The world’s largest airline is…. well, it depends on one’s criteria. This article will consider airlines by available seat miles (ASMs). ASMs are a measure of an airline’s capacity. It is calculated by multiplying the number of available seats on each aircraft by the distance it flies. This is a common measure for airlines, although it comes with its own set of issues. For example, flag carriers with more premium seating options are penalized.
Airlines can be gauged by other measures like market capitalization, revenue, number of flights, fleet size, the number of flights, and other criteria. None of these measures is perfect, with each posing its own set of problems. To highlight one issue with fleet size, this would rank JetBlue Airways (283 aircraft) as a larger airline than Emirates (268 aircraft). This would be irrational, as JetBlue only operates narrowbody fleets of A220s, A320s, and A321s, while 
Emirates operates the world’s largest fleet of Boeing 777 and A380 giants.
Usefulness And Limitations Of ASMs
As stated, ASMs are calculated by multiplying the number of available seats by the flight distance, without regard to class of service or whether the flight is domestic or international. So a 100-seat aircraft flying a 1,000-mile route generates 100,000 available seat miles. The metric is considered useful for determining revenue and cost per available seat mile, and for analyzing and comparing airline capacity.
One limitation of ASMs is that they don’t measure how many passengers are carried or the load factor. It doesn’t factor in how many seats on the aircraft are actually sold. So, a mostly empty Boeing 777 flying internationally around the world, configured in economy class seats, will generate many ASMs, while not actually carrying many passengers and potentially losing a lot of money. Internationally, the typical load factor is around 80%, although North American airlines typically average around 89%.
Airlines flying shorter distances may also be penalized by ASM, as the metric penalizes them for spending more time at airports changing between flights. It’s worth keeping in mind that American Airlines wins the accolade as the largest airline by some measures. However, American also flies shorter routes than United Airlines, giving the Chicago-based airline an edge in ASMs.
Largest Airline By ASMs
According to OAG, the airline with the most ASMs in 2024 was United. OAG listed United Airlines as flying a total of 315 billion ASMs (expressed by OAG as 507 billion ASKs or Available Seat Kilometers). The publication noted this represented a seven percent increase compared with the previous year. This placed United ahead of its main rivals, American Airlines and Delta Air Lines, which came second and third, respectively.
Separately, United Airlines also serves the most (373) destinations of any airline in the world. But that doesn’t mean United is first in the number of routes. That is Europe’s ultra-low-cost carrier, Ryanair, which operated 5,442 routes in 2024. United Airlines did come second by this measure, although with 2,653 routes it was less than half that of Ryanair.
|
United Airlines key metrics (OAG) |
|
|---|---|
|
ASMs |
315 billion |
|
Number of departing seats |
117,209,824 (summer 2025) |
|
Average domestic flight length |
726 |
|
Number of routes |
2,653 |
|
Number of destinations |
373 |
Of the three big US carriers, 2022 data from Cirium shows United has the highest on-average fares. United’s average fare was around $330, while Delta stood at around $295 and American at $282. This is partly a reflection of United flying longer routes, generating more ASMs. United also has the world’s second-largest widebody fleet after Emirates. United is one of the primary US carriers connecting the United States to the rest of the world.
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American Airlines Comes First In Departing Seats
As stated, American Airlines typically operates shorter flights than United. American had the most seats of all airlines —some 275.5 million —and the most flights. In all, American operated some 2.2 million flights in 2024. When it comes to the number of flights, United ranks fourth in the world behind American, Delta, and Southwest. After these four US giants, Ryanair took the fifth spot with 209.6 million seats on 1.1 million flights.
When measuring the number of one-way departing seats on the schedules, American also carried more seats than any other airline in the United States. Using this metric, in the summer of 2025, American had 152 million departing seats, followed by Delta with 137 million, Southwest with 133 million, and United with 117 million. After that, there is a significant drop off with Alaska Airlines having 34 million departing seats.
|
Metric for the Largest Airline |
Largest Airline (per OAG, CompaniesMarketCap) |
|---|---|
|
ASMs |
United Airlines (315 billion) |
|
Number of flights |
American Airlines (2.2 million) |
|
Number of departing seats |
American Airlines (152 million) |
|
Market capitalization |
Delta Air Lines ($40 billion) |
|
Most routes |
Ryanair (5,442) |
American’s edge in these categories becomes clear when considering the airlines’ average domestic flight distance. In July 2024, JetBlue led with domestic flights averaging 1,026 miles. It was followed by Sun Country (972 miles), Alaska (934 miles), Spirit (842 miles), Hawaiian (826 miles), Breeze (803 miles), Allegiant (763 miles), Frontier (759 miles), and then United as the first Big Three carrier at 729 miles. American’s average domestic flight was the shortest of all large US carriers at just 628 miles.
United Edges American With The Largest Mainline Fleet
The three main carriers, Delta, United, and American, are so big and complicated that it is difficult to calculate their true fleet size. This is particularly true of their regional brands, Delta Connection, United Express, and American Eagle, whose aircraft are often operated and, in some cases, even owned by contracting airlines.
Restricting the criteria to just the mainline airlines, data from Planespotters.net shows United has the largest fleet. In total, United has 1,055 mainline aircraft, marginally more than American with 1,001 and Delta with 990. United’s narrowbody fleet is composed of 200 Airbus A320 family airliners, 565 Boeing 737 narrowbody aircraft, and 61 remaining Boeing 757 ‘pencil’ airplanes.
The United widebody fleet is the world’s second-largest, with 53 remaining Boeing 767s, 96 Boeing 777s, and 80 Boeing 787s. This gives United an all-Boeing widebody fleet. This may yet change as United has an outstanding order for continuously pushed back 45 Airbus A350-900s intended to eventually replace its fleet of Boeing 777-200ERs.
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ASMs Are Not Directly Tied To Profitability
Available Seat Miles provide a standardized capacity measurement and a uniform way to quantify an airline and compare it with another. It is considered invaluable for comparing the scale and output of airlines worldwide. It allows airlines to work out Cost Per Available Seat Mile (CASM) and revenue per Available Seat Mile (RASM).
The metric is more meaningful when paired with Load Factor as Revenue Passenger Miles/ASMs. It doesn’t reflect the airline’s revenue or profitability, although United is currently ranked third most valuable behind Delta and Ryanair. The market value of many major airlines, like Emirates and Qatar Airways, is not known as their shares are not publicly traded. According to the most recent complete financial results, United is one of the most profitable airlines in the world with a net profit of $3.15 billion. This placed it behind Emirates and Delta, but ahead of IAG and Turkish Airlines.
In other words, ASMs should be seen as an essential building block to measure the scope and performance indicators of airlines in the industry. But it should never be used as a standalone metric for meaningful financial analysis, or even to rank airlines’ true size. Still, as fate has it, United Airlines does perform well in terms of recent market value and net profit.
Other Airlines Getting Bigger
By most metrics, the Three Big US airlines, American, Delta, and United, dominate the list of the world’s largest airlines. When it comes to ASMs, United’s large fleet and longer flight distances help it to boast the largest ASMs in the world. However, these US-based airlines are also mature airlines operating in a mature market. According to OAG, HK Express was the “Fastest Growing Airline of 2024,” having boosted flights by 46% in that year.
There are plenty of other fast-growing airlines, particularly low-cost airlines around the world. One example is flynas, which grew by 63% between 2019 and 2024. Perhaps the most remarkable airline to watch is India’s IndiGo, which is set to grow its fleet to sizes that could rival those of the largest US airlines. The airline is ranked as one of the most valuable in the world. While IndiGo has around 400 aircraft currently in service, it has over 900 on order. This includes orders for around 870 Airbus A320 aircraft.
Another factor to consider is that European airlines are consolidating into groups. The leading groups are IAG (including British Airways, Iberia, and others), Air France-KLM, and the Lufthansa Group (including Lufthansa, SWISS, Eurowings, Austrian Airlines, and others). The ranking of airlines by number of ASMs also depends on how these companies are treated. As brands are preserved, ASMs are typically provided for individual airlines (such as British Airways), although a case could be made to compare the ASMs of airline groups to airline groups.
