MSL
Mean Sea Level
Last updated: April 20, 2026 · Maintained by Aviatr Editorial Team
What is MSL?
MSL (Mean Sea Level) is the standard reference datum for altitude, expressed in feet or metres above the long-term average ocean surface. Aircraft altimeters calibrated to local QNH display altitude in feet MSL — the measurement used for IFR flight-level assignments, airspace floors, and terrain-clearance limits below the transition altitude.
How is MSL used?
Pilots reference MSL constantly: airport elevations, cloud bases in METARs, airspace ceilings, minimum en-route IFR altitudes, and approach decision heights are all published in feet MSL across EASA. Two related altitude references serve different contexts. AGL (Above Ground Level) governs VFR minimum heights over congested areas, typically 1,000 feet AGL in Europe. Flight Levels (FL) replace MSL above the transition altitude, using the standard pressure datum of 1013.25 hPa instead of local QNH. An altimeter set to QNH reads MSL; set to 1013 hPa it reads pressure altitude or Flight Level. Confusing MSL with AGL or pressure altitude is a common student-pilot error that instructors address early in training, particularly during approach briefings where both references appear on the same chart.