US legacy carriers maintain dominance as biggest airlines by seat numbers for summer 2025

"American accounts for 21% of the capacity in the top ten US airlines and is the largest airline in the world based on the number of passengers flown. Its last major merger was in 2013 when it merged with US Airways. Despite the large number of passengers carried, American is not valued highly compared with its counterparts, and, according to Companies Market Cap, it is only the 19th most valuable airline in the world, with a market cap of around $7.6 billion."

Delta Air Lines follows closely behind with an estimated 137 million seats this summer. The carrier operates a diverse fleet totaling around 990 mainline aircraft including models like Boeing 717s, Boeing 737s, Boeing 757s, and Airbus A320s. Delta has been shifting away from Boeing widebodies toward Airbus A330 and A350 jets after retiring its older long-haul models such as the Boeing 747s and Boeing 777s. Delta is considered by some measures as the world’s most valuable airline at approximately $35 billion.

"Delta is ranked as the most valuable airline in the world at around $35 billion, placing it ahead of Ryanair ($33.8 billion), United Airlines ($29 billion), and IndiGo ($25.9 billion). Its last major merger took place in the late 2000s when it acquired Northwest Airlines, building on its acquisition of substantially-ailing Pan Am's transatlantic routes in the early 1990s."

Southwest Airlines remains third-largest overall with about 133 million seats scheduled for this summer season. It maintains an all-Boeing-737 fleet—the largest such fleet worldwide—and focuses mainly on domestic routes plus select leisure destinations outside mainland USA.

Southwest currently flies more than 800 aircraft: "Southwest's over 800-strong portfolio of aircraft is the largest Boeing 737 fleet in the world." The breakdown includes hundreds each of various generations within that model family; new MAX variants are still pending certification before entering service.

United Airlines comes fourth among US carriers by seat count with approximately 117 million scheduled for this period. United operates one of aviation’s largest fleets—1,041 mainline aircraft—including both narrowbody (Airbus A320/Boeing) and widebody types (Boeing only). United’s international network spans numerous global destinations following its merger with Continental Airlines in 2010.

Among smaller airlines not making OAG’s top ten are names like Avelo Airlines, Breeze Airways, Sun Country Airlines—and many regional operators flying under larger brands’ banners (e.g., Delta Connection or United Express). The combined market share held by airlines outside this top group stands at roughly nine percent.

Looking ahead, several US carriers plan to introduce new technologies such as JetZero Blended Wing Body airliners or Boom Overture supersonic jets into their operations while moving away from ultra-large models like those built by Airbus or older-generation Boeings.