FL = pressure altitude ÷ 100
With 29.92 inHg (1013.25 hPa) in the Kollsman window, the altimeter reads pressure altitude. A flight level is that reading divided by 100, usually written with three digits: 18,000 ft pressure altitude is FL180. Below the published transition altitude you fly QNH altitudes; at or above the published transition level you fly flight levels. In the United States both numbers are 18,000 ft / FL180. In other airspaces they can differ, leaving a transition layer in between so that a climbing aircraft changes to 29.92 at the TA and a descending aircraft changes back to QNH at the TL.
The optional QNH box applies the same 1,000 ft per inHg rule of thumb used on the pressure-altitude calculator: PA ≈ QNH altitude + 1,000 × (29.92 − QNH in inHg). It is a quick offset, not a hydrostatic integration — for a temperature-aware true altitude see the true-altitude page.
What this does not assign
RVSM, hemispheric rules, and the actual flight level you are cleared to are ATC products. A low regional QNH can raise the lowest usable flight level above the published TL; this page does not compute that. Always set the altimeter as required by 14 CFR 91.121 or the equivalent national rule, and use current ATIS or ATC settings rather than a remembered 29.92 below the transition.
Sources
- FAA Aeronautical Information Manual, altimeter setting procedures, flight levels, and the U.S. 18,000 ft transition.
- 14 CFR 91.121, altimeter settings.
- ICAO Doc 8168 (PANS-OPS), transition altitude, transition level, and the transition layer.
Frequently asked questions
Is FL180 always 18,000 feet on the altimeter?
It is 18,000 feet of pressure altitude — the reading you get with 29.92 set. If you still have a local QNH in the window, the same flight level is a different indicated number.
Why can the transition altitude and transition level differ?
Some states publish a TA in feet and a TL as a flight level high enough that even on a low QNH there is a buffer (the transition layer) between QNH altitudes and flight levels. The United States uses the same 18,000 ft / FL180 for both.
Does this change my RVSM or cruising-level rules?
No. It only converts the numbers and shows which altimeter setting a published TA/TL would call for. Cruising levels and RVSM come from the airspace and the clearance.