Each time you prepare to board an airliner, the chances are one in 475 that you may not leave the gate with the other passengers.

That's the time to know about passenger rights if you are ever bumped from an overbooked flight.More people are flying than ever before, and the chances of being bumped, particularly during peak travel periods such as late spring and summer, may increase if traffic remains heavy this year.

Airlines routinely overbook their flights by about 20 percent to compensate for the number of people who never show up at the gate, according to How to Fly, an airline guide published by the Consumer Federation of America Inc., a consumer advocacy group in Washington.

About 7 million passengers were no-shows in 1986, according to the book.

The U.S. Department of Transportation requires airlines to ask for volunteers before kicking people off a flight.

And an airline usually finds enough passengers to relinquish their seats voluntarily when a flight is overbooked.

It's hardly what I would call a severe problem, said Hal Paris, spokesman for the Department of Transportation in Washington. But most passengers, I would say, do not understand their rights.

Nearly 80 percent of the 692,766 passengers bumped from flights in the first nine months of 1987 volunteered to give up their seats, according to the agency.

Volunteers either receive cash compensation or, more often, a free ticket to use in the future.

The Transportation Department has no requirements for airlines to reimburse volunteers.

When an airline doesn't have enough volunteers, a passenger can be bumped against his or her will.

If a passenger volunteers to stay behind, he should check whether the airline will cover other costs such as a hotel stay or meals.

Also, check to see whether a free ticket has travel restrictions and when the next flight with confirmed reservations is available.

It's a negotiable item, Mr. Paris said. It's a matter of selling back your seat.

Most airlines have a last at the gate, first bump policy when they involuntarily bump a passenger.

The airline will try to find another flight if a passenger is bumped involuntarily.

If the airline finds another flight that is scheduled to arrive within one hour of the original flight, a passenger won't receive any compensation.

Airlines, in effect, have a one-hour grace period when they bump someone.

If the delay is longer, the Transportation Department requires an airline to pay a penalty to the passenger.

However, if the carrier finds a flight that is scheduled to arrive at the same destination in more than one hour, but less than two hours after the original arrival time, the passenger will receive an amount equal to the one- way fare up to $200.

If the airline can't find a flight that is scheduled to arrive within two hours of the original arrival time, the Transportation Department requires the airline to give a passenger compensation equal to twice the fare of the oversold flight up to $400.

The agency gives the airlines four hours leeway for international departures.