Americans became increasingly isolated from each other during the 1970s, as the number of households increased 27 percent and the average number of people in them declined from 3.11 to 2.75, according to 1980 census figures released today.

This change in living patterns reflects, among other things, a dramatic increase in young adults who live alone, a rise in the divorce rate and a declining birth rate, according to Census Bureau official James Weed.

Another major factor is the "tendency for elderly people to continue to maintain their own households where in the past they might have moved in with relatives," he said.

The census counted 80.4 million households in 1980, up from 63.4 million in 1970. However, the number of people in the households increased by only 12 percent.

California, the most populous state, also has more households (8.6 million) than any other state and more people in them (23.1 million). New York and Texas are next. However, Utah has the largest average number of people per household -- 3.20. Hawaii follows with 3.15.

Utah has relatively large households at least partly because it has the highest birth rate in the country, Weed said. Some experts have suggested this is because of the large number of Mormons in the state, whose religion encourages them to increase their numbers, and also because of younger families migrating into the state. Hawaii also has an above-average birth rate.

The District of Columbia has the smallest average size of household at 2.39. Florida households are the next smallest at 2.55. Experts are still analyzing the figures and plan a fuller report on their social implications later, Weed said. But it is likely, he said, that the small size of households in Florida is related to the large number of retirees there, many living alone or with one other person.

Maryland households are slightly larger than the national average, at 2.82 persons. Virginia is about average with 2.78 persons a household.

Earlier surveys have shown that older women are the largest group living alone. However, young adults living alone generally have shown the fastest rate of increase since 1970.

Also because of postponement of child bearing and longer periods of married life after children move out of the house, about half of all married couples have no young children in their homes, recent surveys have shown. That was true in 43 percent of married households a decade ago.

In all, some 220.8 million people live in households, the latest report says. And 5.7 million live in group quarters such as institutions, rooming houses, military barracks and college dormitories.

The Census Bureau defines a household as "all persons who occupy a group of rooms or a single room wich constitutes a housing unit [and who] live and eat separately from other persons in the building and have direct access from the outside of the building or through a common hall."

The Census Bureau released these figures before the fuller analysis was complete, Weed said, becuase of a "tremendous demand" for the information from various businesses such as broadcasters and market researchers, as well as city and county planners and others who make million-dollar decisions based on the data.

These are the first data to be broken down by households since the 1970 census, he added.