Nearly 100,000 of metro Washington's white-collar federal workers are overpaid and carry too much rank, according to a new study by the Office of Personnel Management, which is continually finding warts and blemishes on the civil service it is supposed to preserve and protect.

If the OPM data is correct, it means that about one of every three civil servants here in Grades 1 through 15 is a notch or two higher on the totem pole than he (she) deserves, and accordingly being paid anywhere from $1,000 to $2,000 a year too much.

Reason for the government-wide grade imbalance--which OPM says is much worse here than in Bremerton or Baltimore--is said to be sloppy job classification that puts too many people in jobs where they are either being paid for work they do not do, or are unqualified for duties assigned them.

Nationwide, OPM says, it estimates that about 14 percent of the government's 1.3 million white-collar aides,--from stenos to managers,are overgraded, while fewer than 2 percent are under-graded and underpaid. OPM says the classification costs the taxpayers an unnecessary $682 million a year.

In an advisory that federal agencies will receive shortly, OPM Director Donald J. Devine says the administration plans "a number of steps to improve classification accuracy and position management" and that a crackdown on so-called grade creep will begin later this year.

OPM draws its conclusions from an audit last year of "randomly selected jobs" in 20 agencies. OPM said it checked 713 positions (out of 1,312,000 white-collar jobs) and used "established sampling techniques to achieve a 95 percent level of confidence that the actual government-wide misclassification rate lies within an estimated interval of approximately plus or minus three percent." That means OPM thinks its conclusions are right.

Earlier this year Devine said OPM would push agencies to review job classifications in 1983 with a view to downgrading positions that are overgraded. Devine also said an alternative to downgrading is adding new duties to jobs to justify the grades they carry.

Under present federal law, federal workers hit by no-fault demotions keep their current grade for two years even though their position is downgraded. They also keep indefinitely the salary they were earning before they were downgraded, although they do not get the same raises as other employes.

OPM officials said yesterday that agencies will be "strongly encouraged" to review jobs to make sure that employes are properly graded, and in the correct occupational title and job series.

If OPM's numbers are accurate and if it sticks to its plan, thousands of workers here and around the nation could get new duties added to their jobs, or be downgraded, in the not-too-distant future.