President Carter said yesterday he will approve an Oct. 1 pay raise for federal employees that will give them full "comparability" with workers in the private sector.

But the White House has not yet decided what percentage raise full comparability will require, though Carter is said to be leaning toward 7.05 per cent, the increase recommended by one advisory committee.

That committee, whose members are known as the President's pay agents, consists of Labor Secretary Ray Marshall, budget director Bert Lance and Civil Service Commission Chairman Alan Campbell.

The pay raise would go to 1.4 million white-collar civilian government employees and to 2 million members of the military.

A 7.05 per cent pay raise would cost about $3.4 billion a year, according to White House figures.

Carter has also received a recommendation for a 8.8 per cent pay raise from the Federal Employees Pay Council - a five-member group of federal employee union leaders. A raise of that size, according to the White House, would cost $4.2 billion a year.

The President is waiting to announce the size of the raise until he receives a third recommendation from the President's Advisory Committee on Federal pay, a group of experts from the private sector of the economy established by law to offer advice on federal pay matters.

The group's recommendation is due to be made next week and Carter is expected to announce the size of the pay raise shortly after Congress returns from its recess next Wednesday.

In the complex federal pay system, the President had until today to decide whether to go ahead with a regular annual pay raise, designed to make government salary levels comparable with those in private employment, or to take some other action, such as ordering a pay freeze. Yesterday he said he will approve a regular comparability pay raise. He will decide how much that should be, based on recommendations from advisers, and it will automatically go into effect in October.

The pay raise will affect about 400,000 federal civilian and military employees in the Washington metropolitan area.

The Federal Pay Comparability Act provides that the President annually adjust the salaries of federal white-collar workers and military personnel so that they are comparable to salaries paid to private employees. The measure of comparability is based on Bureau of Labor Statistics figure.

Pay for federal blue-collar workers and Postal Service employees is adjusted by separate systems.

Total federal personnel costs in the fiscal year that will begin Oct. 1 are estimated in the budget at about $74 billion, out of total spending projected to reach about $460 billion.

These costs break down into $48.2 billion for civilian employees and $25.8 billion for the military.

The number of federal civilian employees, including employees of the Postal Service, will be just over 2.8 million next fiscal year, about where it has been for the last five years. There now are 13 federal civilian employees for every 1,000 people in the country, which is about the same as the ratio in 1960 and 1950.