Enter the density, flow speed, characteristic linear dimension, and dynamic viscosity of a fluid to determine the Reynold’s number.

## Reynolds Number Formula

The following formula is used to calculate a Reynolds number.

Re = (p*v*L / u)

• Where Re is Reynolds number
• p is the density of the fluid
• v is the flow speed of the fluid
• L is the diameter of the tube
• u is the dynamic viscosity of the fluid

## Reynolds Number Definition

Reynold’s number is a unitless factor that describes the ratio of inertial forces to viscous forces of an object moving through a fluid or gas.

## Reynolds Number Example

How to calculate Reynolds number.

1. First, determine the density.

For this example, we will say the density is 100 slugs/ft^3.

2. Next, determine the diameter of the tube.

The diameter is found to be 5 ft.

3. Next, determine the flow velocity.

For this example the velocity is found to be 10 ft/s.

4. Next, determine the viscosity.

Using the calculator above, the viscosity is found to be 10 lb-s/ft^2.

5. Finally, calculate the Reynold’s Number.

Using the formula we find the Reynolds number to be 100*5*10/10 = 500. This is a laminar flow.

## FAQ

What is Reynold’s Number?

Reynold’s Number is a dimensionless number that represents the ratio of the inertial forces to viscous forces acting on a fluid that is moving through an area, most often a tube.

What is Reynold’s Number used for?

Reynold’s number is used to classify whether fluids are moving in a laminar or turbulent flow. A number less than 2100 indicates laminar and a number greater than 2100 indicates turbulent flow.

What is laminar flow?

Laminar flow is flow described as viscous flow in which the liquid flows along the lays of a tube without causing eddies or turbulence.