Minimum Self-Cleansing Slope Calculator

What grade does this sewer need to hit self-cleansing velocity? Enter the diameter and target velocity to get the minimum slope from Manning’s equation.

Pipe inside diameter (inches)

Manning’s nDefault 0.013 for concrete pipe.

Target self-cleansing velocity (ft/s)2 ft/s is the usual minimum (Ten State Standards).

Minimum Slope

Item	Value

How the calculation works

Manning’s equation rearranged for slope (full flow, US units)
V = (1.486 / n) × R^2/3 × S^1/2
S = [ V × n / (1.486 × R^2/3) ]²
R = D / 4 for a circular pipe flowing full
Slope in percent = S (ft/ft) × 100

Diameter (in)	Min slope at 2 ft/s, n = 0.013 (ft/ft)
8	0.0033
10	0.0025
12	0.0019
15	0.0014
18	0.0011
24	0.0008

Notes & limitations

The 2 ft/s self-cleansing velocity at design flow is the long-standing Ten State Standards (GLUMRB) criterion for sanitary sewers; the slope formula is Manning’s equation solved for S with R = D/4. The reference slopes above are the exact Manning solutions for the listed inputs — try them in the tool to confirm.

This is the slope for the target velocity at full flow. At partial depths the velocity differs — the full-flow check is the standard design simplification.
Flatter grades than the computed minimum need justification (and usually more frequent cleaning); steeper is fine but watch maximum velocity and drop structures.
Use the design n, not the new-pipe n — aged sewers run rougher, which demands a steeper slope for the same velocity.
Minimum diameter and minimum slope rules vary by state — confirm against the applicable standards before finalizing plans.

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