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

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## Reynolds Number Formula

The following formula is used to calculate a Reynolds number.

Re = (p*v*L / u)

- Where Re is Reynold’s 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

To calculate the Reynolds number, multiply the density, flow speed, and diameter, then divide by the dynamic viscosity.

## 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.

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

**Next, determine the diameter of the tube.**The diameter is found to be 5 ft.

**Next, determine the flow velocity.**For this example the velocity is found to be 10 ft/s.

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

**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.