DrainageCalculators

Manning's Channel Calculator

Calculate open channel flow using Manning's equation. Supports rectangular, trapezoidal, triangular, and parabolic channel shapes. Free professional-grade hydraulic engineering calculator.

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

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.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

Manning's Channel Calculator

Calculate open channel flow using Manning's equation. Select your channel shape and enter the hydraulic parameters to determine discharge, velocity, and flow characteristics.

Input Parameters

Channel Shape

Flow Parameters

ft

Depth of water in the channel

ft/ft

Longitudinal slope of channel bed

Roughness coefficient for channel material

View Manning's n Reference Table

Channel Geometry

ft

Width of the channel bottom

Ready to Calculate

Enter your channel parameters and click Calculate to see results.

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the Manning's Channel Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

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

Resources

Manning's n Reference Table

Look up Manning's n values for various channel materials and conditions.

Open Channel Hydraulics Guide

Learn the fundamentals of open channel flow and hydraulic design.

Channel Design Tutorial

Step-by-step guide to designing efficient open channels.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.