93%+ Full: Southwest's 11 Busiest Routes With Seats Nearly Sold Out [Map & List]

Published Jul 31, 2026, 4:00 PM EDT James lives and breathes route development. Educated in Air Transport Management at Loughborough University and Cranfield University, James has a PhD in airline strategy. James was Market Opportunity Analyst at London Luton Airport and Chief Analyst at anna.aero. He taught airline strategy and economics to undergraduate and postgraduate students for five years and has worked closely with multiple carriers on route and market intelligence projects. He is based near London, UK. In the 12 months to April 2026, Southwest Airlines transported 170 million passengers. The US Department of Transportation (DOT) shows that the hybrid operator's traffic fell by two million passengers compared to the prior 12 months. Nonetheless, around 465,000 people traveled daily. Between May 2025 and April 2026, Southwest's overall load factor was just 77.4%, down from 79.1% in the previous 12 months. Additional traffic did not keep pace with the extra capacity. The DOT indicates that the international market outperformed the vast domestic market, at 78.9% and 77.4%, respectively. Both results were lower than for all US carriers combined. Southwest's 11 Routes With Loads Of 93% Or More To help avoid anomalies, only airport pairs with at least 3,000 round-trip passengers are included. The following 11 routes collectively had 645,000 round-trip passengers. Only one in every 263 of Southwest's passengers flew them, whether on a local basis or to connect elsewhere. Some of the markets, such as St. Louis Lambert International Airport (STL) to Salt Lake City and Phoenix Sky Harbor International Airport (PHX) to Ft. Lauderdale, used to have many more flights (and thus considerably more passengers) than they do nowadays. The loads vary from a high of 95.3% — which was Southwest's best-performing link in this sense out of its entire considerable network — to 92.6%. As always, load factors are just one performance measure, and should not be considered in isolation. It is always about how they were achieved, including the fares needed to achieve the results. None of them are from Denver International Airport (DEN) or Harry Reid International Airport (LAS), serving Las Vegas, which are Southwest's two busiest airports for flights. STL, which is the carrier's 11th-busiest facility for departures and has been important for connecting flights for years, featured the most. Load Factor (May 2025 To April 2026)* Route Round-Trip Passengers** 95.3% STL to Seattle-Tacoma International Airport (SEA) 30,076 94.3% Los Angeles to Lihue 7,593 94.0% STL to Salt Lake City 3,424 93.5% PHX to Ft. Lauderdale 5,914 93.1% Houston Hobby to San Juan 29,196 93.0% Las Vegas to Honolulu 287,889 93.0% Baltimore to SEA 36,794 92.9% STL to Portland (Oregon) 10,147 92.9% Phoenix to Honolulu 118,345 92.7% Boston to Austin 78,243 92.6% Chicago Midway to Bozeman 39,986 * According to the US DOT ** According to the US DOT Loading map… Drag to explore Southwest From STL To SEA Credit: Shutterstock STL has been part of the huge airline's network for 41 years. Cirium Diio data shows that the route from STL to SEA, which covers 1,485 nautical miles (2,750 km) each way, has been flown since 2010. At the time, it was Southwest's longest nonstop operation from STL, but it is now fourth. In the 12 months to April 2026, Southwest only flew STL-SEA between June and September last year, when the 175-seat Boeing 737-800 was the most-deployed variant. The short operating period helps to explain the relatively low traffic figure (30,076 passengers) and not having a lot of capacity and only flying in the summer explains the very high load factor (95.3%). The route is served in those four months in 2026. In the past, a year-round operation existed. Let's focus on June to September 2025. In those four months, Southwest's load factor varied from 93.4% (September) to 96.8% (August). In contrast, the airline's overall load from STL in that period was just 77.1%, while it was 87.9% for SEA. Alaska Airlines also operates SEA-STL nonstop. In the same four months, that carrier filled 86.7% of seats. Of course, it is not a fully like-for-like comparison. STL-SEA is a decent-sized local market. Between June and September 2025, 111,898 round-trip passengers only flew between the two airports. They did not go elsewhere. Alaska was the largest operator (66,790 local passengers), followed by Southwest (28,284). Importantly, Southwest's figure includes those who flew nonstop and connected on the way. 9,911 passengers did transit en route, especially in DEN and MDW. More Than Six In Ten Passengers Were Local Credit: Shutterstock The previous section showed that 28,284 passengers flew Southwest between STL and SEA from June to September 2025. As 9,911 connected on the way, 18,373 flew nonstop. In other words, the local market accounted for 61.1% of the 30,076 passengers who flew Southwest's nonstop services on this route. The airports will surely have Southwest lounges in due course. The remaining 38.9% of the market (11,703 passengers) transited to another Southwest flight in STL or SEA or at both airports. Given STL's importance as a connecting airport for the carrier, most people did so there. However, Nashville International Airport (BNA) is increasingly superseding STL for this role in this rough geographic area. 66.8% of Southwest's BNA-SEA passengers flew elsewhere.