Created By : Vaibhavi Kumari

Reviewed By : Rajashekhar Valipishetty

Last Updated : May 17, 2023

The Froude number calculator estimates the Froude number for a given flow rate, allowing you to characterize fluid flow in an open channel. Waves, weirs, spillways, wind engineering, ripraps, predicting hydraulic leap, pipe flow, and partially submerged vessels moving through water, i.e. ship design, all require the Froude number formula. Scroll down to learn more about the Froude number and how to calculate it.

Area of cross-section (A)
Width of the channel (W)
Hydraulic mean depth (Hd)
Flow velocity (u)
Acceleration due to gravity (g)

What exactly is the Froude Number?

The Froude number is a dimensionless number that determines how external influences (most notable gravity) affect a fluid movement. Froude number is the ratio of flow inertia to gravity. The Froude number Fr is influenced by gravity's acceleration g, flow velocity u, and hydraulic depth Hd, as follows: Fr = u / √(g * Hd)

The hydraulic depth, Hd, is defined as the ratio of the cross-sectional area of the channel, A, to the channel width, W. Mathematically it is expressed as;  Hd = A /W

The hydraulic depth is replaced with characteristic length L, which represents the length of the ship at the waterline, in various applications, such as ship hydrodynamics. The waterline is defined as the point at which the ship's hull meets the water's surface.

How do you Calculate the Froude Number?

Learn how to compute the Froude number using the instructions below.

  • Step 1: First, enter the cross-sectional area.
  • Step 2: Calculate the hydraulic depth using the width of the channel.
  • Step 3: Calculate the velocity of the flow.
  • Step 4: The Froude number is calculated by the calculator.

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How to Use the Froude's number?

According to the value of the Froude number, the flow is classified as supercritical (Fr > 1) or subcritical (Fr < 1). It's called critical flow when the Froude number is around one.

The number is critical in the hydraulic leap concept, which is used to measure the change in water surface level that occurs under various flow circumstances. To distinguish between laminar and turbulent flow, the Froude number is utilized similarly to how Reynold's number is used.

FAQs on Froude Number Calculator

1. What does the Froude number represent?

The Froude number is used to compare the resistance of bodies of various sizes and forms to make waves. The Froude number determines whether the flow is supercritical or subcritical in free-surface flow.

2. When Froude's number is less than 1, the flow is called?

The critical flow Froude number is 1. Sub-critical flow occurs when a flow's depth in a channel exceeds the critical depth (hc). Sub-critical flow has a Froude number of less than 1.

3. What is the significance of the Froude number over the Reynolds number?

The relationship between frictional and inertial forces is represented by the Reynolds number, whereas the relationship between gravity and inertial forces is represented by the Froude number.