The senior planners for the four military services have told a Senate subcommittee that it will cost even more than the $1.6 trillion the president has earmarked for defense over the next five years to carry out all the military missions for which they have been told to prepare.

Their testimony may lead some in Congress to argue that they need even more money, and certainly every cent that President Reagan has asked for. But it has already led at least one senator influential on defense matters, Sam Nunn (D-Ga.), to ask the opposite question: whether Congress should scale down the program to fit the funds that are available.

The testimony, in which the military service senior planners said the administration has directed them to gear up for more contingencies than projected forces could handle, came at a Senate Armed Services manpower subcommittee hearing two weeks ago. It drew almost no notice at the time, and the Pentagon then classified the hearing transcript, but finally released it with minor deletions over the weekend.

The Washington Post reported two months ago that Pentagon specialists had figured it could cost up to $750 billion more than the Reagan administration had budgeted over five years to carry out all the administration's military objectives as interpreted by the Joint Chiefs of Staff.

Defense Secretary Caspar W. Weinberger dismissed this estimate at the time, saying it reflected only the services' unconstrained wish list. And there was no mention, in the hearing two weeks ago, of any amounts of money. But the thrust of the testimony was the same.

The gloomy tone was set by Fred C. Ikle, undersecretary of defense for policy, who told the subcommittee that it would take "a defense increase considerably steeper than what the administration now proposes" to make up the difference in this decade "in accumulated military assets between the United States and Soviet Union."

Subcommittee member Nunn then zeroed in on the representatives of the military services, asking one after another if he thought the administration had set aside enough money to buy a "reasonable assurance force."

The "reasonable assurance force" as Nunn was using the term is that force "required to implement with reasonable assurance the strategy of the nation as the Joint Chiefs of Staff understand that strategy."

Lt. Gen. William R. Richardson, Army deputy chief for operations and plans, said his service would need more divisions than Reagan's five-year plan provides to carry out all the missions set for it in the administration's defense guidance; Vice Adm. Carlisle A. H. Trost, Navy planning director, said his service would need more than the 15 seagoing battle groups now planned; Lt. Gen. Jerome F. O'Malley, Air Force deputy chief of staff for plans and operations, said the Air Force would need "considerably greater than the 40" tactical fighter wings budgeted.

"So all of you are saying," Nunn pressed, "even with the increases in the defense budget that we now have, that we are not at the end of five years in this budget going to have what Gen. Miller Lt. Gen. J.H. Miller, Marine Corps deputy chief of plans, policy and operations has called a reasonable assurance force. Is that the Air Force view?"

"That is correct," O'Malley replied.

"Army view?" Nunn asked.

"Yes," said Richardson.

"Navy view?"

"Yes," responded Trost.

"Marine view?"

"Yes," said Miller.

The Washington Post on Jan. 8 reported that Richard D. DeLauer, Pentagon research chief, had told fellow members of the Defense Resources Board, a top policy group, that the forces the Joint Chiefs of Staff said they would need to carry out civilian defense policy would cost $750 billion more than the Reagan administration had budgeted for the five-year period from fiscal 1984 to 1988.

DeLauer's staff had priced out the force the Joint Chiefs had listed in their Joint Strategic Planning Document. Although Weinberger dismissed the document as an unconstrained wish list, saying it would not take an extra $750 billion to rearm America under the Reagan blueprint, he approved an internal investigation, including polygraph tests, in an attempt to find out who told The Post about the secret discussion on the Pentagon report entitled, "Planning Defense Resources to Match Strategy."

The military planners who appeared before the Senate manpower subcommittee on Feb. 26 took a more serious view than Weinberger of the Joint Strategic Planning Document.

Gen. Miller called the document the Joint Chiefs' "primary input to the secretary of defense for the development of his defense guidance . . . . This is advice to the civilians . . . . This is based upon the threat, the input from the commanders in chief of the Unified Specified Command and the judgment of the Joint Chiefs as to those programs that are required for reasonable assurance of success."

Nunn, at the end of the hearing, told Ikle: "If the Army, Navy, Marine Corps and Air Force and you, too, are agreeing that even with the next five-year defense plan we still are not going to have a reasonable assurance force, then it seems to me we had better go back to the drawing boards on strategy."

In an interview yesterday Nunn elaborated on what he regards as an over-ambitious military strategy embraced by the Reagan administration with little critical review by Congress.

Some of the assumptions underlying the Reagan administration's defense strategy have such grave foreign policy implications, along with military ones, Nunn contended, that Congress would learn most about it by questioning Secretary of State Alexander M. Haig Jr. and Defense Secretary Weinberger jointly in committee hearings.

One unexplored assumption, Nunn said, is that the United States would be able in a crisis to practice "horizontal escalation" by attacking Soviet interests far from the point of confrontation in a kind of global tit-for-tat.

Nunn said Weinberger set off alarm bells in this regard when he raised the possibility in his posture statement of moving into Poland to retaliate for a Soviet push against American interests somewhere else.

Said Weinberger:

"A wartime strategy that confronts the enemy, were he to attack, with the risk of our counteroffensive against his vulnerable points strengthens deterrence and serves the defensive peacetime strategy. . . . Our counteroffensives should be directed at places where we can affect the outcome of the war . . . . Thirty-seven years after free elections in Poland were promised at Yalta, the imposition of martial law in Poland makes clear how such elections would turn out if they were permitted. Our plans for counteroffensive in war can take account of such vulnerabilities on the Soviet side."

Asked Nunn during yesterday's interview: "Do we really want to move to that posture, being prepared to escalate a conflict horizontally? If it's really to be implemented, it has enormous implications."

For one thing, he continued, to carry out such a horizontal strategy the United States would have to have thousands more people in uniform, including the addition of whole divisions to the Army and wings to the Air Force.

Another assumption that Congress should explore before it decides how many dollars are sufficient for defense is the Reagan decision to prepare for a long, rather than short, war. Nunn said neither the United States nor its North Atlantic Treaty Organization allies have either the forces or supplies to make good on this policy.

The administration's new marching orders greatly broaden what the military services are supposed to be able to do, Nunn said, "but neither the Congress nor the American people has focused in on this yet."