If p is the density and η is coefficient of viscosity of fluid...

Reynolds number is given by, ${R}_{e} = \dfrac{ρvd}{η}$, where, $v =$flow velocity, $d =$hydraulic diameter of the pipe, $ρ =$density of the fluid & $η =$coefficient of viscosity of the fluid.

Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities.