Darcy-Weisbach Equation

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 = Loss of head due to friction in meters.

= Pressure drop in Pascals.

f  = Colebrook friction factor (Moody’s chart)

L = Length of the pipe in meters.

 v = Velocity of fluid in m/s.

= Fluid density in kg/m3.

g = Acceleration due to gravity (9.8 m/s²).

d = Inner diameter of pipe in meters.

Coolebrook equation :

Re = Reynolds number, dimensionless
ϵ = absolute roughness of pipe wall in m.