Friction loss, which is due to the shear stress between the pipe surface and the fluid flowing within, depends on the conditions of flow and the physical properties of the system. These conditions can be encapsulated into a dimensionless number Re, known as the Reynolds number
where V is the mean fluid velocity and D the diameter of the (cylindrical) pipe.  In this expression, the properties of the fluid itself are reduced to the kinematic viscosity ν
where
: μ = viscosity of the fluid (SI kg / m • s)