In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. A flight level is a surface of constant atmospheric pressure used to define an aircraft’s vertical position relative to other aircraft when flying above the transition altitude. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressure of 1013 hPa, or 29.92 inHg. The actual surface pressure will vary from this at different locations and times. Therefore, by using a standard pressure setting, every aircraft has the same altimeter setting, and vertical clearance can be maintained during cruise flight.