The national medical bill in 1980 rose the most in 15 years, hitting $247 billion or 9.4 percent of the gross national product, a new government report says.

That means that medical care took nearly $1 of every $10 that Americans spent last year.

Total health care costs, which had risen 13.4 percent from 1978 to 1979, rose 15.2 percent from 1979 to 1980. About three-fifths of that increase was due to inflation, a tenth was due to population growth and the rest resulted from greater "intensity of care," meaning more doctor visits per capita, more tests and more of all other services, according to the statistics from the Department of Health and Human Services.

The snowballing of health care costs is gloomy news for government planners, who are desperately trying to control medical spending. Federal, state and local governments laid out $104 billion or about 42 percent of all money spent for health care last year, with the federal government accounting for about two-thirds of this, mostly through Medicare and Medicaid.

HHS and the Office of Management and Budget are now developing and are expected soon to announce new proposals to cut Medicare and Medicaid outlays through such possible methods as charging patients more, paying the states less for care of the needy, and paying hospitals less.

In that connection, HHS Secretary Richard S. Schweiker said yesterday that while HHS and OMB are considering a variety of ways to cut federal outlays for Medicaid and welfare as well, the next round of Reagan administration proposals would not include any requests to turn Medicaid or Aid to Families with Dependent Children into a system of "block grants."

Schweiker, at a meeting of a subcommittee of the President's Committee on Federalism, said block grants for Medicaid and AFDC might be proposed "in the future" but "not in this go-round."

Medical care costs were 8.9 percent of GNP in 1979; their increase to 9.4 percent is a sign of how much faster they are growing than the economy generally.

Yesterday's report on health outlays shows that in 1980 the nation spent about $99.6 billion on hospitals, $46.6 billion on doctors, $20.7 billion on nursing homes, $19.2 billion on drugs and medical supplies, and $15.9 billion on dentists.

Medicare and Medicaid accounted for 36 percent of all the hospital outlays.

While overall national health costs were growing 15.2 percent, the giant Medicare program, which is mainly for the elderly, grew 21.4 percent, as days of care for patients and other services averaged more.

The growth of Medicare outlays has already had major and unpleasant repercussions for the entire Social Security system.

Many in Congress had thought they could "borrow" Medicare funds to carry the cash-short old-age insurance trust fund through the 1980s until its revenues catch up with its expected costs, but only last week the Medicare actuaries reported that faster hospital cost increases than they had expected left Medicare with little to lend.

That may necessitate major legislation to keep the old-age fund from going broke next year.