Drain Slope Calculator
Calculate drain pipe slope (grade), vertical drop, run length, and flow velocity using Manning's equation for proper drainage design.
Enter as percentage (e.g. 2 = 2%) — equivalent to inches per foot × 100/12
Results will appear here.
Formulas Used
Slope & Drop:
- Slope (decimal) = Slope% ÷ 100
- Vertical Drop (ft) = Horizontal Run (ft) × Slope
- Vertical Drop (in) = Drop (ft) × 12
- Slope (in/ft) = Vertical Drop (in) ÷ Run (ft)
- Slope Ratio = 1 : (1 ÷ Slope)
Manning's Equation (full-flow circular pipe, US customary):
- V = (1.486 / n) × R2/3 × S1/2
- R = D / 4 (hydraulic radius for full circular pipe)
- A = π × D² / 4 (cross-sectional area)
- Q = A × V (volumetric flow rate)
Where: V = velocity (ft/s), n = Manning's roughness coefficient, R = hydraulic radius (ft), S = slope (ft/ft), D = diameter (ft), A = area (ft²), Q = flow rate (ft³/s)
Assumptions & References
- Slope calculations assume a straight, uniform-grade pipe run.
- Manning's equation assumes steady, uniform, full-pipe flow conditions.
- The US customary form uses the conversion factor 1.486 (= 1.0 m1/3/s converted to ft1/3/s).
- Minimum recommended residential drain slope: 1/4 inch per foot (≈ 2.08%) per IPC/UPC plumbing codes.
- Maximum recommended slope: 1/2 inch per foot (≈ 4.17%) to prevent liquids from outrunning solids.
- Self-cleaning minimum velocity: 2.0 ft/s to prevent sediment deposition.
- Manning's n values: PVC 0.009, ABS/Cast Iron 0.011, Concrete 0.013, Clay 0.015, Corrugated Metal 0.024 (ASCE, Hydraulic Institute).
- Reference: Manning, R. (1891). On the Flow of Water in Open Channels and Pipes. IPC Section 704, UPC Section 708.