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For educational purposes only. Not a substitute for professional engineering judgment.
About Manning's Equation for Open Channels
Manning's equation is an empirical formula used to calculate the velocity of flow in open channels. It relates the flow velocity to the channel's hydraulic radius, slope, and roughness coefficient.
The Manning Equation
V = (k/n) * R2/3 * S1/2
Where:
- V = Mean velocity (ft/s or m/s)
- k = Conversion factor (1.486 for US units, 1.0 for SI)
- n = Manning's roughness coefficient
- R = Hydraulic radius = A/P (ft or m)
- S = Channel slope (ft/ft or m/m)
Supported Channel Shapes
This calculator supports four common channel cross-sections:
- Rectangular - Common for lined channels, flumes, and box culverts
- Trapezoidal - Most efficient for earthen channels and roadside ditches
- Triangular - Used for gutters, V-ditches, and small swales
- Parabolic - Natural channel shape, grass-lined swales
Key Parameters
- Hydraulic Radius (R) - Ratio of flow area to wetted perimeter
- Wetted Perimeter (P) - Length of channel boundary in contact with water
- Froude Number (Fr) - Ratio of inertial to gravitational forces; indicates flow regime
- Specific Energy - Sum of flow depth and velocity head
Reference & standards
What This Solves
Calculates flow rate and velocity in open channels of various shapes (rectangular, trapezoidal, triangular, parabolic) using Manning's equation.
Best Used When
- You are designing or analyzing a ditch, swale, or open drainage channel
- You need to determine the capacity of a roadside drainage feature
- You want to size a channel to carry a specific design flow at a given slope
Do NOT Use When
- You are sizing a circular pipe rather than an open channel — Use Manning's Pipe Calculator
- You need the optimal (most efficient) channel cross-section geometry — Use Best Hydraulic Section Calculator
- You are designing a vegetated swale with specific velocity and residence time criteria — Use Swale Calculator
Key Assumptions
- Flow is uniform and steady (constant depth and velocity along the channel)
- The channel has a constant slope and cross-section
- Flow is gravity-driven with a free water surface (not pressurized)
- Roughness is uniform throughout the channel
- The channel is prismatic (constant shape) along its length
Input Quality Notes
Manning's n values for vegetated channels vary with season, growth, and maintenance. Use higher n values for conservative design of natural or grass-lined channels.
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Last verified: February 2026