President Reagan's plan to commit $1.5 trillion to rearming America over the next five years "will not have anything in the way of an inflationary impact," Defense Secretary Caspar W. Weinberger predicted yesterday.

Speaking and answering questions before the Women's Economic Round Table, Weinberger said the military buildup will take an extra 0.3 percent of the nation's output each year, pushing the share of the gross national product devoted to defense from the current 6.2 to 6.8 percent in five years.

Such an investment would not be inflationary, "result in larger deficits or crowd out investments" in the private sector, thanks to the president's economic program, the secretary said. That program includes cuts in both non-military spending and taxes.

In answering written questions from the audience, Weinberger decried "the leakage of classified documents" revealing that he had asked the military services to assess what would have to be done to enable the defense industry to absorb half the gross national product in an emergency.

He said those instructions "grew out of the need to plan against the contingency of full-scale mobilization," not out of any intention to spend half the gross national product on defense in peacetime.

While Weinberger did not talk about it, the Pentagon is working on an assortment of incentives to encourage defense contractors to expand their facilities so they could handle a surge of orders in an emergency. Tax breaks rather than federal subsidies are expected to be emphasized.

Weinberger indicated he would oppose any direct subsidies for the defense industry, declaring: "I'm an old free-market man," He said he had opposed the direct federal help given to Lockheed, Amtrak and Chrysler.

Early in the day, at a breakfast meeting with reporters, Weinberger reacted coolly to the idea of again trying to protect land missiles with anti-ballistic missiles. He said available hardward still is not proof against incoming warheads.