Enter either the dynamic viscosity and density, or the kinematic viscosity. The Reynolds number is proportional to the ratio of inertial to viscous effects. It relates to the amount of disorder in the flow, with a larger nunber (e.g. over ~2400 in a circular pipe) indicating turbulent flow. For calculation, the "characteristic length" should be taken according to convention. For example, in a pipe it is the hydraulic diameter, and in laminar flow over a plate, it is the plate length.