Flight level (FL)

Definitions

• A measure of altitude (in hundreds of feet) used by aircraft flying above 18,000 feet with the altimeter set at 29.92 "Hg.
• Means 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 250 represents a barometric altimeter indication of 25,000 feet; flight level 255, an indication of 25,500 feet.
• 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. (See ICAO term FLIGHT LEVEL.)

References

• 14 C.F.R. Part 1, Federal Aviation Administration, 2026
• Instrument Procedures Handbook, Federal Aviation Administration, 2017
• Pilot Controller Glossary, Federal Aviation Administration, 2023