Herman Kahn, 61, a leading strategic theorist who declared that nuclear conflict is both inevitable and survivable and a futurist who believed that coming generations would be free of want and economic inequality, died of a heart attack yesterday at his home at Chappaqua, N.Y.

Mr. Kahn, a founder and director of the Hudson Institute at Croton-on-Hudson, N.Y., made his reputation at the RAND Corp., the defense policy center in California where he worked from 1948 to 1961. There he began to consider the consequences of nuclear war as if such a catastrophe were inevitable. In so doing, he laid the foundation of the controversy that surrounds his name and that broadly delineates the debate on nuclear policy today.

According to one group, nuclear war must be avoided because it would destroy mankind. Any suggestion that such a conflict would not destroy all life merely makes the holocaust more likely. According to Mr. Kahn, nuclear war would be terrible indeed, but mankind would survive it. Since such wars are bound to take place, it behooves man to prepare for them.

He stated his case in two books that appeared in the early 1960s. "On Thermonuclear War," which was published in 1960, argued that the policy of deterrence, known officially as "mutually assured destruction" (MAD), was unworkable. Thus, the techniques of survival must take a large place in policy planning.

The second book, "Thinking the Unthinkable" (1962), restated this premise and went on to criticize those who refused to face the possibility of war as acting like "ancient kings who punished messengers who brought them bad news."

An example of the criticism that Mr. Kahn drew is a review by James R. Newman, an editor of Scientific American, who called "On Thermonuclear War" "a moral tract on mass murder: how to plan it, how to commit it, how to get away with it, how to justify it."

When "Thinking the Unthinkable" came out, Stuart Chase said in The Saturday Evening Post that the author was a romantic who refused to admit "the central problem of our age, that nuclear weapons have made war obsolete."

Mr. Kahn retained his views to the end of his life. In an interview with The Washington Post last year, he said he had as little faith in the more recent doctrine of "nuclear utilization target selection" (NUTS), a scenario for limited war, as he had in MAD. His underlying point was that a belief that war is impossible could lead to some policy error that would, in fact, make it happen.

Nations other than the United States and the Soviet Union could start a nuclear conflict, he said in the interview, and this would likely occur sometime in the next 20 years. If the superpowers were to go to war in an unrestrained way, he said, man still would survive.

The real problems, he continued, would be "the unexpected effects, things you don't know. It's not as bad as you think. If you destroy the ozone layer which protects the earth from solar radiation , you're going to go around with coverings like they do in the deserts. And there'll be no change in human health. What it does to animals and vegetables is a complicated story. But human beings? I won't like it. I don't wear hats. But it's not a problem. It's just uncomfortable."

Which is not to suggest that Mr. Kahn, a massive, balding, informal man with a round white beard, a twinkle in his eye, and a fondness for provocative words and phrases, did not take his subject seriously.

"There is no Holy Grail," he told the interviewer when asked how to avoid a cataclysm. "Now we're looking for the first time at the final solution, and there are no final solutions. Up till now you've been asking sensible, reasonable questions. You want an answer, go to your local church."

In recent years, Mr. Kahn has written several books on the future. In "The Coming Boom," which appeared last year, he predicted the disappearance of poverty and unemployment in the United States as a result of zero growth in inflation. By the year 2100, he said, Americans would have average incomes of $50,000 in terms of 1980 dollars.

He also wrote frequently and extensively on the Pacific Basin, the future of Japan, world trade, energy, transportation, population and related topics. He remained an occasional consultant to the highest levels of the government.

Herman Kahn was born on Feb. 15, 1922, at Bayonne, N.J. He grew up in New York City and California. His IQ is said to have been 200. When he was inducted into the Army during World War II, he is said to have scored 181 out of a possible 182 on the military intelligence test.

He graduated from the University of California at Los Angeles and took a master's degree in physics at the California Institute of Technology. He worked for the Northrop and Douglas aircraft companies before joining the RAND Corp.

"Intense interest in economics, public affairs, and science all contrived to lead me into my current work," he recalled in the early 1960s. In his interview with The Post last year, he said that in that work "you've got to be detached, no question about that. I know of no analyst who uses the term megadeath. The peace people use it and they think we use it. But the criticism is basically right. We don't look at those millions of numbers and try to visualize each baby, each 5-year-old kid. We don't go around saying there are 300,000 babies, 250,000 5-year-olds. You don't want to have a super imagination."

Mr. Kahn's survivors include his wife, Rosalie Jane of Chappaqua; two children, Deborah Yetta of Brooklyn, N.Y., and David Joshua of Washington, and two brothers, Julian, of Chicago, and Morris, of Los Angeles.