Flight level and altitude

The altitude on the local altimeter setting that a flight level puts you at, or the other way round, with the US lowest usable flight level and transition altitude.

Planning and education aid. Not for primary navigation. Full disclaimer

Experimental: not yet fully verified. How results are checked

A published result changed on 2026-09-23: Station pressure now comes from the altimeter-setting relation instead of a sea-level ratio shortcut. The worked example moved from 5,108 to 5,112 ft pressure altitude and 7,932 to 7,937 ft density altitude; at an 8,000 ft field with 28.20 inHg the old answer was 90 ft low. Changelog

Computing your own values needs JavaScript and WebAssembly. The worked example below was computed when this page was built, and the documentation under "How we got this" applies.

185

The lowest usable flight level is FL185.

Transition altitude (US)
Rule
Altitude of the flight level

Provenance

Computed by: aviation.altimetry.flight-level 1.1.0, core 0.1.0
Model: A flight level is a pressure altitude in hundreds of feet. On a QNH the altimeter reads that pressure altitude less PA(QNH), the pressure altitude of the setting itself (the altimeter-setting relation that defines QNH). The US lowest usable flight level is FL180 at 29.92 inHg or higher and 500 ft higher for each 0.50 inHg lower, to FL210 at 26.92 (14 CFR 91.121(b))
Accuracy: Exact for ISA; real temperature moves the true altitude (see the cold-temperature tool). The US table covers settings down to 26.92 inHg
Notes: 1 shown with the answer
Cites: Federal Aviation Administration, 14 CFR Part 91, General Operating and Flight Rules; International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet))

Something look off?

How we got thisFormula, worked example, sources, and proof

Model: A flight level is a pressure altitude in hundreds of feet. On a QNH the altimeter reads that pressure altitude less PA(QNH), the pressure altitude of the setting itself (the altimeter-setting relation that defines QNH). The US lowest usable flight level is FL180 at 29.92 inHg or higher and 500 ft higher for each 0.50 inHg lower, to FL210 at 26.92 (14 CFR 91.121(b))

Show your work

Below standard
29.92 − setting, in hundredths of an inch
29.92 − 29.42 = 50
Lowest usable flight level
FL180 + 5 for each started 0.50 inHg below 29.92
180 + 5 × ⌈50 ÷ 50⌉ = 185

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Exact for ISA; real temperature moves the true altitude (see the cold-temperature tool). The US table covers settings down to 26.92 inHg

Worked example: QNH 29.42 inHg, FL185. Source: add-aviation-suite lowest-usable-flight-level scenario: FL185 at 29.42 inHg, with the FAA table cited. It is golden test vector v008, and every build checks the tool still gives its answer within its tolerance.

You enter

Altimeter setting (QNH): 29.42 inHg
Flight level: 185

You get

Lowest usable flight level: 185
Transition altitude (US): 18,000 ft
Rule: 14 CFR 91.121(b), rules as of 2026-09-22. Not legal advice: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-B/subject-group-ECFRe4c59b5f5506932/section-91.121
Altitude of the flight level: 18,033 ft

Review: Not yet independently reviewed by a flight instructor (CFI).

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.1.0, core 0.1.0. See this tool in the verification report.

Changes

2026-09-23, result change: Station pressure now comes from the altimeter-setting relation instead of a sea-level ratio shortcut. The worked example moved from 5,108 to 5,112 ft pressure altitude and 7,932 to 7,937 ft density altitude; at an 8,000 ft field with 28.20 inHg the old answer was 90 ft low. Changelog

Checked against: 7 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

14 CFR Part 91, General Operating and Flight Rules, Federal Aviation Administration, eCFR, current. § 91.121 (altimeter settings; paragraph (b), lowest usable flight level). Read free. Rules as of 2026-09-22.
Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), International Civil Aviation Organization, 3rd edition, Doc 7488/3. Section 2 (defining constants and equations) and Table 1. Paid only.

Terms

FL: flight level: A pressure altitude in hundreds of feet, flown with the altimeter set to 29.92 inHg. FL185 is 18,500 ft of pressure altitude. Source: Aeronautical Information Manual
QNH: altimeter setting (Q code): The pressure setting that makes an altimeter read the airport's elevation on the ground, and altitude above sea level in the air. Source: Aeronautical Information Manual

Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.

Learn the concept: What is pressure altitude?

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