Simple rule can be used to calculate friction loss in piping. A.A. Durand. OGJ, 26 May 1997, 95(21), 77, 78.
The simple rule described, called the `Rule of Fours' is designed to be easily memorized for use in the field. It states that---the pressure drop of 400 gpm of water at 60 degrees F in a 4-in, clean, commercial, schedule-40 steel pipe is almost 4 psi/100 ft. The accuracy of the rule can be improved considerably by applying another `four'---that is, to change 400 gpm to 400 gpm plus 4% (416 gpm).