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.

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

**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.