§ 91.121 Altimeter settings
14 CFR Part 91 — Subpart B—Flight Rules
Official text — 14 CFR § 91.121
REGULATION — LEGALLY BINDING(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—
(1) Below 18,000 feet MSL, to—
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
(2) At or above 18,000 feet MSL, to 29.92″ Hg.
(b) The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in the following table:
(c) To convert minimum altitude prescribed under §§ 91.119 and 91.177 to the minimum flight level, the pilot shall take the flight level equivalent of the minimum altitude in feet and add the appropriate number of feet specified below, according to the current reported altimeter setting:
Reproduced from the eCFR. Text as amended through 2026-07-31. Verify against the official source before relying on it operationally.
SkeyeMentor explanation
Educational — not an FAA sourceIn plain English
Altimeter setting rules: which setting to use below the lowest usable flight level, what to do when no station is within 100 nautical miles, and the requirement above that altitude.
Why it matters
Everyone in the same airspace using the same reference is what makes vertical separation work.
Quick example
You are climbing through 17,000 feet. State the altimeter setting you must use.
Written and reviewed by the SkeyeMentor editorial team. This explanation is a study aid, not legal advice, and it is not part of the official text above. Verify current requirements through the official source before relying on them operationally.
Is this a regulation?
Yes — this is regulatory text and is legally binding.
Text published in the Code of Federal Regulations. Regulations are legally binding on pilots and operators.
Yes — this is the text of a Federal Aviation Regulation. It is legally binding. The explanation below it is SkeyeMentor educational content, not part of the regulation.