 Friction loss is then the change in pressure Δp per unit length of pipe "L"
When the pressure is expressed in terms of the equivalent height of a column of that fluid, as is common with water, the friction loss is expressed as "S", the "head loss" per length of pipe, a dimensionless quantity also known as the "hydraulic slope".where

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.