If you thought legroom on commercial airlines was already cramped, get set to be squeezed some more.

New Boeing 737-800s now being delivered to American Airlines have the same-size cabins as the existing 737-800s in American's fleet. But the new planes have 12 more coach seats, pushing the total number of seats to 160. Delta Air Lines Inc. has also added 10 seats to its 737-800s, raising the total to 160. So has Continental Airlines Inc.

The seat squeeze shows how airlines are aggressively cramming more seats into jets. The trend has been going on for years, but has picked up momentum of late as airlines take food galleys out of airplanes since they've stopped serving free meals. Some carriers also are replacing seats with new ones made with slimmer frames and cushions, creating additional rows.

American Airlines Capt. Jim Kaiser shows off the new interior of the airline's 737-800 planes. Associated Press

Slimmer seats free up space. But instead of giving it to customers, airlines are using it to try to make their fleets more profitable, taking all those inches and adding more seats to jets. A few extra passengers on each trip can spell the difference for tight-margin airlines between losing money and making money.

In American's case, some customers will lose some legroom. The airline says it standardized the "seat pitch" -- the distance from a point on one seat to the same point on the seat in the next row -- at 31 inches throughout the coach cabin. Some rows in the old configuration had as much as 33 inches of seat pitch, and American's Web site says the old configuration averaged 32 inches.

Exit rows still have more legroom in the new layout -- about the same 39-40 inches as in the old configuration. But "bulkhead" seats in the first row of the coach cabin won't be as roomy as frequent fliers are accustomed to. A spokesman for American says there's a "slight reduction" in legroom for the first row of the economy section.

AMR Corp.'s American says the room for the two added coach rows was freed up by several changes besides just newly designed seats with thinner seatbacks squeezed closer together. Two service-cart storage cabinets behind the last row of seats were eliminated because, well, there's not as much food and beverage service onboard flights these days. The space between the two cabins was shrunk using a new contoured divider.

Despite the squeeze, American believes the new seats won't feel more crowded to travelers. The seat bottoms pivot forward a bit like movie-theater seats to give the person behind you more knee room when reclining.

"The new seat design does not make you feel cramped," said spokesman Charles Wilson. "It has everything to do with a combination of both the way the seat reclines and the new material used for the seat cushions."

There are other benefits as well. The new 737s come with regular power ports available at all seats, bigger overhead bins and 20 drop-down LCD video units for entertainment programming. The 737-800s will also burn 35% less fuel per-seat than the MD-80s they replace.

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Still, seat pitch at 31 inches may well feel tight to many travelers. American once had 34 inches in coach when it marketed itself as the "More Room" airline from 2000-2005. The MD-80s being phased out in favor of the 737-800s will replace MD-80 jets that have 31- to 33-inch pitch in coach.

While some low-cost airlines still offer 32-34 inches of seat pitch on planes, 31 inches has become the standard in coach at many carriers. Delta, for example, had 32 inches in its 737-800s when it had 150 seats. A reconfiguration completed last summer on all 71 737-800s in Delta's fleet pushed that to 160 seats, using slimmer seats. But seat pitch did decline: The first seven rows in coach have 32-inch pitch, but the 15 rows behind the exit doors have 31-inch pitch.

More passengers on a plane means more travelers competing for the same overhead bin space. And bathrooms, too. Most 737-800s are delivered from Boeing with three bathrooms -- one in the front for first-class passengers and two in the rear for coach customers.

Continental and Alaska Airlines are notable exceptions. Continental opted to add an extra mid-cabin bathroom to some of its 737-800s and use those planes primarily on trans-continental flights. That version also has an extra row of first-class seats and carries a total of 155 seats.

The bathroom retrofit began after the airline's president at the time, Greg Brenneman, was quoted in a 1999 page one story in this newspaper acknowledging that the new 737-800 was "one bathroom shy of what it needs." With only two lavatories in the rear for then 140 coach seats (and today 144 coach seats), lines for the bathroom, especially when movies end, can be long.

Alaska, a unit of Alaska Air Group Inc., received its first three 737-800s from Boeing with 160 seats and three bathrooms, but customers complained there weren't enough bathrooms. So Alaska decided to remove three seats and add an extra bathroom in the rear of the plane.

American will have to add a flight attendant on flights with the new layout compared with staffing on its current 737s. The 148-seat version gets three flight attendants. The new layout, with 160 seats, requires four since federal rules require one flight attendant for every 50 passengers.

To keep the two versions separate, the newly delivered 737s with the dozen added seats will be based in Chicago, American says. Right now they are flying between Chicago and Los Angeles, Las Vegas, Denver, Fort Lauderdale, Philadelphia and Minneapolis.

The airline started taking 737 deliveries again in March after a hiatus of more than seven years, and the first two planes with the new configuration went into service on April 14. American plans to take delivery of two or three per month until its order of 76 is complete in the first quarter of 2011. The 77 737-800s already in American's fleet with the older seat configuration will eventually be upgraded to the new design, the airline says.

Write to Scott McCartney at middleseat@wsj.com