Reynolds Number Calculator

Measure the balance of inertia and viscosity

Reynolds number compares inertial transport with viscous resistance using density times speed times characteristic length divided by dynamic viscosity. This calculator returns the dimensionless value and provides the familiar internal-pipe regime label. Engineers use Reynolds number to select correlations, anticipate transition, compare scaled experiments, and judge whether viscous or inertial effects dominate a flow.

For density 1.225 kilograms per cubic meter, speed 10 meters per second, characteristic length 0.5 meter, and dynamic viscosity 0.0000181 pascal-second, Reynolds number is 338398. That is well above the common smooth-pipe turbulent threshold of about 4000. For an external body, the same number remains valid but pipe labels do not determine boundary-layer transition.

Apply regime labels only to the right geometry

Characteristic length depends on the problem: pipe diameter, chord, hydraulic diameter, or another geometry scale may be appropriate. Density and viscosity must describe the same fluid state and temperature. The calculation runs in your browser. Transition thresholds vary with geometry, roughness, disturbances, and measurement conditions, so the displayed pipe classification is guidance rather than a universal verdict.