A flight level (FL) is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa (29.92 inHg), the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either above mean sea level or above ground level.
Read more about Flight Level: Background, Transition Altitude, Quadrantal Rule, Semicircular/Hemispheric Rule, Reduced Vertical Separation Minima, Metric Flight Levels, Flight Levels in Russian Federation
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