The Hazen-Williams equation for calculating the pressure drop due to friction for a given pipe diameter and flow rate is as follow
The equation in SI units
P = frictional pressire drop, kPa/m
Q = flow rate, m^3/h
D = pipe inside diameter, mm
C = Hazen-Williams C factor, dimensionaless
In general an average value of C=100 is used for most applications.
The Hazen-Williams C factor depends on the type of the pipe material and the internal condition of the pipe.
A low value such as C=75 may be used for pipe that is 10 to 15 yeas old
The equation in U.S. customary units
P = pressure loss due to friction, psi per ft of pipe length
Q = flow rate, gal/min
D = pipe inside diameter, in
C = Hazen-Williams roughness coefficient factor, dimensionaless