Southwest Airlines, the nation’s largest carrier of domestic travelers, announced plans Tuesday to add six extra seats on each plane under a new interior design of lighter and more eco-friendly materials.

To increase the number of seats from 137 to 143 on each plane, Southwest said it will install low-profile cushions on each seat and will reduce the recline from three inches to two inches. The leg room, the space between the seat backs, will also shrink from 32 inches to 31 inches.

Southwest said the new design will cost about $60 million to install but will help generate about $10 million a year in extra revenue from the added ticket sales.

Installation of the new seats will begin in March and should be completed throughout the fleet in 2013.

Southwest said the seats are about six pounds lighter than the current seats, cutting the overall weight of each aircraft by 635 pounds.

The new seats are made of “E-Leather,” a combination of leather fibers and “a high-performance core,” which the airline hopes will also improve durability.

“The cabin upgrade also allows us to create significant revenue opportunities without adding unwanted fees,” said Southwest Executive Vice President Bob Jordan.

Southwest also plans to install the new seats on the fleet of Boeing 700s and 717s from its subsidiary airline, AirTran Airways. AirTran is expected to be absorbed into Southwest in the next few years.

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