• Airlines worldwide have been on a rollercoaster of schedule and operational challenges since March 2020.
  • Carriers have shrunk and expanded throughout COVID-19, and today's industry looks different than it did pre-pandemic.
  • Aviation data firm OAG shared with Insider the world's biggest airlines based on seats available in 2021 — take a look.

Over the past two and a half years, the airline industry has changed dramatically.

Starting in March 2020, the COVID-19 pandemic forced carriers around the globe to shrink and modify their schedules as travel halted and countries closed their borders. Pilots sought early retirement, employees were furloughed, and aircraft were parked. 

However, the strong post-pandemic demand is flipping the industry on its head. With travel rebounding faster than expected, airlines are making moves to keep up, like buying new planes, flying new routes, and launching aggressive hiring campaigns to get more feet on the ground and in the air to keep the operation running smoothly. 

Globally, airlines carried an estimated 2.2 billion people and flew 22.2 million flights in 2021, reported research company Finances Online, citing data from the International Aviation Transportation Agency (IATA). That compares to 1.8 billion in 2020, which was just 40% of the 4.5 billion carried in 2019, per IATA.

With all the upheaval, which airlines are now the largest in the world? Turns out the US' "Big Four" airlines, including American Airlines, United Airlines, Delta Air Lines, and Southwest Airlines, as well as several Chinese carriers, made the top-20 list with hundreds of millions of seats offered in 2021, according to data from London-based aviation analytics firm Official Airline Guide (OAG).

The organization also provided 2018's data set to Insider in 2019, which revealed American Airlines as the world's largest carrier. Since then, the rankings have changed, with Southwest Airlines becoming the world's largest carrier in terms of seats. Several airlines that made the top 20 list in 2018 offered significantly fewer seats in 2021.

Meanwhile, there are a handful of carriers, like JetBlue Airways and Hainan Airlines, that were not part of the 2018 data set but increased capacity last year to make the list.

Here are the world's 20 biggest airlines in the world by the number of seats made available to customers:

20. Alaska Airlines

Alaska Airlines. Foto: Alaska Airlines

Country: USA

2021 Capacity: 33,645,148 seats

2018 Capacity: 54,574,295 seats

19. Shandong Airlines

Shandong Airlines. Foto: Markus Mainka/Shutterstock

Country: China

2021 Capacity: 33,735,751 seats

2018 Capacity: Not in the 2018 top 20 dataset

18. Hainan Airlines

Hainan Airlines. Foto: TonyV3112/Shutterstock

Country: China

2021 Capacity: 38,230,597 seats

2018 Capacity: Not in the 2018 top 20 dataset

17. Xiamen Airlines

Xiamen Airlines. Foto: PomInOz/Shutterstock

Country: China

2021 Capacity: 39,087,022 seats

2018 Capacity: Not in the 2018 top 20 dataset

16. Sichuan Airlines

Sichuan Airlines. Foto: Rebius/Shutterstock

Country: China

2021 Capacity: 39,294,914 seats

2018 Capacity: Not in the 2018 top 20 dataset

15. Easyjet

EasyJet. Foto: Simone Previdi/Shutterstock

Country: UK

2021 Capacity: 39,490,376 seats

2018 Capacity: 100,082,969 seats

14. Shenzhen Airlines

Shenzhen Airlines. Foto: TonyV3112/Shutterstock

Country: China

2021 Capacity: 39,727,188 seats

2018 Capacity: Not in the 2018 top 20 dataset

13. JetBlue Airways

JetBlue Airways. Foto: Lukas Wunderlich/Shuttestock

Country: USA

2021 Capacity: 40,356,063 seats

2018 Capacity: Not in the 2018 top 20 dataset

12. Spirit Airlines

Spirit Airlines. Foto: Carlos Yudica/Shutterstock

Country: USA

2021 Capacity: 40,622,515 seats

2018 Capacity: Not in the 2018 top 20 dataset

11. Turkish Airlines

Turkish Airlines. Foto: Markus Mainka/Shutterstock.com

Country: Turkey

2021 Capacity: 45,249,215 seats

2018 Capacity: 93,109,025 seats

10. Ryanair

Ryanair. Foto: Jason Cairnduff/Reuters

Country: Ireland

2021 Capacity: 48,696,471 seats

2018 Capacity: 142,540,776 seats

9. Skywest Airlines

SkyWest Airlines. Foto: Heather Dunbar / Shutterstock.com

Country: USA

2021 Capacity: 48,696,471 seats

2018 Capacity: 142,540,776 seats

8. IndiGo

IndiGo. Foto: Nicolas Economou/NurPhoto via Getty Images.

Country: India

2021 Capacity: 72,444,166 seats

2018 Capacity: 74,570,054 seats

7. Air China

Air China. Foto: Dmitry Rukhlenko/Shutterstock

Country: China

2021 Capacity: 75,905,153 seats

2018 Capacity: 90,531,776 seats

6. United Airlines

United Airlines. Foto: EQRoy/Shutterstock

Country: USA

2021 Capacity: 97,220,490 seats

2018 Capacity: 193,981,547 seats

5. China Eastern Airlines

China Eastern Airlines. Foto: VCG/VCG via Getty Images

Country: China

2021 Capacity: 115,625,762 seats

2018 Capacity: 122,917,175 seats

4. China Southern Airlines

China Southern Airlines. Foto: Airbus

Country: China

2021 Capacity: 123,472,958 seats

2018 Capacity: 131,972,745 seats

3. Delta Air Lines

Delta Air Lines. Foto: On The Run Photo/Shutterstock

Country: USA

2021 Capacity: 143,180,744 seats

2018 Capacity: 233,758,798 seats

2. American Airlines

American Airlines. Foto: Nicolas Economou/Getty Image

Country: USA

2021 Capacity: 151,146,489 seats

2018 Capacity: 256,928,663 seats

1. Southwest Airlines

Country: USA

2021 Capacity: 168,852,981 seats

2018 Capacity: 209,526,838 seats

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