United Airlines will retrofit its Boeing 757-300 fleet with slimline seats by the middle of 2018, increasing the number of economy seats on the aircraft to more than 200.

The Chicago-based carrier will add 21 seats to the economy cabin on the aircraft for a total of 210 seats, a notice to pilots on 17 December seen by FlightGlobal shows. First class will continue to have 24 seats.

United's reconfigured 757-300s will have 234 seats, up from 213 currently. Delta Air Lines, the only other US carrier flying the aircraft, also configures its 757-300s with 234 seats.

The airline was not immediately available to comment on the reconfiguration, including whether it will continue to offer personal TV screens and power outlets in economy.

The 757-300 is the latest aircraft in United's fleet to receive additional economy seats. The carrier is reconfiguring its legacy United Boeing 767-300ERs with 214 seats from 213 as it installs its new Polaris business class, and its Boeing 777-200s with 292 seats from either 267 or 269 with the Polaris installations.

United began installing slimline seats on narrowbody mainline aircraft in 2012, beginning with its Airbus A320 family fleet. It now has them on most of its A319s, A320s and Boeing 737s.

Almost all US airlines are in the process of adding, or have added, seats to certain aircraft models in their fleets. For example, American Airlines is reconfiguring its Airbus A321 fleet with 190 seats from either 181 or 187, and its 737-800s with 172 seats from 160.

Adding seats to an aircraft in a carrier's fleet is a cheaper way to increase capacity – and reduce unit costs – than purchasing new or additional aircraft.

United operates 21 757-300s, Flight Fleets Analyzer shows. It operates the aircraft on high-density routes primarily from its Chicago O'Hare, Denver and San Francisco hubs, FlightGlobal schedules show.

Updated with Delta 757-300 seat count. An earlier version of this story said United is adding 20 seats, it is adding 21 seats.

Source: Cirium Dashboard