Monthly military retirement benefits will top the billion-dollar mark for the first time in March, when a new 6 percent cost-of-living raise goes into effect for former military and federal personnel.

The monthly annuities of federal retirees will jump $10 million in the District of Columbia, Maryland and Virginia for the 211,737 civilian retirees in those jurisdictions. Nationwide the monthly cost of civilian federal pensions will jump to $1.2 billion when the before-taxes raise goes into checks that will be delivered in April.

When the new 6 percent goes into effect federal civilian retirement checks here will go to $183.8 million a month, one of the largest single sources of income in metro Washington. Millions more will be reflected in monthly pension checks for military retirees.

Percentage raises for retirees in recent years have been much higher than pay increases for active duty personnel, thanks to a combination of whittled-down pay increases and rampant inflation.

Government retirees -- civilian and military -- get increases every six months to help them keep current with the rate of inflation. Last September they got a 6.9 percent raise. The March boost will be for a flat 6 percent. By contrast, President Carter held most active duty federal workers and military personnel to a 7 percent raise in 1979. He has projected a 6.2 percent raise for this October when retirees will be getting another catchup-with-inflation as measured by the Consumer Price Index.

Federal officials anticipate a big jump in civilian retirements this month -- plus about 5,000 new military retirees -- because of the big back-to-back inflation increases. They anticipate many high-paid officials who have gotten smaller pay raises than regular employes in recent years may decide to quit in February to take advantage of COL boosts, which come twice as often and at about double the percentage rate of pay raises.