Jimmy Carter certified himself as a big defense spender yesterday, urging Congress to start down a path that would increase Pentagon outlays $100 billion in the next five years.

The same president who came into office three years ago with hopes of reducing defense costs and improving relations with the Soviet Union unfurled an old banner from the Cold War to justify his new direction. "Our forces must be increased if they are to contain Soviet aggression," he said.

In his 1976 campaign, Carter called for $5 billion to $7 billion in cuts in the Pentagon budget; but yesterday the president told Congress that "I am committed as a matter of fundamental policy to continued real increases in defense."

And Defense Secretary Harold Brown said Saturday that even these budgeted increases may not be enough. If the Soviets break through the proposed ceilings in the SALT II accord stalled in the Senate, Brown said, the United States would have to follow suit with a "substantial expansion" of its own strategic weaponry.

Carter's fiscal 1981 budget calls for $161.8 billion to be made available for defense. About half of that total would go to pay military people and civilians on the Pentagon payroll rather than to defense contracts.

The budget makes provision for a 7.4 per cent military pay raise next October, as against 6.2 per cent for civilian government employes.

The weapons' buying account in the budget is $40.5 billion, or about one-fourth of the total. Research and development is $16.5 billion.

Included in these sums are funds for a new land-based MX nuclear missile; 17 new ships for the Navy; stepped-up production of XM1 tanks for the Army; and CX cargo planes for Carter's proposed "Rapid Deployment Force" that could be shifted easily around the world.

The government expresses the dollar totals for defense in a number of ways.

In the White House category called national defense, which includes everything from the nuclear warheads the Energy Department makes for missiles to the General Services Administration guards at the doors of the Pentagon, the Carter budget would lift spending authority from this year's $141.6 billion to $161.8 billion in fiscal 1981 and $253.2 billion by fiscal 1985.

Actual spending in the national defense categroy would jump in that same period from this year's $130.4 billion to $146.2 billion next year and $229.7 billion in 1985.

In assessing how much is enough for defense, Congress concentrates on the money requested for the Pentagon itself, not including contributions from the Energy Department, GSA or other agencies.

In the Carter budget, money available to the Pentagon would jump from $139.3 billion in fiscal 1980 to $158.7 billion in fiscal 1981, a 14 percent jump. The Pentagon figures that the "real" increase would be 5.4 percent because of the money eaten up by inflation. Carter proposes to keep raising this budget authority category every year until it hits $248.9 billion in fiscal 1985. This would represent real increases, according to the Pentagon, ranging from 4.2 to 4.8 percent.

Spending, which is lower than the money available for obligations because bills for major weaponry are paid over a number of years rather than all at once, would climb from $127.4 billion to $142.7 billion between fiscal 1980 and 1981, a 12 percent hike. The real increase, the Pentagon computes, would be 3.3 percent in that one-year period.

The annual increases Carter is proposing beyond fiscal 1981 would push the spending total to $224.8 billion by 1985. The annual increases, after allowing for inflation, would run from 3.3 to 4.4 percent by Pentagon calculations.

As recently as September, Carter and Brown were resisting congressional demands to uncrease the defense budget by the 5 percent now recommended by the president. Several senators, when notified of Carter's reversal, said the president was trying to buy votes for SALT II. Carter has since called off his campaign to win early approval of that pact.

Asked why he switched from opposing to championing the 5 percent increase, Brown on Saturday cited "increasing Soviet self-confidence and the recognition that the Soviets were going to continue their buildup."

The defense secretary added that "if we carry out a steady, significant and sustained growth" in defense programs, "I believe the Soviets would be more cautious in throwing their weight around."

The President's five-year defense plan is a blend of old programs to modernize strategic forces and build up nonnuclear ones, as well as a new effort to build a quick reaction force to intervene in distant trouble spots such as the Persian Gulf.

The fiscal 1981 budget contains an $80.7 million down payment for the CX cargo planes that would fly tanks and other heavy armor to distant trouble spots, like the Persian Gulf. The planes would serve the 110,000-man Rapid Deployment Force the president is organizing from existing Marine and Army units.

Carter also plans to anchor cargo ships near likely trouble spots to resupply ammunition, food and equipment for the forces who would fly directly into a country threatened by takeover. The new Carter budget earmarks $213.8 million for the first two of these ships.

Another part of Carter's blueprint for containing communism in distant countries calls for arming strategically located ones such as Pakistan, Saudi Arabia, Egypt and Israel. The money set aside for such military assistance -- which includes training foreign officers as well as selling modern arms -- is no longer carried as a separate item in the Pentagon budget. mThe funding is spread among several agencies, including the White House and the State Department.

President Carter disclosed in his budget message that he will ask Congress for an extra $10 million in supplemental fiscal 1980 money to enable Pakistan to guarantee $100 million in loans to buy American arms. Carter said this would help "meet the increased threat resulting from events in Afghanistan."

The Pentagon does a brisk business in selling American weapons abroad, about $13 billion in fiscal 1979.

On the economic assistance front, the Carter budget proposes $6.2 billion in fiscal 1981, compared to $5.9 billion in fiscal 1980.