In aviation, a flight level is the nominal altitude of an aircraft referenced to a standard pressure datum, as opposed to the real altitude above mean sea level.

To see why flight levels are used, a discussion of the means of measuring altitude is necessary.

Historically, altitude has most easily been measured using an altimeter, which is essentially a calibrated barometer - it measures air pressure, which decreases with increasing altitude. To display altitude above sea level the pilot must recalibrate the altimeter according to the local air pressure from time to time, to take into account natural variation of pressure over time and in different regions. If this isn't done, different aircraft may be flying at different heights even though their altimeters show the same altitude. More critically, different aircraft may be flying at the same height even though their altimeters show different heights. Clearly this is a safety issue.

Flight levels solve this problem by defining altitudes based on a standard pressure of 1013.2 mb (29.92 inHg used in U.S. and Canada). All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure. Flight levels are then assigned a number which is the apparent altitude ("pressure altitude") to the nearest thousand feet, divided by one hundred. Therefore an apparent altitude of 12,000 feet is referred to as Flight Level 120 (except in the United States and Canada -- see note below). Note that aircraft may be at some other actual height than 12,000 feet, but since they all agree on a standard pressure, no collision risk arises.

Flight levels are not used close to the ground, for perhaps obvious reasons - obstacles are fixed to the ground and so their absolute height needs to be known. A vertical region extending from a defined transition altitude to the lowest available flight level is known as the transition layer - pilots will use altitude based on the local pressure below this level, and flight levels above. (In the U.S. and Canada the transition altitude is 18,000 ft msl, in other parts of the world it varies)

Flights above transition altitude being directed by air traffic control will be given flight levels to fly.

Quadrantal rule

(This does NOT apply in the United States or Canada) Flight levels are 500 feet apart, but to further ensure the separation of aircraft, aircraft travelling in different directions in level flight (i.e. not climbing or descending) below FL 245 (24,500 feet) are required to adopt flight levels according to the quadrantal rule, as follows:

• Track 000 - 089° - odd thousands of feet (FL 70, 90, 110 etc)
• Track 090 - 179° - odd thousands + 500 (FL 75, 95, 115 etc)
• Track 180 - 269° - even thousands of feet (FL 80, 100, 120 etc)
• Track 270 - 359° - even thousands + 500 (FL 85, 105, 125 etc)

Semicircular rule

Above FL245 (In the U.S. and Canada this applies above 3000 AGL (above ground level)), the semicircular rule (also known as the hemispheric rule) applies, which increases separation to allow for inaccuracies that creep into the altimeter at higher altitudes:

• Track 000 - 179° - odd thousands (FL 250, 270, etc.)
• Track 180 - 359° - even thousands (FL 260, 280, etc.)

At FL 290 and above, 4000 ft. intervals are used to separate same-direction aircraft (instead of 2000 ft. intervals below FL 290), and only odd Flight Levels are assigned, depending on the direction of flight:

• Track 000 - 179° - odd flight levels (FL 290, 330, 370, etc.)
• Track 180 - 359° - odd flight levels (FL 310, 350, 390, etc.)

Next time you fly, listen to the captain say what flight level you're at - it will obey this rule according to what direction you are flying in. On the return trip, notice the altitude difference (e.g., FL 290 or FL 330 eastbound, and then perhaps FL 310 or FL 350 westbound).

In the U.S. and Canada, note that the foregoing information applies to flights under instrument flight rules (IFR). Different altitudes will apply for aircraft flying under visual flight rules (VFR) above 3000 AGL.

Alternative explanation of first part

An estimate of the real altitude is based on air pressure at the aircraft and the reported local air pressure at sea level (if there is no sea, this is a virtual value by adjusting the value at the ground for its elevation). However, to avoid collisions between two planes, their real altitudes are not important, but only the difference between them. This difference solely depends on the air pressure at both planes, and does not require knowledge of the local air pressure on the ground.

Therefore air traffic control assigns a plane a "flight level" (a nominal altitude), based on an altitude scale with a one-to-one correspondence with air pressure at the plane. Thus basically the plane is assigned an air pressure. The flight level corresponds to the real altitude that would be concluded from the air pressure, if the air pressure at sea level were 1013.2 mbar (29.92 inHg or 101.32 kPa).