Uncle Sam pumps more than $800 million into the Metropolitan Washington area each month in the form of federal and military paychecks. Come October another raise is due white collar U.S. employes and military personnel.

This is the time for guessing, and nail-biting, not only for the 400,000 people directly affected, but also for local-merchants and others who live directly off Washington's biggest industry.

Thousands of nonfederal workers here -- with trade associations, unions and private firms -- follow Uncle Sam's play lead. All of them are anxious to know what the magic number will be this October.

At this point your guests is as good (or bad) as any of the so-called experts.

The numbers floating around are:

6.2 percent. That is the amount originally budgeted by President Carter last January.

7.8 percent. That is the new budget estimate issued last week by the Office of Management and Budget for the likely amount of the October federal pay raise.

9.1 percent. That is the average white collar industry pay increase noted by the Bureau of Labor Statistics in its March 1979-March 1980 survey. That BLS data is used by the White House to determine how much of a catch-up U.S. workers are due to keep even with the private sector.

13.49 percent. That is the figure the president's pay advisers -- Office of Management and Budget, the Labor Department and the Office of Personnel Management -- say is necessary for a full catch-up-with-industry raise this October. That is based on industry wage gains since the last U.S. raise and takes into account portions of increases denied federal workers over the past two years.

President Carter will make the final pay decision. And Congress is sure to go along with whatever amount he sets for white collar federal workers.

Aides to Carter say he has not studied the pay situation closely, and will not make a decision until the last week in August.

Until Carter makes his announcement, and Congress endorses it, all the numbers you hear churching from the rumor mill are meaningless.