Over the past century, the airline industry has changed the way humans travel. It has effectively made the world a smaller place making it possible for billions of people everywhere to explore the world.

According to the International Air Transport Association (IATA), consumers spent an estimated $871 billion or roughly 1% of the global GDP on air travel.

The trade group, whose membership is made up of 290 airlines, expects the total number of airline passengers around the world to reach 8.2 billion people by 2037 that's double today's passenger count.

As a result of the industry's growth, many of the world's airlines have become household names. Through recent mergers, American, Delta, United, and Southwest have grown to become colossal mega-carriers.

Read more : The 11 longest flights in the world, ranked .

This got us at Business Insider thinking about how they stack up against the world's largest airlines.

Fortunately, good folks at OAG were able to provide us the data sets that answered our question.

The London-based aviation data and intelligence firm ranked the world's airlines based on the total number of seats each carrier made available to customers during 2018. OAG also included the total number of aircraft in each airline fleet.

Some airlines on the list are able to offer substantially more seats while operating fewer aircraft.

According to OAG senior analyst John Grant, the reason for this variation is due to the differences in the planes that comprise a carrier's fleet and the seat configurations of each aircraft. As a result, airlines that operate larger aircraft and aircraft with higher seat density will be able to make more seats available to customers.

Here's a closer look at the 20 biggest airlines in the world:

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