Definition
An altitude expressed in hundreds of feet, indicated on an altimeter set to the standard pressure of 29.92 inches of mercury (1013.2 hPa). For example, FL250 represents an indicated altitude of 25,000 feet on an altimeter set to 29.92. Flight levels are used at and above 18,000 feet MSL in the United States, where all aircraft use the standard altimeter setting so they share a common reference for vertical separation.
Plain English
A way of stating altitude in the upper airspace where every aircraft sets its altimeter to the same standard pressure. Once everyone is using the same setting, altitudes are written as flight levels — FL180 means 18,000 feet, FL350 means 35,000 feet — and pilots can be sure they are vertically separated from other traffic.
Context Anchor
You will see FL in air traffic control clearances, instrument flying, and airspace descriptions, especially at higher altitudes where pilots use the standard pressure setting.
Derivation
The word 'level' comes from the Latin 'libella,' meaning a small balance or horizontal line. A 'flight level' is simply a horizontal layer in the sky — but one referenced to a fixed standard pressure rather than to local conditions, so every aircraft in the high-altitude system is measuring against the same yardstick.
Why Pilots Care
Provides a common pressure reference that maintains safe vertical separation between aircraft in high-altitude airspace.
Analogy
It is like everyone measuring from the same zero mark on the same ruler. Once everyone uses the same starting reference, the numbers line up.
Intuition Check
Do not read “level” here as “flying straight and level,” and do not treat a flight level as height above the ground. In FL, “level” means an altitude reference based on the standard pressure setting.
Example Sentence 1
Example Sentence 2
Class A airspace begins at FL180 and all operations are conducted on flight levels.