Reynolds Number Calculator

The Reynolds number is the ratio of inertial to viscous forces in a flow, Re = rho v L / mu, and it decides whether the flow is laminar or turbulent. Enter a pipe diameter with either a velocity or a flow rate, a rectangular or annular duct, or any geometry with the density and viscosity you have, and it returns the Reynolds number, the flow regime and the characteristic length it divided by. Water and air properties come from temperature through Kell's density equation, the Kestin viscosity correlation and Sutherland's law, so you do not have to look them up. The pipe results also carry the velocity at which the flow turns laminar and the entrance length before the velocity profile settles. Re is a ratio and nothing more: it tells you which friction correlation applies, but it is not itself a pressure drop.