The voluntary turnover rate of government employes is so low that Uncle Sam could cut salaries 5 percent without forcing an exodus of the best and the brightest people, contends a staff report on compensation being prepared by the Office of Personnel Management.

OPM's report says the "quit rate" in government (the number of employes who leave voluntarily other than to retire) is 3.8 percent. That, OPM says, is less than a third of the quit rate in the private sector, where it varies from about 12 percent to 20 percent, depending on the industry.

An OPM official said yesterday that the quit rate -- "and not any pay surveys" -- is the real measure of how well, or how badly, you are paying.

"If your quit rate is high, it means you are paying less than the competition; if it is the same as the competition it means your pay is on par. If the quit rate is low, and ours is very low, it means you are paying very well," the official said.

Earlier this year when government pay agents surveyed civil service and private sector pay, they concluded that federal salaries needed to be increased by 18 percent to make them comparable with industry levels. President Reagan rejected that report and ordered a 3 1/2 percent pay raise, which he delayed until January.

Last week a study prepared for the House Post Office-Civil Service Committee concluded that the government's total pay and fringe benefit package is worth 7.2 percent less than the package offered in the private sector.

"We don't care about the pay surveys," the OPM official said. "We have done surveys showing government pay is 2 percent less than the private sector, and have done others that show it is 11 percent higher than industry. It depends on the base you use.

"The important thing, the thing that tells you where you stand in the competitive market, is the quit rate. Ours is lower. That means federal workers know that they cannot do any better in the private sector.

"I don't care what the other pay surveys show. You can take the House report and fold it five ways. It doesn't matter. You can take the pay agents' report and stuff it."

Federal unions have long contended that the low quit rate is more a reflection of the job market, of employes' dedication to their jobs and to their vesting in the retirement system than it is a measure of how government pay levels stacked up against industry's.

The Reagan administration is expected to use the data about the low turnover rate to push its case in Congress for a 5 percent pay cut. An OPM spokesman said yesterday that if the pay cut (due to go into effect Oct. 1) is defeated by Congress, the only way to make the same savings in personnel outlays over the next three years is to cut employment by 125,000 in the 12 months beginning Oct. 1.