# Fanning Friction Factor

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Fanning friction factor, abbreviated as f, a dimensionless number, is the ratio of shear stress acting on the surface of a solid.  Fanning friction factor is not the same as the Darcy friction factor.  It is equal to 1/4 the Darcy friction factor.  Since they are basically the same, you need to note which one is being used when doing calculations. This can be done by using the subscript F, fF

### Fanning Friction Factor Formula

$$\large{ f = \frac{ \tau }{ \rho \; \frac{Q^2}{2} } }$$

Where:

$$\large{ f }$$ = Fanning friction factor

$$\large{ \rho }$$  (Greek symbol rho) = fluid density

$$\large{ \tau }$$  (Greek symbol tau) = shear stress

$$\large{ Q }$$ = bulk flow velocity