Manning's Equation Drainage Slope

Self-cleansing slope and slope to carry target flow at half-full.

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Example

You enter

Pipe diameter (in) 4
Target flow (gpm) 50
Pipe material pvc
Required scour velocity (ft/s) 2
As-built slope to check (in/ft, 0 = skip) 0.25

You get

Self-cleansing slope 0.0484
Slope for flow (half-full) 0.0788

Details, formula, and sources

English Manning V = (1.486 / n) × R^(2/3) × S^(1/2), solved for self-cleansing slope at 2 ft/s and the slope to carry target flow at half-full with R = D / 4.

Manning equation; n-values from USGS WSP-2339 (public domain).

USGS WSP-2339 free at pubs.usgs.gov.

Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.

Field names used by the API: pipe_diameter_in, target_flow_gpm, material, scour_velocity_fps, actual_slope_in_per_ft, slope_self_cleansing_in_per_ft, slope_for_flow_in_per_ft

Half-full hydraulic radius R = D / 4circular cross-section geometry
Self-cleansing velocity 2 ft/s for sanitary sewerspublic engineering practice