Flight level vs altitude explained: understand standard pressure, QNH, transition altitude, FL notation and why FL180 may not be 18,000 feet AMSL.
In aviation and real-world flight simulation, a flight level is a pressure-based vertical reference shown with the altimeter set to standard pressure, 1013.25 hPa or 29.92 inHg, and stated in hundreds of feet. Altitude normally means distance above mean sea level with local QNH set, so FL180 need not equal 18,000 feet AMSL.
Flight level vs altitude at a glance
The difference is the pressure reference: altitude uses local QNH, while a flight level uses one shared standard-pressure setting.
| Reference | Altitude | Flight level |
|---|---|---|
| Altimeter setting | Local QNH | Standard pressure: 1013.25 hPa or 29.92 inHg |
| Datum | Mean sea level | A standard atmospheric pressure surface |
| Example | 18,000 feet | FL180 |
| Normal use | Below the transition altitude | Above the transition altitude |
An altitude with the correct QNH set gives an indicated position relative to mean sea level. It is not height above the ground, which is called height or AGL, and it is not necessarily identical to GPS-derived geometric altitude.
How is a flight level calculated?
Set the altimeter to standard pressure and express the indicated pressure altitude in hundreds of feet. An indication of 10,000 feet becomes FL100; 18,000 feet becomes FL180; and 35,000 feet becomes FL350.
Do not simply divide an altitude shown with QNH by 100. The altimeter must first be on standard pressure. Also, not every numbered pressure surface is available as a cruising level; ATC, direction-of-flight rules and local airspace determine which levels may be used.
When do pilots switch from altitude to flight level?
Pilots change to standard pressure while climbing through the published transition altitude, then restore local QNH while descending through the transition level, following local procedures and ATC instructions.
The transition altitude is normally fixed for an area. The transition level is the lowest usable flight level above it and can change with atmospheric pressure. The space between them is the transition layer, which prevents aircraft using different pressure references from being assigned too close together.
- Before departure: obtain the local QNH from ATIS, ATC or the weather report. Our guide to finding the altimeter setting in a METAR explains the
QandApressure formats. - During the climb: use QNH and report altitude until reaching the transition altitude.
- Passing the transition altitude: select standard pressure and verify that the primary and standby altimeters show the correct setting. Vertical position is then reported as a flight level.
- During descent: obtain the destination QNH and set it while passing the transition level, or at the point specified by ATC or the local procedure.
Transition values vary by country and sometimes by airport, so they should never be guessed from habit. We cover the underlying pressure change in more detail in our explanation of how QNH and standard pressure alter the altimeter. The complete departure-to-arrival sequence also forms part of a normal IFR flight-simulation workflow.
Does FL180 always mean 18,000 feet above sea level?
No. At FL180, an altimeter set to standard pressure indicates a pressure altitude of 18,000 feet, but the aircraft's actual height above mean sea level can be higher or lower.
Atmospheric pressure and temperature move pressure surfaces relative to mean sea level. This is why flight levels work for aircraft separation: every aircraft uses the same pressure datum, even though the geometric height of that datum changes with the weather.
Flight levels do not remove cold-temperature errors or guarantee terrain clearance. Published minimum altitudes, QNH procedures and any required temperature corrections still apply during departures, approaches and other terrain-critical phases.
How are flight levels written and spoken?
Flight levels are normally written with three digits and spoken digit by digit. FL350 is “flight level three five zero”, while FL070 is “flight level zero seven zero”.
Below the transition altitude, pilots use an altitude such as “seven thousand feet”, not “flight level seven thousand”. Calling an altitude by a flight-level name implies that standard pressure is set, so mixing the terms can conceal an incorrect altimeter setting.
Why does a simulator or ATC show the wrong altitude?
The mistake we see most often is using the wrong pressure reference for the phase of flight. Check the barometric setting before changing pitch or chasing a GPS altitude.
- QNH left set during the climb: select
STD, 1013 hPa or 29.92 inHg when crossing the transition altitude. - Standard pressure left set during descent: enter the reported QNH before operating below the transition level. Otherwise, approach and terrain indications can be wrong.
- Wrong pressure units: 1013 hPa and 29.92 inHg are the same standard setting. Confirm which unit the instrument is displaying.
- Inconsistent weather information: use the QNH supplied by the same simulated weather or ATC source governing the flight. Raw station pressure is not a substitute for QNH.
- Different altitude types: barometric, GPS and radio altitudes measure different things and are not expected to agree exactly. Radio altitude is height above the terrain directly below the aircraft.
- Independent instruments: airliners may have separate captain, first-officer and standby settings. Verify each one rather than assuming a barometer shortcut changed them all.
- Transition mismatch: simulated ATC and a published procedure may use different transition values. Follow the active ATC system for an ATC-controlled simulator flight, or the published procedure when recreating it independently.
A discrepancy of several hundred feet usually points to the pressure setting or selected reference. A smaller difference may come from instrument modelling, rounding or non-standard temperature. Do not alter a correct reported QNH merely to force the barometric altimeter to match GPS altitude.