A dimensionless friction factor used to determine the frictional pressure drop resistance of a moving fluid in a pipe or over a surface. The friction factor is the proportionality constant in which the wall shear stress, τ_w, of a fluid at a surface is proportional to the kinetic energy of the fluid per unit volume

where ρ is the density and v is the fluid velocity. It is named after American engineer John Thomas Fanning (1837–1911)