Many folks calculate 4 times greater head loss (or 4 times less) than the actual friction loss. This comes from confusion between Moody and Fanning Friction factors. Some friction factor graphs are for Moody Friction factor, which is 4 times Fanning friction factor. That is, f = 64/Re is Moody and f = 16/Re is Fanning.
Be careful with your hydraulic calcs. It is easy to mix the two and calculate 400% greater (or 25% less) head loss. The calculation for head loss in feet is:
where,
h(friction) = head loss by friction in feet
f(M) = Moody Friction factor
f(F) = Fanning Friction factor
L = length in feet
D = pipe inside diameter in feet
v = velocity in ft/s
g = 32.174 ft/s^2, acceleration due to gravity
The Colebrook-White equation is an iterative method that calculates Fanning friction factor.
f(F)^2 = 1 / ( -4 * Log(eps / (3.7 * D) + 1.256 / (Re * ?f(F) )
where,
eps = pipe roughness in feet
Re = Reynold's number