Subject
The Colebrook friction factor applies to fluids flowing over any external surface subject to drag and added mass coefficients.
The Fanning friction factor applies to fluids flowing through a circular pipe resulting in a loss of head.
Whilst Colebrook’s and Fanning’s factors normally apply to particularly rough surfaces and turbulent flow, they may legitimately be applied to any surface (incl. smooth) and exposed to any type of flow (incl. laminar) where safety and sensitivity are considered sufficiently important. However, as Reynold’s number decreases (flow becomes laminar) and/or the surface becomes smoother, their magnitude reduces considerably. Even so, their use should never be discounted without good reason.
Calculator
In addition to the calculation of the Colebrook and Fanning factors, Colebrook can also be used to calculate simply the Reynolds number, i.e. to identify fluid flow condition; Laminar, Transient or Turbulent.
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Hardness Calculator  Options
For all of the calculation options, the input and output data are as follows:
You enter any one of the following: 
and the hardness calculator will provide all of the following: 


Colebrook Friction factor

Fanning Friction factor

Reynold's Number

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