The Census Bureau said yesterday that 73 percent of the nation's population lives in households headed by married couples, down from 82 percent in 1970.

The percentage would be higher if the bureau had made allowance for such groups as unmarried young adults and elderly widows and widowers, who traditionally are more likely to live alone.

The bureau, however, has always calculated on the basis of total national population.

The new census survey was taken last March, and excluded people who were institutionalized.

The new figures show that, for all the talk of new life styles, the traditional family is still the norm in this country. If divorce is common, so is remarriage, and about three-fourths of all children under 18 live in traditional two-parent households.

But it is still true that appreciably more people are living alone or in one-parent families headed by women than in 1970.

The bureau also found that since 1970 there has been a sharp drop in the size of the average household, from 3.14 persons to 2.72 in 1982.

The bureau said relatively low birth rates, declining marriage rates and high levels of separation and divorce caused this decline.

In its basic figures on the settings in which people live, excluding those living in nursing homes and other institutions, the bureau reported that in March, 1982:

* 165.2 million people, 72.8 percent of the population, lived in married-couple households that consisted of 49.6 million couples (99.3 million people), 46.3 million children under 18 and 19.6 million others, both relatives and nonrelatives.

* Another 35 million people, 15.4 percent of the population, also lived in families, but families headed by one man or woman with no spouse present. About three-quarters of these families were headed by women, often single parents with several small children. But some of these families contained no children and consisted of brothers or sisters or other relatives living together.

* In addition, 19.4 million people, 8.5 percent of the non-institutional population, lived alone, up from 6 percent in 1970.

* Finally, nearly 7.4 million (3.3 percent) lived in households of two or more persons not related by family ties. A large number of these households consisted of people of the same sex sharing living quarters, such as group houses or friends sharing an apartment.

The bureau said that in March there were about 1.9 million unmarried couples consisting of a man and a woman living together; in four-fifths of these cases one or both parties were under 35.

Of the 25.2 million people 65 and over, two-thirds lived in families of some type, and one-third, mainly widowed females, lived entirely alone. This reflects womens' longer life expectancies.