Those alphanumeric codes on your flight confirmation are far from random, and they do much more than just help you confirm the correct gate when you arrive at the airport. From indicating the direction you’re flying to signaling an important route — or even avoiding numbers associated with historical events — airlines use a complex logic to assign flight numbers. We'll decode what a flight number is and what it means so you can feel like an insider the next time you're at the airport.

Who assigns flight numbers?

Flight numbers are assigned by both the International Air Transport Association, or IATA (a trade association of the world’s airlines), and the airline itself.

You’ll notice that every flight number starts with two-letter code. These letters represent the airline that is operating the flight and are assigned by the IATA. In some instances, these two letters are easy to decipher. For example, DL indicates Delta, AA indicates American Airlines, and UA indicates United. But then there are some confusing ones, such as NK to represent Spirit Airlines and WN for Southwest. What's going on? Well, when these airlines started, the "more obvious" letters, like SW for Southwest, was already taken. (Check out airline codes by searching the IATA database.)

That’s easy enough, so what does the numerical part of the flight number mean? The airlines actually assign that value to their own flights. However, there isn’t a straightforward formula that they all follow and there are, of course, exceptions to the rules.

What do the numerals in flight numbers mean?

Flight numbers can only be up to 4 digits long. That means airlines can choose any number from 1 to 9999. The lower the flight number, the more important the route is to that particular airline (such as flight number 1). The smaller flight numbers could also indicate one of the airline’s original routes, a popular route, or one of its most-flown and highest-revenue routes.

Typically, flights flying north or east will be given an even number (like 428), while those going west or south will be given an odd number (like 429). Although this logic is generally followed by the industry, it's one of those “rules” that is sometimes broken depending on an airline’s internal process and its scheduling system.

Another noteworthy fact is that an outbound flight number is generally one digit below the return flight. For example, Southwest’s WN 1 from DAL to HOU will increase to WN 2 when it returns HOU to DAL. And you guessed it — given that the flight numbers are so low, those are two of Southwest’s most-beloved routes.

Popular flight numbers and their meaning

Can you guess what popular annual event United Airline’s UA 500 from Indianapolis to San Francisco pays homage to? The Indy 500, of course!

Did you know that 7s are usually associated with “lucky” flight numbers, particularly those heading to Las Vegas? Here are some examples:

• JetBlue #777: Boston to Las Vegas

• Spirit Airlines #777: Ft Lauderdale to Vegas

• Frontier #777: Denver to Vegas

What flight numbers aren't used?

There are many flight numbers that are typically avoided for a variety of reasons. First, there are those that match aircraft models, such as 737 and 757, which can cause confusion. Due to superstition, airlines avoid using flight numbers 13 and 666. Similarly, they retire flight numbers that represent a route that encountered a headline-producing incident. For example, American Airlines used flight number AA11 for its daily westward route from Boston to Los Angeles until the attacks of September 11, 2001. Afterward, the flight number was changed. The same is true of Malaysia Airlines flight 370, which disappeared en route from Kuala Lumpur to Beijing.

Difference between a confirmation number and flight number

A flight number is not the same as the confirmation number you receive when booking a reservation. A confirmation number is typically an alphanumeric code used to identify your reservation and expedite the check-in process. You might use this code to check in online or at the airport, or if you call into the airline’s customer service center, you’ll likely provide the code so they can quickly pull up your reservation.

What happens to a flight number during a layover?

Although your confirmation number stays the same for your entire itinerary, you'll notice a different flight number assigned to each route you fly. And if you happen to be switching carriers on your layover, you'll notice that the two-letter code at the start changes in addition to the numbers that come after it.

Final thoughts: What do flight numbers mean?

Although a flight number meaning may seem like a mystery to the average eye, you can see that it's not so mysterious after all. First two letters: airline. The following numbers: route significance, direction, or flight order. Now that you know how to decode what a flight number means, practice figuring out the direction of the flights around you the next time you're at the airport.