Darcy's Equation
The frictional head loss can be computed by using a mathematical relationship which is termed as Darcy's equation for head loss. The equation takes two dissimilar forms. The first form of Darcy's equation establishes the losses in the system related with the length of the pipe.
Here:
f = friction factor
L = length of pipe (ft)
D = diameter of pipe (ft)
v = fluid velocity (ft/sec)
g = gravitational acceleration (ft/sec2)