How this calculator works
Reynolds number compares inertia with viscosity. Choose a length relevant to the geometry, such as internal diameter for a full round pipe. Kinematic viscosity already includes density; dynamic viscosity needs density separately. The displayed regime thresholds apply to developed circular-pipe flow, not every flow geometry. Water reference options use the cited saturated-water property equations; custom data should match temperature, composition and shear conditions. Non-Newtonian foods and blood do not have a universal constant viscosity.
Formula
Re = U L/nu = rho U L/mu; nu = mu/rho. 1 cSt = 0.000001 m2/s; 1 cP = 0.001 Pa s.
Worked example
With U = 1 m/s, L = 0.01 m and nu = 0.000001 m2/s, Re = 10000. With U = 0.1 m/s, Re = 1000.
Advertisement
Frequently asked questions
Which length should I enter?
Use the scale defining your flow problem. Circular pipe classifications require internal diameter.
Why do temperature and composition matter?
They change density and viscosity, sometimes strongly, so a fluid name alone is insufficient.
Can Reynolds number be zero?
Yes for zero speed. Recovering length, density or viscosity from zero motion is indeterminate.