As far as the federal government is concerned, the Washington metropolitan area may soon not be Washington anymore, but Washington-Arlington.

The Office of Management and Budget, which is in charge of formally defining the nation's metropolitan areas for census counts and federal programs, has combed through 1980 census data--particularly information on jobs and commuting patterns--and decided that the Washington area should have its boundaries enlarged and its name changed.

Three outlying counties--Stafford in Virginia and Frederick and Calvert in Maryland--would join the new Washington-Arlington Metropolitan Statistical Area, adding about 190,000 people, 1,152 square miles, and more than 100,000 cows.

In 1980 the official Washington metropolitan area--including the District, seven surrounding counties, and five cities--had 3.1 million people, 2,810 square miles, and an estimated cow population of about 30,000.

Richard Forstall, the Census Bureau official in charge of preparing the new definitions and analyzing the data to apply them, said the changes were necessary "to keep things consistent with the rest of the country."

Forstall said the changes reflect the growth of jobs in the suburbs and shifts in commuting patterns, with many more workers living on the metropolitan fringe. The proposals are based on a revised set of rules for defining, dividing, and naming metropolitan areas that were adopted in 1980 by a 25-member panel called the Federal Interagency Committee on Metropolitan Statistical Areas.

The proposed changes, which will go into effect June 30 unless OMB reverses itself after hearing any local appeals, were greeted yesterday with pleasure in Arlington and bemusement or objections from almost every other place involved.

Putting Arlington in the name "is the most meaningless thing I've heard in my life," said John F. Herrity, chairman of the Fairfax County Board of Supervisors. "It sounds like some bureaucrat tripped over his tongue."

Forstall said Arlington is being added to the Washington area's name for two reasons. The county has more than 100,000 jobs--some 112,000 in 1980, according to the census count--and also because the Census Bureau calls it a "census designated place," which is equivalent to a city.

Forstall said Fairfax, Montgomery, and Prince George's don't qualify for the designation even though each has more jobs than Arlington, because these more sprawling counties aren't regarded as city equivalents.

Under the old rules Arlington, which has a population of 152,000, would have been too small to get into the area's name.

Officials of OMB said the name change would indeed have no effect. The new counties added to the metropolitan area would come under new rules for some federal programs, but Edwin Dale, spokesman for the agency, said "very little in federal grants" would turn on it.

Depending on how other agencies interpret the changes, he said, some counties might be cut out of Farmers Home Administration programs, which provide low-interest loans, while becoming eligible for community development block grants under HUD. There might also be some increases in the wages that must be paid on federally aided projects under the Davis-Bacon Act.

The changes will have an impact on the statistical complexion of the area, as reported by the Census Bureau, making it appear slightly less wealthy and also less black, because the outlying areas to be added have lower average incomes and a smaller proportion of blacks than the present metropolitan area.

Because of another twist in the new designations, data reported in some census surveys for the central city of the area, which used to be the District of Columbia alone, will also include information for Arlington County and the city of Frederick, Md.

"That's ridiculous," said George Grier, a Bethesda demographer who has written major studies on the area's population. "It is hard to imagine a more diverse combination."

The metropolitan areas are designated by OMB, and are used by the Census Bureau to organize its data and by some other federal agencies for distributing funds. Basically, they are central cities and their surrounding suburbs. The United States now has 318 of them, accounting for about 16 percent of its area and three-quarters of its population.

For the past 30 years each one has been called a Standard Metropolitan Statistical Area or SMSA, a designation that is changed under the new rules to Metropolitan Statistical Area or MSA.

The rules governing what the areas are named, which counties get into an area and what is called a central city are complex.

The three outlying counties--Frederick, Calvert, and Stafford--got into the new metropolitan statistical area because of a change in where commuters commute to. Under the old rules 15 percent of the workers in any county included in a metropolitan area had to work in the area's core--defined here in 1970 as D.C., Arlington, and Alexandria. Now 15 percent must work in the "central counties," which for Washington also include Montgomery, Fairfax, and Prince George's.