A dimensionless number used in fluid flow calculations that accounts for the permanent energy loss as a fluid moves over a surface. It is related to shear stress at a surface and kinetic energy of the fluid per unit volume as:

Proposed by John Thomas Fanning (1837–1911), the Fanning friction factor for laminar flow in pipes and tubes is related to Reynolds number as:

For turbulent flow, numerous correlations exist for both smooth and rough-walled pipes. A number of charts have been prepared such as those by Moody, and by Stanton and Pannell, in which friction factor is correlated against Reynolds number for differing pipe surface roughness. It is important to note that this Fanning friction factor has a value of one-quarter of the Darcy friction factor.