Metropolitan Washington's $38 million daily civilian federal payroll will jump this month as a 4 percent increase begins for most of the area's 350,000 federal workers.

A 4 percent raise for military personnel -- including nearly 60,000 here -- is effective today.

The increase for the government's million-plus scientists, secretaries, engineers and other white collar personnel starts with the first pay period beginning on or after Oct. 1.

President Reagan announced the raise last month. He will make it official within the next few days by executive order.

Federal pay is supposed to be adjusted each October to keep it current with wage scales for similar jobs in industry. According to pay data collected by the government, this year's increase should have been about 18 percent.

But the president and Congress approved a 4 percent ceiling on the October pay raise. Reagan, like Presidents Nixon, Ford and Carter, feels the system used to match federal and private pay is out of whack, because is does not compare the value of fringe benefits, consider geographical pay differentials in industry, or consider the salaries paid 12 million state and local government workers.

The federal pay law allows the president to propose an "alternate" pay raise, which is what Reagan did. That figure goes into effect unless Congress overturns it. It did not.

Not included in this pay raise are senior government officials (making $57,500 or more), members of Congress, postal employes or the nearly 500,000 federal wage board (blue collar) employes whose wages are tied to local private industry pay. However, the 26,000 blue collar workers here are expected to get a similar 4 percent raise that will be announced later this month.

Although most white collar workers in the Legislative Branch of government (Government Printing Office, Library of Congress and General Accounting Office) will get the raises automatically, the employes of senators, representatives and congressional committees will not get the raise unless their boss approves it.

The 4 percent pay increase will amount to just over $1,000 a year for the typical Washington area white collar fed, whose average salary is just over $26,000. (A chart of the new pay schedule appears today on Page A25.)

Employes who are near the $57,500 pay ceiling will get only that part of the raise that will move them to the maximum rate.

A big chunk of the raise will be eaten up next year by the new Medicare tax and by expected higher insurance premiums for government employes.

Beginning in January, federal and postal workers will be required to pay the Medicare portion of Social Security. That will be 1.3 percent of their salary on amounts up to about $32,000, costing workers between $100 and $450 a year.

In addition, federal workers, who now pay anywhere from $300 to $1,000 a year for health insurance, will be hit by a rate increase in January. New rates, which will vary among the 100-plus plans in the program, will be announced shortly.

The 150,000 government supervisors and managers (in Grades 13, 14 and 15) who are under so-called merit pay are guaranteed only half of the October pay raise. Any additional raises they get will be based on performance appraisals they get this year. Some of those merit pay raises will not be determined until as late as December, but they will be retroactive to the first pay period beginning on or after Oct. 1.