How this calculator works
The Darcy factor relates distributed pipe loss to velocity head. In a circular laminar flow it is independent of wall roughness and equals 64/Re. The turbulent factor is found by solving Colebrook rather than using an explicit approximation. Transitional flow from Re 2300 to below 4000 is rejected because disturbances can change the flow state. Relative roughness is limited to 0.05 for this model. Fluid properties must refer to the same operating temperature.
Formula
Re = rho v D/mu. Laminar fD = 64/Re. Colebrook: 1/sqrt(fD) = -2 log10[epsilon/(3.7D) + 2.51/(Re sqrt(fD))]. Fanning factor fF = fD/4.
Worked example
At Re = 1000 the Darcy factor is 0.064. At Re = 100000 and relative roughness 0.001, Colebrook gives approximately 0.02217453594.
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Frequently asked questions
Can I enter a Fanning factor?
Multiply it by four first. This page and the related Darcy-Weisbach page use the Darcy convention.
Why is transitional flow rejected?
A smooth interpolation would imply more certainty than the simple model provides. Establish the flow regime or use measured losses.
Is hydraulic diameter always the pipe bore?
For a full circular pipe, yes. For another section it is four times hydraulic radius, with additional limits on applying circular-pipe correlations.