Reynolds Number Calculator

Andrew Wood • 28 September 2022

The Reynolds number is an important dimensionless quantity 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 can either be used in manual mode (density, viscosity or kinematic viscosity known); or alternatively use it in conjunction with the built-in 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 Temperature
Altitude
Temp.
Kinematic Viscosity
Use Kinematic Viscosity
Density
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.

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