Metropolitan Washington's 22,000 government carpenters, electricians, auto mechanics, janitors and other blue-collar workers will be getting a pay increase effective Oct. 18. Size of the raise has not been determined. But it cannot exceed the 4.8 percent limit Congress has put on white- and blue-collar (wage board) raises that take place in the new fiscal year.

Last year blue-collar (wage board) feds here got an average 8.5 percent raise, while their 300,000 white-collar counterparts got a 9.1 percent increase. This year, because of pay caps imposed by Congress and the White House, the maximum increase for the 1981 fiscal year (that begins October 1) is 4.8 percent.

Unlike white-collar salaries, which are based on a nationwide comparison with industry pay, wages for blue-collar workers are linked to home-town industry pay. The half million blue-collar workers are divided into 135 separate geographic wage areas. Local pay surveys and resulting raises for them are staggered throughout the year. Government-industry pay comparisons here (run by Defense, the largest employer) are made in August. The blue-collar raises here are effective each October. Similar increases for several thousand workers paid under the lithographic wage system go into effect in November. They are subject to the same 4.8 percent cap that applies to all salary increases beginning with the new (fiscal) year.

Blue-collar wage increases already made in other parts of the country have averaged just under 9 percent this year, representing a catch-up with home-town industry scales. Workers in other parts of the country will be subject to the same 4.8 percent pay cap next year.

Federal officials declined to speculate on how much the blue-collar increase, which will cover trades and craft workers, leaders and supervisors will be. But it cannot exceed the 4.8 percent limit, and some employes could get even less than that. The exact amount of the blue-collar raise will be announced on or about Oct. 13.

The October increases leave federal workers -- white-collar or blue-collar -- little to cheer about. The government's own data says it would take an average pay raise of about 15 percent to bring white-collar government pay up to par with private industry. But Congress (in the Budget Reconciliation Act) and the president already have said 4.8 percent is the most those workers can get.