---
title: "Froude Number Calculator"
description: "Calculate Froude number for open channel flow from velocity, depth, and gravity to classify subcritical and supercritical flow."
image: "https://onlineminitools.com/tools/froude-number-calculator/logo.svg"
---

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# Froude Number Calculator

Calculate Froude number for open channel flow from velocity, depth, and gravity to classify subcritical and supercritical flow.

      O M T

## What Is the Froude Number?

The Froude number (Fr) is a dimensionless quantity that compares inertial forces to gravitational forces in open-channel and free-surface flow. It helps classify whether flow is subcritical (tranquil), critical, or supercritical (rapid). It is widely used in hydraulic engineering, ship design, and river modeling.

## Froude Number Formula

Fr = V / √(g × L), where V is flow velocity, g is gravitational acceleration, and L is a characteristic length. For open channels, L is typically the hydraulic depth or flow depth. When Fr < 1, flow is subcritical; when Fr > 1, flow is supercritical; at Fr = 1, flow is critical.

## Flow Classification

- **Subcritical (Fr < 1):** Disturbances propagate upstream. Depth is relatively large compared to velocity.
- **Critical (Fr = 1):** Minimum specific energy for a given discharge. Transition between regimes.
- **Supercritical (Fr > 1):** Rapid, shallow flow. Disturbances cannot propagate upstream.

Related tools: [Flow Rate Calculator](https://onlineminitools.com/flow-rate-calculator) and [Reynolds Number Calculator](https://onlineminitools.com/reynolds-number-calculator).

## Frequently Asked Questions

   **What is critical velocity?**

Critical velocity is V = √(gL), the speed at which Fr = 1. Below this speed flow is subcritical; above it, supercritical.

   **What length should I use for L?**

For rectangular open channels, use the flow depth. For complex sections, use hydraulic depth A/T, where A is cross-sectional area and T is top width.

   **How is Froude number used in ship design?**

Ship hulls are compared using Fr based on waterline length. High Fr indicates wave-making resistance dominates, which matters for speed and fuel efficiency.

   **Is Froude number the same as Reynolds number?**

No. Reynolds number compares inertial to viscous forces. Froude number compares inertial to gravitational forces. Both are important but describe different physics.

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### Recent Visits

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