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A flight level (FL) is the altitude an altimeter shows when it is set to standard pressure, 1013.25 hPa (29.92 inHg), written in hundreds of feet. FL350 is the level at which the altimeter reads 35,000 ft on that setting. Above the transition altitude every aircraft uses the same setting, so they all measure their height against the same pressure surfaces. That shared reference is what makes vertical separation work, whatever the local weather is doing.
Reading a flight level
Take the pressure altitude and drop the last two zeros. A level below FL100 keeps a leading zero.
| Pressure altitude (altimeter on 1013.25 hPa) | Flight level |
|---|---|
| 4,500 ft | FL045 |
| 10,000 ft | FL100 |
| 18,000 ft | FL180 |
| 35,000 ft | FL350 |
| 41,000 ft | FL410 |
Our Pressure Altitude Calculator gives the pressure altitude for any field elevation and altimeter setting.
A flight level is a pressure surface, not a fixed height
An altimeter measures pressure, so a flight level follows a pressure surface. That surface rises and falls with the weather, which means the true height of a given flight level changes from day to day.
- Sea-level pressure. Near sea level, 1 hPa is worth roughly 27–30 ft. With QNH 1033 the 1013 hPa surface sits about 20 × 27 ≈ 540 ft above mean sea level, so FL100 is close to 10,540 ft. With QNH 993 it is about 540 ft lower, near 9,460 ft. This is the same effect behind “from high to low, look out below”, explained in our QNH, QFE and QNE guide.
- Temperature. Cold air is denser, so pressure surfaces sit lower than in the standard atmosphere. A common rule of thumb is about 4% of the height for every 10 °C below ISA. See cold temperature altimeter correction and the ISA calculator.
Aircraft at FL350 and FL360 are always 1,000 ft apart on their altimeters, because both use the same reference. That is the whole point. For terrain clearance, though, the true altitude is what matters, so pilots check minimum safe levels against QNH and temperature.
Transition altitude, transition level and the transition layer
- Transition altitude (TA): at and below it, aircraft fly altitudes with QNH set. Climbing through it, pilots set 1013 and start flying flight levels.
- Transition level (TL): the lowest flight level available above the transition altitude. Descending through it, pilots set QNH.
- Transition layer: the airspace between the two. Aircraft pass through it but do not cruise in it.
Transition altitudes are set by each State and sometimes by each airport. The United States uses 18,000 ft and FL180 across the country, while many other States use much lower values. ATIS normally broadcasts the current transition level.
Why the transition level moves with QNH
Take a transition altitude of 5,000 ft and look at where FL060 sits on different days (standard temperature, about 27 ft per hPa):
| QNH | True altitude of FL060 | Room above a 5,000 ft TA |
|---|---|---|
| 1030 hPa | about 6,460 ft | about 1,460 ft |
| 1013 hPa | 6,000 ft | 1,000 ft |
| 995 hPa | about 5,510 ft | about 510 ft |
| 975 hPa | about 4,970 ft | below the TA, so FL060 cannot be used |
As QNH falls, every flight level sinks. The transition level is therefore raised on low-pressure days so that the lowest usable flight level stays safely above the transition altitude, and it can be lowered when pressure is high. Each State sets the minimum buffer it requires.
Which flight levels can you cruise at?
ICAO’s table of cruising levels assigns levels by magnetic track, not heading. The short version is “east is odd, west is even”:
| Magnetic track | IFR | VFR |
|---|---|---|
| 000° to 179° | Odd thousands: FL050, FL070… FL290, FL310, FL330… | Odd thousands + 500 ft: FL055, FL075, FL095… |
| 180° to 359° | Even thousands: FL060, FL080… FL300, FL320, FL340… | Even thousands + 500 ft: FL065, FL085, FL105… |
Example: an IFR flight on a magnetic track of 095° that wants to cruise near 30,000 ft would plan FL290 or FL310. On a track of 275° it would plan FL300 or FL320. Above FL410 the steps widen: eastbound FL450, FL490, FL530 and westbound FL430, FL470, FL510.
Some States publish their own variations, and controllers can assign any level, so the AIP and the clearance always take priority over the general table.
RVSM: 1,000 ft between FL290 and FL410
Between FL290 and FL410 inclusive, Reduced Vertical Separation Minimum (RVSM) airspace separates approved aircraft by 1,000 ft instead of 2,000 ft. This doubles the number of usable levels in the busiest part of the sky. Aircraft need approved altimetry and an altitude-hold system, and operators need RVSM approval. Non-approved aircraft are normally kept out of this band or separated by 2,000 ft.
Common mistakes
- Using heading instead of track. A strong crosswind can put your heading in the other semicircle. The rule uses magnetic track.
- Forgetting to set 1013 at the transition altitude, or forgetting QNH at the transition level. Both put the aircraft at a different height from everyone else and are a classic cause of level busts.
- Treating a flight level as a true altitude. On a cold, low-pressure day a low flight level can be hundreds of feet lower than its number suggests.
- Assuming the transition level is fixed. Where it varies with QNH, check ATIS before descent.
Tools for this topic
- Pressure Altitude Calculator
- QNH hPa / inHg Converter
- ISA Standard Atmosphere Calculator
- Feet / Metres Converter for States that assign levels in metres
Educational content only. Follow your local AIP, ATC clearances and operator procedures.
Sources and further reading
- AIM 7-2, Barometric Altimeter Errors and Setting Procedures
- 14 CFR 91.159, VFR cruising altitude or flight level
- 14 CFR 91.179, IFR cruising altitude or flight level
- ICAO Annex 2, Rules of the Air, Appendix 3 (tables of cruising levels)
- ICAO Doc 8168, PANS-OPS, Volume I (transition altitude and transition level)
Sources checked November 2026. Rules differ between countries and change over time: for real flights, use the current publications of the authority that applies to you, your aircraft flight manual and your operator’s procedures.