Continental Airlines Inc., the fourth-biggest US carrier, will begin charging for seats with extra legroom, boosting revenue from sources other than ticket sales.

The fee will vary depending on the market, time and day, and length of flight, the Houston-based airline said Tuesday in a statement, without providing examples. The coach seats can be purchased starting March 17, Continental said.

Airlines are expanding fees for optional products and services to increase revenue without raising fares. Carriers collected at least $2 billion in so-called ancillary charges in 2009’s third quarter, rising 36 percent from a year earlier, according to the US Bureau of Transportation Statistics.

“Seats with additional legroom are higher-value seats, and we want to offer them to customers who recognize that value,” Jim Compton, Continental’s chief marketing officer, said in the statement.

US Airways Group Inc. and JetBlue Airways Corp. are among other carriers that charge for seats with extra legroom.

Customers will be able to purchase the seats at Continental.com or airport kiosks when they check in as much as 24 hours before a flight, the airline said. The seats can be reserved by Continental’s  Onepass Elite frequent fliers without charge, said Julie King, a spokeswoman for the airline.

Seats included in the offer vary by aircraft type and are a “limited number” in all cases, King said. The extra legroom ranges from 7 inches in a regional jet to as much as 20 inches in larger aircraft, she said.

Continental Airlines Inc., the fourth-largest US carrier, said revenue from each seat flown a mile rose as much as 8.5 percent in February.

So-called unit revenue for Continental’s main jet operations and regional partners increased in a range of 7.5 percent to 8.5 percent from a year earlier, the Houston-based airline said Tuesday in a statement.