Most of the federal government's 1.4 million white collar workers will take a pay cut in October because of a little-noticed bookkeeping change Congress passed last year to hold down federal payroll costs.

The typical Washington-area federal worker will lose about $3.40 per pay period, according to government payroll officials. The change, which affects how hourly wages are calculated for professional, technical and clerical employes, will reduce government salaries in the Washington area about $24 million a year.

About 340,000 federal employes in the Washington-Baltimore area will be affected by the pay cut.

The salary-cutting formula--tucked away in last year's massive budget reconciliation bill--will be in effect until 1985 unless Congress repeals or extends it.

Government officials estimate that the bookkeeping change will save the government at least $120 million for each full year it is in effect.

It will not mean any pay change for the 30,000 wage board (blue collar) workers in this area, or for postal employes. In addition, members of Congress, agency heads and other government officials who are paid monthly will not be affected by the cut.

The change was made last year when Congress was searching for ways to cut federal spending programs. The House Post Office-Civil Service Committee, trying to keep money in other federal pay and retirement programs, suggested that a substantial savings could be made by temporarily adopting a computation formula along the lines of a proposal made earlier by the General Accounting Office.

Under the current system, pay for most federal employes is computed on the basis of a work year with 2,080 hours. That works out to a year of 364 days, one day short of the typical calendar year.

GAO suggested that the government could save from $120 million to $130 million a year by increasing the work year to 2,087 hours. Adding seven hours has the effect of decreasing the hourly pay rate, thereby lowering salaries for most people. The government workers will continue to work the same number of hours as they do now, and only their hourly wage rate will change as a result of the hours added to the work year for the purpose of computing salaries.

Congress ordered agencies to make the change in October, when the new federal fiscal year begins.

Employes can figure out what kind of cut they will receive by doing the following:

Divide the amount of annual salary by 2,087 (the new work hours figure) and multiply the answer by 80 hours. The result will show the new biweekly gross pay after the change.

Then subtract all current paycheck deductions to get the new biweekly net pay. That figure, when subtracted from the current net pay figure, will show the approximate size of the cut.

The paycheck bite comes as a surprise to most workers. They are used to getting modest pay increases each October to keep up with industry. President Reagan has said he wants to defer the next increase until at least next April.

Congress has tentatively approved a 4 percent increase to go into effect in January. Meantime, pay for most people will decrease in a few weeks.