As many as 15,000 bus riders a day may be cheating Metro by paying less than full fare, a top Metro official estimated yesterday.

Furthermore, an undetermined but small number of subway riders are cheating on fares simply by leaving through elevators that bypass the fare-collecting area at six of Metro's 28 stations, the official said.

There is no official estimate of how much Metro is losing. But assuming the average cheater costs Metro 20 cents, it could easily be $3,000 a day or roughly $750,000 annually for weekday use of the system.

Nicholas J. Roll, Metro's assistant general manager for transit services, provided those estimates after the Metro Board meeting yesterday and disclosed that Metro officials are reviewing stepped-up enforcement options to reduce bus and subway fare evasion.

Fare evaders, Roll said, live in all three jurisdictions - Maryland, Virginia and the District of Columbia - and cheat Metro "without regard to their income or social standing," as he put it.

He said that top Metro officials began discussing ways to solve the bus fare evasion problem about two weeks ago when "we just began to get too many reports of people cheating on the fares. There wasn't any bolt of lightning or anything that got us interested in this."

Roll was directed to report to the Metro Board on fare evasion problems at yesterday's meeting after board member Cleatus Barnett referred to a Washington Post story that told about subway fare cheating at the Rosslyn station.

It is particularly easy for suburban commuters to cheat on the way home in the evening after transferring to the bus from the subway. When they board the bus, their transfer is good for only one bus zone. If they cross a zone line - and thousands do - they are supposed to pay an extra zone-crossing charge which, can range from 15 cents to 30 cents, depending on the location.

Basically it is an honor system. On a crowded rush-hour bus it is easy for the rider to claim that his destination is in a cheaper zone than it actually is.

"We also know that maybe 5 per cent of our bus drivers don't work that hard to enforce fare collection," Roll said. He said Metro was considering some kind of occasional check system where passengers would have to buy a zone ticket from the bus driver, then return it when leaving the bus. If it were the wrong color, the passenger would have to pay more.

Some District of Columbia bus riders have avoided fares for years by participating in a cooperative transfer sharing game that everybody knows.

A rider boards the bus and gets a transfer whether he needs it or not. Upon leaving the bus, he hands the transfer to somebody else, often a total stranger. Metro has tried to deal with that problem twice in the past by proposing a five-cent transfer charge. That has been shot down after public hearings as politically impossible. Transfers remain free.

The elevators at the six subway stations present another kind of problem. It is possible, if the station attendant is busy or slothful, to find an operating elevator to the outside and to leave without paying full fare.

"Most people are honest," Roll said. "It's a question of how much money we want to spend to collect the fares from those who are not."

On another matter, the board yesterday also approved a $90,000 program for shelters at outdoor Metro stations, and gave a final approval to personnel actions that will add 20 Metro police to patrol operations.