Reynolds Number Calculator

laminar turbulent transition

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 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.
Kinematic Viscosity:
Kinematic Viscosity

Characteristic Length:

Reynolds Number:

The Reynolds number is given by the following formula.

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

\( \rho: \) 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.

Disclaimer (the simple version):

Every effort has been made to ensure that the calculations that form part of this tool are correct and accurate. HOWEVER, AeroToolbox cannot and does not guarantee that the results output from the calculator are correct. You should always double check the results – especially if you are using the results for engineering calculations. Finally, if you do discover any bugs in the calculations please send an email to and let me know so I can make the correction.

Disclaimer (the legal version):

Aerotoolbox has made this calculator freely available to use by accessing it from the address. Be aware that the content on this site is copyrighted and redistribution or copying of this content is prohibited. The release of this calculator does not and cannot warrant that any functions contained within are accurate or error free. The use of this software comes with NO WARRANTY or LIABILITY of any kind. The entire risk as to the software applicability, performance and quality lies solely with you as the user and not Aerotoolbox. It is the responsibility of the user to ensure that the correct values have been selected at all times and to cross-reference the results obtained with a second source. In no circumstance or event shall, or the domain owner associated with the website, be liable for any damages, whether direct or indirect, incidental, special, or consequential arising out the use of or any defect inherent in the software. By using this calculator, you as the user expressly agree to the terms of this disclaimer in full.