Froude Number Calculator.

Calculate the dimensionless Froude number.

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Velocity

Gravity

Characteristic length

Froude number

0.000000

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Use Cases

Hydraulic Structure Design

When designing weirs, spillways, and stilling basins, the Froude number helps determine energy dissipation requirements and flow conditions to prevent erosion and ensure safety.

Example: Determine if flow over a spillway is supercritical by inputting the flow velocity and characteristic length.

Frequently Asked Questions

What is the Froude number used for?: The Froude number is a dimensionless value used in fluid mechanics to characterize the influence of gravity on flow. It helps determine flow regimes (e.g., subcritical, critical, supercritical) in open channels, rivers, and hydraulic structures.
How is the Froude number calculated?: The Froude number is calculated as the ratio of the flow velocity to the square root of the product of gravitational acceleration and characteristic length. In this calculator, you input velocity (m/s), gravity (m/s²), and characteristic length (m), and it computes the Froude number.
What does a Froude number greater than 1 indicate?: A Froude number greater than 1 indicates supercritical flow, where the flow velocity exceeds the wave celerity. This typically occurs in fast, shallow flows and is important for designing spillways, weirs, and other hydraulic structures.

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units: velocity in m/s, gravity in m/s², and characteristic length in meters.
For open channels, the characteristic length is typically the hydraulic depth (flow area divided by top width).
Use the standard gravity value 9.81 m/s² unless you are working on a different planet or in a specific context.
Interpret the result: Fr < 1 (subcritical), Fr = 1 (critical), Fr > 1 (supercritical).

Common Mistakes to Avoid

Using inconsistent units, such as velocity in km/h or length in feet, without converting to meters and seconds.
Confusing characteristic length with total channel length; it should be the relevant depth or hydraulic radius.
Forgetting to use the correct gravity value, especially when working in non-standard conditions.

Last updated: August 13, 2026

Use Cases

Hydraulic Structure Design

When designing weirs, spillways, and stilling basins, the Froude number helps determine energy dissipation requirements and flow conditions to prevent erosion and ensure safety.

Example: Determine if flow over a spillway is supercritical by inputting the flow velocity and characteristic length.

Frequently Asked Questions

What is the Froude number used for?

The Froude number is a dimensionless value used in fluid mechanics to characterize the influence of gravity on flow. It helps determine flow regimes (e.g., subcritical, critical, supercritical) in open channels, rivers, and hydraulic structures.

How is the Froude number calculated?

The Froude number is calculated as the ratio of the flow velocity to the square root of the product of gravitational acceleration and characteristic length. In this calculator, you input velocity (m/s), gravity (m/s²), and characteristic length (m), and it computes the Froude number.

What does a Froude number greater than 1 indicate?

A Froude number greater than 1 indicates supercritical flow, where the flow velocity exceeds the wave celerity. This typically occurs in fast, shallow flows and is important for designing spillways, weirs, and other hydraulic structures.

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units: velocity in m/s, gravity in m/s², and characteristic length in meters.
For open channels, the characteristic length is typically the hydraulic depth (flow area divided by top width).
Use the standard gravity value 9.81 m/s² unless you are working on a different planet or in a specific context.
Interpret the result: Fr < 1 (subcritical), Fr = 1 (critical), Fr > 1 (supercritical).

Common Mistakes to Avoid

Using inconsistent units, such as velocity in km/h or length in feet, without converting to meters and seconds.
Confusing characteristic length with total channel length; it should be the relevant depth or hydraulic radius.
Forgetting to use the correct gravity value, especially when working in non-standard conditions.

Use Cases

Hydraulic Structure Design

When designing weirs, spillways, and stilling basins, the Froude number helps determine energy dissipation requirements and flow conditions to prevent erosion and ensure safety.

Example: Determine if flow over a spillway is supercritical by inputting the flow velocity and characteristic length.

Frequently Asked Questions

What is the Froude number used for?

How is the Froude number calculated?

What does a Froude number greater than 1 indicate?

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units: velocity in m/s, gravity in m/s², and characteristic length in meters.
For open channels, the characteristic length is typically the hydraulic depth (flow area divided by top width).
Use the standard gravity value 9.81 m/s² unless you are working on a different planet or in a specific context.
Interpret the result: Fr < 1 (subcritical), Fr = 1 (critical), Fr > 1 (supercritical).

Common Mistakes to Avoid

Using inconsistent units, such as velocity in km/h or length in feet, without converting to meters and seconds.
Confusing characteristic length with total channel length; it should be the relevant depth or hydraulic radius.
Forgetting to use the correct gravity value, especially when working in non-standard conditions.