# Reynolds Number Calculator

The Reynolds number is an important dimensionless ratio used to predict whether a flow is laminar or turbulent and is defined as the ratio of inertial to viscous forces in a fluid. The calculator below can either be used in manual mode (density, viscosity or kinematic viscosity known); or alternatively use it in conjunction with the standard atmosphere calculator to quickly determine the Reynolds number in a standard atmosphere for any altitude and temperature combination.

Standard Atmosphere
Manual Calculation

Use Standard Temp.
Use Kinematic Visc.
Altitude:
Density
Temperature:
Viscosity
Kinematic Viscosity:
Kinematic Viscosity
Velocity:

Characteristic Length:

Reynolds Number:

The Reynolds number is given by the following formula.

$$Re = \frac{Inertial Forces}{Viscous Forces} = \frac{\rho V L}{\mu} = \frac{V L}{\nu}$$

Where:
$\rho:$ Density of the fluid
$V:$ Velocity of the fluid passing over the object
$L:$ Characteristic length of the body (often wing chord or fuselage length in aeronautical design)
$\mu:$ Dynamic viscosity of the fluid
$\nu:$ Kinematic viscosity of the fluid $(\nu = \frac{\mu}{\rho})$

Note: The calculator will automatically convert the units of your choice into SI units in order to maintain the non-dimensional nature of the formula. You can input into the formula with the units of your choice – no need to be consistent: e.g. you can specify altitude in feet, characteristic length in m and velocity in knots.