Beginning in January, the 347,000 federal white-collar, blue-collar and postal employes here will have to start paying the Medicare portion of Social Security, or 1.3 percent of salary on amounts up to $35,700.

The typical ($26,000) white-collar civil servant in the Washington area will pay $338 -- nearly a dollar a day -- next year in taxes for Medicare, the government health insurance plan for retirees.

The bite will work out to $4.34 each pay period for the lowest-paid white-collar workers (Grade 1) whose biweekly paycheck, before taxes or other deductions, is $333.60.

Top-paid civil servants whose gross biweekly paycheck is $2,240.40 will pay the maximum Medicare tax next year of $464.10.

Employes who earn around $15,000 in 1983 will have $7.57 taken out of each paycheck to cover Medicare.

Workers who get $20,000 a year will feel a biweekly Medicare bite of around $10 that will last all year.

Only employes whose pay exceeds the $35,700 limit (the cutoff for Social Security and Medicare taxes) will notice a slight increase in take-home pay, toward the end of 1983, when their Medicare tax obligation cuts out.

In addition to the Medicare tax, federal workers will also have to contribute 7 percent of their total annual salary into the Civil Service Retirement fund. For the lowest-paid worker that comes out to $23.35 every two weeks.

Employes at the $26,000 level (the white-collar average here) will have about $70 deducted from each paycheck, to cover their retirement program.

Top-grade civil servants will contribute $157.53 every two weeks into the retirement fund.

Unless Congress puts them under full Social Security, federal workers will not have to pay the entire Social Security tax, which next year will be 6.7 percent of salary on amounts up to $35,700. Government employes who have second, nonfederal jobs that are covered by Social Security (most are) will still have to pay the Medicare tax on the federal salary, even if Social Security taxes are being deducted from their nonfederal paycheck. Government employes who have already qualified for Social Security benefits (including Medicare) through prior private industry service will also have to pay the Medicare tax.