What Are Flight Levels? Why Pilots Say FL350 Instead of 35,000 ft

The short answer

A flight level is the altitude an aircraft is flying at, measured against a fixed reference pressure (1013.2 hPa) rather than the local atmospheric pressure. FL350 means approximately 35,000 feet above that reference, which translates to about 10,668 meters above mean sea level under standard atmospheric conditions. The “FL” prefix is just a label that tells you the value uses the standard pressure reference, not the local one.

Pilots say “FL350” instead of “35,000 feet” because the two are not always the same. The actual height above the ground varies with the weather, while the flight level stays constant regardless. That predictability is what makes safe vertical separation possible across thousands of aircraft in the same airspace.

This guide explains how flight levels work, why aviation kept feet as its unit, and gives the exact meter equivalents for the flight levels you will see on a flight tracker.

Why flight levels exist (the QNH problem)

At low altitude, pilots set their altimeter to the local barometric pressure (called QNH). With that setting, the altimeter shows the aircraft’s true altitude above mean sea level. As long as everyone in the same region uses the same local QNH, separation works.

The problem appears at high altitude. A pressure system can shift the actual pressure at sea level by 30 hPa or more between locations only a few hundred miles apart. If an aircraft over Boston is using Boston QNH and another aircraft over New York is using New York QNH, their altimeters read slightly different altitudes for the same true height. Over short distances and low altitudes, the error is small enough to ignore. Above 18,000 feet, where aircraft routinely overfly multiple weather systems, the error becomes a safety hazard.

The fix is the standard pressure setting. Above the transition altitude (18,000 ft in the United States, varies in other regions), every aircraft sets its altimeter to 1013.2 hPa, also written 29.92 inHg. The reading on that setting is the flight level. It is not the true altitude. It is the altitude the aircraft would be at if the atmosphere were exactly standard. Two aircraft at the same flight level might be at slightly different true altitudes, but they are always 1,000 ft apart from another flight level above or below, regardless of weather. That predictability is what makes air traffic control workable.

How flight levels are written and read

The flight level number is the altitude in hundreds of feet. FL350 is 35,000 ft of indicated altitude. FL100 is 10,000 ft. FL430 is 43,000 ft.

Pilots and controllers say each digit out loud: “flight level three five zero” for FL350. The American conversational shorthand “thirty-five thousand” works in casual contexts but the digit-by-digit form is mandated on the radio.

Common flight levels and their approximate metric equivalents:

The “approximate” qualifier matters because the meter equivalent assumes standard atmosphere. The true altitude in meters varies a little day by day with the actual atmosphere. For technical aviation calculations, use the exact conversion calculator or the precomputed pages linked in the table.

The 1,000 ft step and RVSM

Vertical separation between aircraft on different cruise altitudes is 1,000 ft (about 305 m). That is why flight levels increment in 10 (which is 1,000 ft, since the FL number is in hundreds). Pilots never cruise at FL355, only FL350 and FL360.

Between FL290 (29,000 ft) and FL410 (41,000 ft), the airspace uses Reduced Vertical Separation Minimum, called RVSM. Before RVSM was introduced in the 1990s, the standard above FL290 was 2,000 ft of vertical separation because altimeters at high altitude were less precise. Improvements in altimetry let ICAO halve that separation, which roughly doubled the usable airspace in the busiest cruise band. Above FL410, the 2,000 ft separation is still in force (FL410, FL430, FL450, FL470, etc.).

Aircraft that fly in RVSM airspace need certified altimetry. A small private aircraft without the certification cannot legally cruise between FL290 and FL410, even if it has the performance to climb that high.

Cruise altitudes by aircraft type

Different aircraft prefer different flight levels based on engine performance, weight, and route length.

Narrow-body airliners (Boeing 737, Airbus A320 family): typically cruise at FL330 to FL370. The 737-800 on a transcontinental US flight is most often at FL360 or FL370. Empty repositioning flights climb higher.

Long-haul widebodies (Boeing 777, 787, Airbus A330, A350): cruise band is FL370 to FL410. The Boeing 787 is famous for cruising high (often FL390 to FL410) because its composite airframe handles the lower-pressure thin air efficiently.

Older widebodies (Boeing 747, 767, Airbus A340): usually cruise FL330 to FL390. The 747’s four-engine design and high weight make it less efficient at the highest cruise altitudes than newer twins.

Business jets (Gulfstream G650, G700, Bombardier Global 7500, Dassault Falcon 8X): cruise FL430 to FL510. These aircraft were designed for the 45,000 ft band, which is above the weather and above most commercial traffic. The Gulfstream G700 service ceiling is 51,000 ft, which is FL510.

Regional jets (Embraer E175, CRJ-700): cruise FL280 to FL370. Lower than the big iron because shorter routes do not justify the climb time to the higher band.

Supersonic (historical, Concorde): cruised at FL550 to FL600, far above any subsonic traffic. Concorde service ceiling was about 60,000 ft.

For more on the physics of why cruise altitude matters, see our companion guide Why aircraft cruise at 36,000 feet.

Why aviation uses feet and not meters

The simple answer is path dependence. The Wright brothers worked in feet. The American and British aviation industry of the 1910s through 1930s established practices that ICAO codified after World War II. Once the system was in place, switching costs were enormous. Altimeters, charts, radio phraseology, training manuals, certification standards, and air traffic control software all reference feet.

There are a few countries that fly altitudes in meters domestically: Russia, China, Mongolia, North Korea, and a handful of central Asian states. International flights into and out of those countries convert to feet at the border. The Russian metric system uses 300 m steps roughly equivalent to 1,000 ft (300 m equals 984 ft, close enough), so the operational picture is similar even though the unit is different.

The case for switching is weak. Feet work, the system is safe, and a transition would cost billions of dollars of training and equipment changes for no functional benefit. The ICAO standard remains feet for altitude, knots for speed, and nautical miles for distance, with meters and kilometers used only where they have always been (visibility, runway dimensions in many countries).

What altitude really means at flight level

The flight level is what the altimeter shows when set to standard pressure. It is not the actual height above the ground, and it is not exactly the height above mean sea level. The difference matters in two situations.

Terrain and obstacle clearance: pilots in mountainous regions track minimum sector altitudes that account for the actual terrain plus a safety margin. Flight level alone is not enough.

Vertical position relative to other aircraft: this is the case where flight level shines. Because every aircraft uses the same pressure reference, two aircraft at FL350 and FL360 are 1,000 ft apart in true altitude as well as 1,000 ft apart in indicated altitude. That is the only number air traffic control needs to know to keep them separated.

Once below the transition altitude on descent (typically 18,000 ft in the United States, 5,000 to 10,000 ft in much of Europe depending on the airport), pilots switch from the standard pressure setting to local QNH. From that point, the altimeter reads true altitude above mean sea level, which is what matters for approach and landing.

Quick reference

The shortest version of this guide for a screenshot:

Flight level = altitude in hundreds of feet, measured against a fixed pressure reference (1013.2 hPa).

Above 18,000 ft in the US, all aircraft fly on flight levels for consistent vertical separation.

FL350 ≈ 10,668 m. Other common values are in the table above.

For an exact conversion of any flight level to meters (or vice versa), use the meters-to-feet calculator or the precomputed conversion pages for the values you care about.

Sources and further reading:

• Wikipedia: Flight level
• SKYbrary: Altitude, Flight Level and Height
• Flightradar24 blog: Flight levels explained
• IVAO: Altitude, height and flight level
• Our guide: Why is one foot exactly 0.3048 meters?