Although metropolitan Washington's federal payroll is fast approaching the $1 billion-a-month mark, new and bigger bites from their 1983 paychecks mean that many of the area's 341,000 federal workers will get less take-home pay than they did last year.

To add insult to injury, Uncle Sam is reaching back to apply the new 1983 Medicare tax to the end-of-1982 salaries of workers.

Effective this month, feds from U.S. senator to U.S. stenographer have to pay a 1.3 percent Medicare tax portion of Social Security (D.C. government aides are exempted). That tax will be applied to the first $35,700 of salary for a maximum tax of $464.10 for 1983.

The average white-collar worker here, who earns $26,000 a year, will pay a Medicare tax of slightly less than a dollar a day, or about $338 this year.

Civil servants who had hoped that the first paycheck this month (reflecting work performed last month) would escape the Medicare tax are in for an unhappy surprise.

In addition to paying higher health insurance premiums--up an average of 24 percent this year--federal workers are paying the 1983 Medicare tax deduction on wages earned last year but paid this year.

The law that extended Medicare coverage and taxes to federal workers (the Tax Equity and Fiscal Responsibility Act of 1982) says the new tax "shall apply to remunerations paid after Dec. 31, 1982." That means this tax is paid on income received this year, even if the income was for work performed in December before the tax was applied to feds.

Keeping track of take-home pay has become a challenge to many civil servants. In October, most rank-and-file employes got a 4 percent raise. Then more than 30,000 career and political executives got big increases in December, thanks to the lame-duck session of Congress. Congress also boosted the salaries of House members more than $9,000 and raised the outside income limit for senators, whose salaries remained at $60,662.50.

The Medicare-health insurance double whammy this year has offset, and in some cases actually wiped out, the recent pay gains of many federal workers. Employes stuck with high-cost health insurance plans--with premiums as high as $1,600 a year--are hardest hit.

While the new Medicare tax is an added burden to federal workers (who already contribute 7 percent of their gross salary into their own retirement fund), it has been a bonanza for thousands of workers who are retiring this month.

Thanks to a "grandfather" clause in the law, employes who retire from government after working only one day this month will be covered by Medicare. In most instances, non-federal employes must pay the full Social Security tax (now 6.7 percent of salary up to $35,700) for at least eight years to qualify for Medicare coverage.

Because of the special Medicare break, federal personnel offices that normally handle about 8,000 retirements a month are swamped with paperwork from employes who held off retiring last year to work that one magic day this month.

January retirees are delighted with the Medicare coverage law. But their colleagues left behind to pay the bills see it as just another addition to their already overcrowded list of deductions.