The Congressional Budget Office says that if changes are not made the price tag on the federal retirement program will more than double in the next five years to $2.5 billion a month because of inflation and the addition of 462,500 new retirees by 1986.

CBO has given Congress a series of options to cut retirement costs, now $14.7 billion a year, either by forcing employes to kick in more for their pension program or reducing both the size and frequency of future cost-of-living adjustments.

If the current system is left intact, the CBO says, the cost of the retirement program for U.S. aides will jump to $30 billion by 1986, and nearly 70 percent of the increase will result from twice yearly COL raises retirees now get to keep pace with inflation.

The Senate and House are working on legislation that would limit federal, postal and military retirees to one inflation catch-up each year. Retirees now get COL adjustments each March and September.

CBO says the civil service retirement fund is in good financial shape. Its $73.6 billion in reserves is five times the annual payout. But it says the fund will continue to shrink over the years unless contributions (employes and the government each pay 7 percent of salary into the fund) are raised, or COL raises are held to one a year. One of the options identified includes limiting future COL raises to 70 percent of the inflation rate as measured by the Consumer Price Index.

CBO data given to Congress indicate that the federal retirement program is generally much more generous than pension systems for private workers under Social Security because federal employes can retire earlier and get two guaranteed COL raises each year. CBO says the true cost of the federal retirement program amounts to nearly 30 percent of payroll.

If Congress wants to change the federal retirement program, CBO lists the following options:

1) Increase the amount of moeny that agencies contribute to the retirement fund. They now match employe contributions. CBO says the government contribution, to be borne by the agencies (and taxpayers) in this option, should be 29.5 percent of payroll, up from the current 7 percent.

2) Increase contributions by employes and by agencies. Employes who now pay 7 percent into the fund would be required to pay 9 percent of their total salary for retirement benefits. Agency contributions would be raised to 27.5 percent.

3) Keep the current contribution rate but limit retirees to one COL raise a year, with the increase being held to 70 percent of the rise in living costs as measured by the Consumer Price Index. The CBO option would penalize employes who retire before age 65, with a 10 percent annuity reduction for those retiring at age 60 and 20 percent for persons retiring at age 55. CBO says this should be phased in gradually over a 20-year period.

The Reagan administration, so far, has resisted plans to give retired COL raises that are below the actual rate of inflation. But it is backing Senate and House legislation that would limit retirees to a single COL adjustment each year.