Flight Level

Flight Level (FL) is a standard nominal altitude based on the standard pressure setting of 1013.25 hPa (29.92 inHg). Flight levels are used above the transition altitude to ensure vertical separation between aircraft, since all aircraft use the same pressure reference.

Flight levels are expressed in hundreds of feet: FL350 = 35,000 ft, FL100 = 10,000 ft. In non-standard atmospheric conditions, the actual altitude may differ from the indicated flight level.