The head loss   (or ) expresses the pressure loss due to friction in terms of the equivalent height of a column of the working fluid, so the pressure drop is
:  = The head loss due to pipe friction over the given length of pipe (SI units: m);
:  = The local acceleration due to gravity (m/s).
It is useful to present head loss per length of pipe (dimensionless):
where  is the pipe length (m).
Therefore, the Darcy–Weisbach equation can also be written in terms of head loss: