AN (Army-Navy) sizes were established by the aircraft industry and designate the outside diameter of rigid tubing that the corresponding fittings are used with. Each dash size equals 1/16 of an inch. (ie -8 AN = 8 x 1/16" = 1/2" OD of the metal tube) Each standard AN size has its own standard thread size. Since the tube sizes do not equate with the hose sizes, due to the variation in wall thickness, the ID of the hose and tubes are not the same.  The table below shows the AN size and the equivalent tubing size.