Atkinson friction factor is a measure of the resistance to airflow of a duct. It is widely used in the mine ventilation industry but is rarely referred to outside of it.
Atkinson friction factor is represented by the symbol  and has the same units as air density (kilograms per cubic metre in SI units, lbfmin^2/ft^4 in Imperial units).  It is related to the more widespread Fanning friction factor by
in which  is the density of air in the shaft or roadway under consideration and  is Fanning friction factor (dimensionless).  It is related to the Darcy friction factor by 
in which  is the Darcy friction factor (dimensionless).
It was introduced by John J Atkinson in an early mathematical treatment of mine ventilation (1862) and has been known under his name ever since.