DrainageCalculators

Manning's Equation for Pipe Flow Calculator

Calculate pipe flow discharge, velocity, and hydraulic properties using Manning's equation. Supports full and partial flow in circular pipes with imperial and metric units. Free online tool for drainage engineers.

What This Solves

Calculates the flow rate and velocity in a circular pipe using Manning's equation for gravity-driven (non-pressurized) conditions.

Best Used When

  • You are sizing a storm sewer or drainage pipe for a known design flow
  • You need to check whether an existing pipe has enough capacity
  • You want to evaluate partial-flow depth and velocity in a circular pipe

Do NOT Use When

Key Assumptions

  • Flow is uniform and steady (not rapidly changing)
  • The pipe has a constant slope and cross-section along its length
  • Flow is gravity-driven with a free water surface (not pressurized)
  • Roughness is uniform throughout the pipe
  • Fully developed turbulent flow conditions exist

Input Quality Notes

The Manning's n value has the largest effect on results. Use published values for the specific pipe material and condition (new vs. aged). Slope should be the actual energy grade line slope, not just the pipe invert slope, though they are equal for uniform flow.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

Input Parameters

Pipe Properties

Internal diameter of the pipe

Don't know this value? Look it up
ft/ft

Pipe slope (rise/run)

Roughness Coefficient

Select a material to auto-fill Manning's n, or enter a custom value below

Typical values: 0.009-0.015 for smooth pipes, 0.022-0.030 for corrugated metal

Don't know this value? Look it up

Flow Conditions

Enable to specify a flow depth less than the full pipe diameter

For educational purposes only. Not a substitute for professional engineering judgment.

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.