Marriott Corp. soon will announce a significant expansion of its 372-room Key Bridge Hotel.

What this development indicates when put in the context of other hotel construction or additions in metropolitan Washington, is a continued growth in the sector of the local economy called tourism.

With or without a convention center that would house the largest meetings, exhibits and conventions, Washington will remain one of the strongest magnets for vistors in the world.

With some 35,000 hotel rooms in the area, Washington ranks as the third-largest mecca for travelers in the entire world, despite the absence of casinos and beachfront. The total number of rooms has increased about 15 percent in the past decade and a further 15 percent gain in hotel space is expected in the near future, according to Austin Kenny, executive vice president of the Washington Convention and Visitors Association.

Marriott plans to begin work in June on an addition of 200 rooms, a new tower, pool and ballroom at Key Bridge that will be completed early in 1980. The Bethesda company also will open a 40-room addition to its Crystal City hotel this summer, as well as a new pool, sauna and ballroom.

"Business has been excellent . . . so good, in fact that we have to expand," said spokeswoman Barbara Van Blarcum of Marriott.The firm's hotel occupancy rates here generally exceed the areawide average (68 percent in the past year).

New hotels also are planned or possible at the old Willard site, several downtown redevelopment complexes, in Georgetown, and around the Beltway. If a Capital Hill confrontation ends in approval of a D.C. convention center - as now expected by leading business leaders - even more hotels would be constructed.

Thus, it is evident that travel to Washington is a popular thing to do. But the mass influx of tourists at cherry blossom time and the long lines to get into the White House provide misleading indicators about the state of the local tourism industry.

Despite the apparent importance of tourism to the local economy, no method ever has been developed to determine how many people visit here at any one time or over any period.

For example, Kenny believes Washington can look for a "a relative boom again" in the business of catering to visitors. "But we just don't know how to measure," says Kenny, a veteran of tourist promotion here.

What is known is that many visitors to Washington aren't tourists. The Washington area last year was host to 824 convention or group meetings, attracting 714,000 delegates and adding $214 million of revenues to the local economy.

Overall, Kenny's staff estimates, 4.5 million visitors stayed over night at commercial establishments in the area last year. The 1977 figures indicate an increase of about 5 percent in the number of visitors over 1976, when a projected deluge of Bicentennial travelers never materialized.

The increase in convention business amounted to $60 million of new revenues and $2 million of new taxes for local government.

This much, the Convention and Visitors Association will say. Beyond that, it is impossible to say how many other persons come to Washington as tourists. There have been estimates ranging up to 20 million a year annually for all visitors but there is no specific headcount.

Visitor counts at national monuments of museums cannot be translated into useful data because as new museums open, the relationship with previous years is distorted. Museum volume soars during a King Tut exhibit or an Air and Space Museum opening. Many of the people counted are area residents on repeat trips.

Len Hickman, who heads the 40-member Hotel Association of Washington (13,000 rooms in D.C.), estimates that 80 percent of the registrants at city hotels are here to take part in conventions or attend to their businesses.

Business at D.C. hotels in the early weeks of 1978 is up some 6 percent over last year and bookings for meetings indicate a good year ahead on many days. Traditionally, Washington hotels are most crowded on Mondays, Tuesdays and Wednesdays and in the spring and fall. Rooms are available most of the time.

A large number of the uncounted millions alleged to visit here annually are thought to stay with friends and relatives or to make one-day trips into the city without staying overnight.

Meanwhile, growing uncertainty about the economy and major legislative efforts of the Carter administration may be having a positive impact on the tourism business. Kenney speculates that farmer lobbyists and persons interested in tax or energy legislation may have swelled some hotel registers in 1978.

Speculation, guesswork, some real figures. These are what the Washington community uses as benchmarks to assess the health of an important economic sector. Given the level of research capability and statistical knowledge in this capital city, this is an obvious shortcoming that some institution - perhaps the Metropolitan Washington Council of Government? - should seek to improve.