Metropolitan Washington, which has about 12 percent of the total federal work force, lost nearly one-third of the 28,060 federal jobs that disappeared between September of 1981 and September of this year.

New data from the Office of Personnel Management says that the federal work force shrunk to 2,700,568 during that 12-month period, with 8,880 of the cuts made in the District and its Maryland and Virginia suburbs.

Nearly 10,000 federal workers have been RIFfed (fired) for economy reasons since President Reagan took office. The number of individuals RIFfed here was just under 3,000. But many thousands more employes suffered because of the Reductions in Force, being either demoted or transferred when senior workers whose jobs were abolished bumped less senior employes out of their positions.

OPM says that 98,842 non-Defense federal jobs have been eliminated since Reagan was sworn in. But those cuts in the domestic side of government have been partially offset by about 20,000 new civilian hires in the Defense Department since January, 1981.

Even with the job cuts reported by OPM the federal payroll has been rising, mainly because of the 4.8 percent pay raise in October 1981 and longevity pay raises granted automatically to most federal workers during the 12-month period.

Other changes in the federal head count:

* Despite the decline in federal employment here to 343,801, the monthly payroll in this area rose from $752,669 for September 1981 to $860,296 in September of 1982.

* Nationwide, despite the loss of 33,853 jobs, the overall monthly federal payroll was up from $5,145,818 in September of 1981 to $5,571,809 for the month of September 1982.

* During the 12-month period ending September 1982, employment in the legislative branch (Congress, Government Printing Office, the Library of Congress and General Accounting Office) went up by 175 jobs, while employment in the Judicial branch (the courts) increased 341.

The head count is confusing when compared with data from previous years because OPM now uses the "Full Time Equivalent Work Years" system. This measures hours worked, with 2,080 representing a full year, rather than the number of people actually employed.

But any way you slice it, add it or subtract it, the Reagan administration is claiming major success in its program to trim federal employment.