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 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.

  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.


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.

dynamic viscosity calcluator
dynamic viscosity formula