any two fields and click Calculate.
Formulas Used
Decimal Slope = Vertical Fall (in) ÷ [Horizontal Run (ft) × 12]
Grade (%) = Decimal Slope × 100
Slope (in/ft) = Decimal Slope × 12
Slope Ratio (1 : X) = 1 ÷ Decimal Slope
Angle (°) = arctan(Decimal Slope) × (180 / π)
Vertical Fall (in) = Decimal Slope × Run (ft) × 12
Horizontal Run (ft) = Vertical Fall (in) ÷ (Decimal Slope × 12)
Assumptions & References
- All slope representations (%, ratio, in/ft) are mathematically equivalent and interconvertible via the decimal slope.
- The International Plumbing Code (IPC) §704.1 and IRC P3005.3 require a minimum slope of ⅛ in/ft (1.04%) for 3-inch and larger drain pipes, and ¼ in/ft (2.08%) for pipes smaller than 3 inches.
- The commonly recommended residential drain slope is ¼ in/ft (≈ 2%).
- Slopes above 4% may cause liquid to outrun solids in drain pipes, leading to blockages (per plumbing engineering guidelines).
- Horizontal run is measured along the horizontal plane; vertical fall is the true elevation change.
- For very steep slopes (>15°), the small-angle approximation breaks down; the angle formula uses exact arctan.
- Always verify with local building codes and a licensed plumber for permitted installations.