The Census Bureau said yesterday that a third of the households in this country now receive non-cash federal benefits such as Medicare, food stamps and low-cost housing.

Millions more receive cash benefits from the government. The degree of dependency on the federal treasury is thus higher than a third, but the bureau hasn't calculated that yet.

The new census report, the first of its kind, provides the most detailed information so far on the age, sex, race, location, income and other characteristics of the recipients of these benefits, the cost of which has doubled since 1976.

It also describes many of the Americans who will be affected by President Reagan's proposed budget cuts, because almost all of the non-cash benefits, with the exception of Medicare, are on the budget cut list.

Of the estimated 27.2 million households receiving one or more of the benefits, 51 percent received only Medicare, a medical insurance program for the disabled and aged. The numbers in this category are increasing as the population ages.

Spared the Reagan shears, this program benefits middle-class people as well as the poor. Almost one in every four households, or 18.5 million, included at least one person covered by Medicare, the survey showed. More than 80 percent of the recipient household heads were 65 years old and over.

About 40 percent lived alone or with non-relatives and four-fifths of this last group were women.

Families headed by women with no husband present received a significant portion of the non-cash benefits targeted specifically at the poor.

Of food stamp benefits, they made up 42 percent of the recipients; of school lunch beneficiaries, 47 percent; of public or other subsidized housing, 66 percent; and of Medicaid, which goes to needy families with children, the aged, blind and other severely disabled people, 36 percent.

Female-headed families, which tend to have among the lowest income levels of any group, have increased by 53 percent since 1970, to a total of 8.5 million, according to John Coder of the Census Bureau. As a proportion of black households, they have doubled from 20 to 40 percent.

Some of the survey's other findings:

About 7 percent of American households (5.9 million) received food stamps in 1979. About 60 percent had incomes below the poverty level, and 77 percent made less than $10,000. The average annual value of the benefits was $810.

Free or reduced-cost school lunches benefited 4.9 million households, or 18 percent of all those with school-age children. The median income among recipients was $9,190, and 43 percent of them were below the poverty level.

About 3 percent of American households (2.5 million) lived in public or subsidized housing. The group's median household income was $4,980.

About 40 percent of all poor households and 55 percent of all poor children received Medicaid. Their median income was $5,990.

Of the households that received some kind of non-cash benefit, 20 percent received two types. The most frequent combination of two was Medicare and Medicaid. Some 12 percent of the recipients got three or moe kinds of non-cash benefits. The most frequent combination in this category was food stamps, school lunches and Medicaid.

Of all the non-cash household recipients, 14 percent received cash public assistance as well. Two thirds of the non-cash recipients (17.9 million households) got Social Security cash benefits in addition.

Since 1976, the cost of non-cash benefits has risen from $41 billion to $83 billion, says the Congressional Budget Office. Cash income transfer programs total nearly $230 billion.