Free parking, now enjoyed by about one in every 12 federal workers here, is about to end. The government shortly will issue guidelines requiring the typical downtown federal parker to begin paying $25 per month beginning in October.

New parking guidelines will establish rates that are the equivalent of parking fees charged by commercial lots. They will cover about 27,000 slots downtown, at the Pentagon and in free spaces in suburban federal agencies.

Under President Carter's April 6 directive, federal workers who now get free parking must begin paying half the commercial rate this fall. They will begin paying the full fee for parking spaces in October 1981.

Federal officials say the exact amount of the rates charged will depend on the wording of the guidelines, now being completed by the Office of Management and Budget. Once OMB clears them, they will go to the General Services Administration, which will set the half and full rates.

Government employes in suburban locations could pay considerably less than their downtown colleagues, since monthly commercial fees in suburbia are often one-third to one-half the rates charged downtown.

The end of free parking will not affect some VIPs. They will continue to be allowed to park free. Nor will it eliminate thousands of free parking spaces for Capitol Hill employes, or garages on Capitol Hill for senators and House members.

Also left untouched by the pay-for-parking order will be hundreds of free spaces reserved for members of Congress, the Supreme Court and diplomats at National and Dulles airports. The Federal Aviation Admistration - which controls both taxpayers-supported airports - also provides especially good rates for some private employes who work for food services or airlines.

In contrast to the $4.50 to $10 per day rates charged for the 4,000 public parking spaces at National airport, members of Congress park free for as long as they want in protected, close-in spaces. There also is private, reserved parking which costs little more than $1 per month for 3,500 private industry workers at the airports.