Which airline has the biggest or oldest fleet? Here's how Delta, United, American and Southwest compare.

With 1,023 planes, United Airlines has the largest fleet in the world, according to data from Ch-aviation.

More than half of those are Boeing 737s, which is unsurprising given that United is the planemaker's biggest customer.

Unlike its legacy competitors, United shows a clear preference for Boeing planes. Airbus jets make up less than a fifth of its fleet.

United's oldest plane that's still flying is a 34-year-old Boeing 767. Registered as N641UA, it was delivered in April 1991.

While this is nearly two years younger than the oldest plane operated by a US airline, United has the oldest fleet with an average age of 15.8 years.

It also has the highest proportion of wide-body planes at 22% of its fleet, showing how its extensive network offers numerous long-haul flights.

Delta Air Lines has the oldest plane still flying in the US — a Boeing 757 that was delivered just over 35 years ago. It's registered as N649DL and appears to typically operate charter flights.

In fact, Delta also owns the next 21 oldest aircraft, which are a mix of 757s and 767s delivered between 1989 and 1991.

However, the average age of its fleet is still slightly younger than United's, at 15.2 years.

About half its 979 planes were made by Boeing, and half by Airbus, per Ch-aviation data.

Boeing jets make up the bulk of its narrow-body aircraft, but if you're flying long-haul with Delta, it's more likely you'll find yourself on an Airbus.

In fact, Delta is the only carrier on this list to operate wide-body Airbus planes, namely the A330 and the A350.

Tariffs have complicated its orders from the European planemaker. Earlier this month, Airbus sent an A350 from its delivery center in Toulouse, France, to Tokyo, so Delta avoided paying tariffs.

A similar tactic was used five years ago, when the World Trade Organisation permitted tariffs on aircraft as high as 15%, as the US and European Union accused one another of unfair subsidies for Airbus and Boeing.

American Airlines also has a roughly even split between Airbus and Boeing planes.

Although the carrier only uses narrow-body Airbus planes, it has both short and long-haul Boeing jets.

Still, American uses more narrow-body planes than its competitors, representing 87% of its fleet.

Its average jet is also more than a year younger than its two rivals, at 14.1 years old. It has the smallest fleet of the three mainline carriers, totaling 988, per Ch-aviation data.

However, when its regional subsidiaries are included, the American Airlines Group carries the most passengers in the world, totaling 226.4 million last year.

The Dallas-based carrier was the progenitor of the budget airline business model, so its fleet makeup is quite different from that of the mainline carriers.

Southwest's fleet is made up entirely of Boeing 737 jets, numbering 802, according to data from Ch-aviation.

So while it has the smallest fleet of the big four, they all have a similar number of narrow-body planes.

Budget airlines typically only operate one type of aircraft because it saves on maintenance and training costs.

However, its traditional business model has come under threat in recent years. Budget airlines have had to contend with changing consumer habits, increased fuel and labor costs, and domestic overcapacity that has made it harder to fill planes.

So, Southwest has announced an array of changes, scrapping signature policies such as "two bags fly free" and bringing in assigned seating.