Prsident-elect Reagan may want to rethink his promise to freeze federal hiring when he takes office. He is going to inherit a much skinnier federal bureaucracy than he imagined. Down perhaps as much as 100,000 jobs from this spring.

The big surge in vacancies is just around the corner. Federal officials estimate that 50,000 to 70,000 U.S. employes may retire next month. That would be more than Uncle Sam normally loses to retirement in one full year.

Reason for the big jump in retirement parties: Federal employes who retire early next year will be able to get a 7.7 percent cost-of-living (COL) benefit. Under current law, federal and postal employes can retire and take advantage of a "look back" provision that allows them to get the benefit of the COL reais that went into effect before they retired. Congress is planning to abolish the look-back feature. The compromise they have worked out, mainly to benefit lame-duck members of Congress, will allow a 45-day grace period after the bill has been signed into law for person to retire and get the 7.7 percent COL raise. Once the full Senate and House approve the retirement law change, and the president signs it, the 45-day countdown begins. Thousands are expected to retire to take advantage of the last look-back. In normal circumstances, a typical federal worker would have to put in another two to three years to increase his or her pension by 7.7 percent.

If predications of a mass retirement exodus come true, the number of federal job vacancies next month could jump, almost overnight, between 70,000 and 100,000.

Some Reagan aides have already hinted that the Defense Department could be exempted from any hiring freeze. Since about half the civilian federal workforce is in Army, Navy, Air Force and Marine Corps support jobs, that is a big exemption. And the Professional Air Traffic Controllers Organization (which endorsed candidate Reagan) says he has promised that controllers and other key air safety personnel will be exempt from any hiring freeze.