The Fanning friction factor, named after John Thomas Fanning, is a dimensionless number used as a local parameter in continuum mechanics calculations.  It is defined as the ratio between the local shear stress and the local flow kinetic energy density:
where:
* is the local Fanning friction factor (dimensionless)
* is the local shear stress (unit in  or  or Pa)
* is the bulk dynamic pressure (unit in  or )
where the dynamic pressure is given by:
where:
* is the density of the fluid (unit in  or )
* is the bulk flow velocity (unit in  or )
In particular the shear stress at the wall can, in turn, be related to the pressure loss by multiplying the wall shear stress by the wall area (  for a pipe with circular cross section) and dividing by the cross-sectional flow area  (  for a pipe with circular cross section). Thus