Federal managers and supervisors covered by merit pay will continue to be eligible for length-of-service raises of 3 percent and merit increases of up to 10 percent, despite the general pay freeze announced yesterday by President Reagan.

Institution of the pay freeze, called for in the budget tentatively approved by the Senate and House, means there will be no across-the-board pay adjustment for white-collar federal workers until January 1987. This year, employes got a 3 1/2 percent raise, and a year earlier, 4 percent.

But the pay freeze does not affect longevity raises due rank-and-file workers or supervisory personnel, or merit raises for the 115,000 managers and supervisors at Grades 13 through 15.

The step increases are worth about 3 percent, while merit pay raises can range from 2 to 10 percent, with special raises in exceptional cases of up to 20 percent.

The Office of Personnel Management estimates that about 40 percent of the federal work force will be eligible for longevity pay raises next year. Those increases, for employes who are not covered by merit pay, are contingent on the workers having enough time in grade (either one, two or three years) and job ratings of "fully successful" or better. Ninety-nine percent of all federal workers are rated fully successful or better.

There are 43,000 workers in the Washington area who are covered by merit pay because they are called supervisors or managers. Another 31,000 at the same pay levels are excluded from merit pay because they are not considered supervisory personnel.

Under the merit pay system, however, supervisors and managers can get a full longevity raise (worth about 3 percent) in any year they are rated "outstanding."

Merit pay managers get one-half of the longevity raise each year if they are rated "exceeds fully successful," and one-third of the raise (or about 1 percent) if they are rated "fully successful." Employes at the 10th step (top) of their grade are not eligible for any additional longevity raises.

In addition to the length-of-service or step increases, workers under the merit pay system are rated each year for possible merit raises.

The law says they must get a minimum of 2 percent for an "outstanding" rating, although that can go up as much as 10 percent. The amount of merit raises depends on rating systems set up by bosses, and the amount of money available for such increases.

According to a recent OPM study, the average salary for Grade 13 employes in the Washington area outside the merit pay system is $42,699. The average salary for GM (merit pay) 13 employes here is $43,072.

The average for GS 14 employes here is $51,340 and for GM 14 (merit pay) workers, the average is $51,544. For GS 15 the average is $60,929, and for persons under merit pay at the same level the average is $60,538.