President Reagan signed an executive order yesterday that will raise the pay of most white-collar federal civil servants 4.8 percent, and raise the average salary of metropolitan Washington's 300,000 bureaucrats to $512 a week, or slightly more than $26,000 per year.

The raises, which represent a partial catch-up with private industry, will go to nearly 1 million GS (General Schedule) government clerks, administrators, scientists and technical aides scattered from Huntsville, Ala., to Hong Kong. The increase is retroactive to Oct. 4 for most workers and will show up in biweekly paychecks they get later this month or in early November.

Postal employes, blue-collar workers, executives making more than $50,112.50 and federal retirees are not included in this pay raise, which is less than half the amount that presidential pay agents and government union leaders said is necessary to bring government pay up to industry levels.

Data based on comparing salaries of private industry workers with counterparts in government indicated that the civil service pay raise should have been more than 12 percent this year. White-collar federal workers got a 9.1 percent increase in October 1980.

President Reagan rejected the advice of his pay experts, arguing that the system government uses to measure its own salaries against counterpart pay outside of government does not include the value of U.S. fringe benefits, or take in wages paid to the nation's 12 million state and local government employes.

Although the federal pay raise was not official until yesterday -- and has still not been reflected in paychecks -- most people in Washington have already been hit with rent increases, higher parking fees and other other price rises that usually accompany (or precede) a pay raise for the town's biggest industry. The 26,000 blue-collar workers here -- mechanics, laborers, skilled trades workers and others -- are due to get a raise this month that cannot, by congressional order, exceed the 4.8 percent white-collar salary increase.

The new 4.8 percent raise brings the starting salary for Grade 1 (which has only a few hundred people in it) to $8,342. Because of a congressionally imposed ceiling on career civil service salaries, top federal executives in Grade 18 who should be getting $75,177 will remain at the $50,112.50 level. Thousands of workers in Grades 14 and 15 will get partial raises bringing them up to the $50,112.50 level.

Congress next month may vote to lift the so-called pay cap to give federal executives increases denied them in 1979 and 1980 plus the new 4.8 percent raise.

More than 125,000 federal managers and supervisors -- including about 50,000 here -- under a new merit-pay system will get the 4.8-percent raises plus merit increases that will range from $8 to $16 per week. Under President Carter's civil service reform act, managers and supervisors in Grades 13, 14 and 15 went under a merit-pay system Oct. 1. Those employes cannot get length-of-service raises (worth 3 percent) that go to other employes automatically. Longevity increases are granted every one, two or three years depending on the individuals' time in grade.

The pay chart on this page shows the new white-collar pay scale. There are 18 pay grades. The numbers across the top of the chart show the 10 longevity steps for most grades. Although the chart shows many steps in Grades 14, 15, 16 and 17 at higher rates, nobody can be paid more than $50,112.50 unless and until Congress raises the executive pay ceiling.

Military personnel, who once got the same October percentage pay raise as white-collar federal civilians, have been unhooked from the civilian pay cycle. On Wednesday, President Reagan signed legislation that grants the military raises ranging from 10 percent to 17 percent. That $4.5 billion raise is retroactive to Oct. 1 and should show up in checks received on the 30th.

American Postal Workers Union and Office of Personnel Management are still negotiating over the status of the union health plan for 1982. OPM has said that unless OPM can keep its 1982 premium increase in the "modest" range it will drop the AFL-CIO union from next years' health program. APWU is one of the six plans in the program whose average premiums are used to determine the government share of payment for all employe health premiums. OPM wants the APWU to raise its deductible to $225 next year, and cut certain other benefits to reduce costs, if it wants to stay in the program.