Pressure drop is related inversely to pipe diameter to the fifth power. For example, halving a pipe's diameter would increase the pressure drop by a factor of  (e.g. from 2 psi to 64 psi), assuming no change in flow.
Pressure drop in piping is directly proportional to the length of the piping—for example, a pipe with twice the length will have twice the pressure drop, given the same flow rate. Piping fittings (such as elbow and tee joints) generally lead to greater pressure drop than straight pipe. As such, a number of correlations have been developed to calculate equivalent length of fittings.
Certain valves are provided with an associated flow coefficient, commonly known as  or . The flow coefficient relates pressure drop, flow rate, and specific gravity for a given valve.
Many empirical calculations exist for calculation of pressure drop, including:
* Darcy–Weisbach equation, to calculate pressure drop in a pipe
* Hagen–Poiseuille equation