Flight Level

A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet. For example, flight level (FL) 250 represents a barometric altimeter indication of 25,000 feet; FL 255, an indication of 25,500 feet.

Source: FAA Pilot/Controller Glossary (definition from the 2023 edition; current: Change 1 of 7 August 2025); 14 CFR 1.1, General definitions.

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Glossary last updated 2024-07-25.

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