Manning's Equation Calculator
Full-bore discharge, velocity and hydraulic radius for a circular gravity sewer pipe.
Inputs
Results
Full-flow discharge
96.7L/s
At a flow velocity of 1.37 m/s
Flow velocity1.37m/s
Hydraulic radius0.0750m
Flow area0.0707m²
Wetted perimeter0.942m
Grade used1.000%
Full-bore flow assumed. Results update as inputs change.
Calculation details
Engineering equations
Q = (1 / n) · A · R^(2/3) · S^(1/2)
V = Q / A
A = πD² / 4, P = πD, R = A / P = D / 4
Variables
- Q
- Liquid discharge (m³/s)
- V
- Mean flow velocity (m/s)
- n
- Manning's roughness coefficient
- A
- Full-bore flow area (m²)
- P
- Wetted perimeter (m)
- R
- Hydraulic radius (m)
- S
- Pipe grade (m/m)
- D
- Internal pipe diameter (m)
Assumptions
- Full-bore flow in a circular pipe.
- Steady, uniform flow with no minor losses, bends or junctions.
- A grade entered as a percentage is converted to m/m (1% = 0.010).
- Roughness is represented by a single Manning's n value.
Worked example
D = 300 mm, S = 1% (0.010), n = 0.013
A = π × 0.3² / 4 = 0.0707 m², R = 0.3 / 4 = 0.075 m
V = (1 / 0.013) × 0.075^(2/3) × 0.010^0.5 = 1.37 m/s
Q = 1.37 × 0.0707 = 0.0967 m³/s = 96.7 L/s
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Results are preliminary engineering estimates based on the stated assumptions. Confirm inputs, assumptions and requirements against the applicable standards and authority requirements before use in design or construction.