The Boeing 737 is the workhorse of US skies. Eight passenger airlines fly nearly 2,500 of them domestically, and yet the identical airframe can hold anywhere from 124 to 190 seats depending on whose livery it wears. That spread comes down to business model: legacy carriers hedge with first class cabins, hybrids like Alaska give everyone a bit more room, and budget operators cram in as many rows as physics allows.

The big three diverge in ways worth knowing before you book. On the 737-800 and MAX 8, American, Delta, and United all install 16 first class seats, but the economy sections tell a different story. Delta fits 144 economy seats, United 150, and American 156, with American squeezing in extra rows behind the overwing exits. On the stretched 737-900ER, Delta manages 160 economy seats against United's 159, while United runs the widest 737 fleet of all, from the compact 737-700 (126 seats) up to the MAX 9.

Alaska Airlines takes the roomier route. Its 737 MAX 8s originally seated just 159 passengers, the least dense configuration in the country, thanks to extra first class pitch and an additional lavatory. A retrofit is trimming that back slightly, but its 737-900ERs and MAX 9s still carry 16 first class seats where Delta and United fit 20, trading seats for legroom. Southwest, the world's largest 737 operator with over 800 aircraft, stays all-economy but finished adding extra legroom rows across its fleet in January 2026.

At the budget end, Avelo fits 189 seats into its 737-800s, Sun County runs 186-189 on the same type, and Allegiant's MAX 200s pack in 190. The future gets more interesting: American and Delta have confirmed 24 first class seats on the upcoming MAX 10, and Alaska stunned observers in September 2026 by announcing a three-class MAX 10 subfleet with 12 lie-flat 'Aurora Suites', the first lie-flat 737 ever in the US. That puts it in direct competition with the premium Airbus A321s flown by JetBlue, American, Delta, and United on transcontinental routes.

For travellers, this matters more than it might seem. Two flights on the 'same' aircraft type can differ by 20-plus seats, meaning noticeable differences in legroom, cabin feel, and the odds of an upgrade. If space matters, Alaska's configurations are currently the most generous of the major carriers, while Allegiant, Avelo, and Sun Country offer the tightest pitch. And with the MAX 10 arriving from 2027, premium domestic flying is about to change shape for the first time in a decade, so it may pay to watch how American, Delta, United, and Alaska spec their new flagships.