A chart yesterday suggested that the number of unmarried couples living together was almost as great as the number of married couples. The total number of unmarried couples living together in 1982 was 1,863,000, the number of married couples 49,600,000 and the number of divorced Americans 11,504,000.

The number of unmarried couples living together has tripled since 1970, according to a statistical snapshot of the United States released today by the Census Bureau.

At the same time, Americans are marrying later in life and divorcing more often, trends that have increased the number of children living with single parents by two-thirds in the last dozen years, the census also found.

"One of the most striking shifts we saw was the trend toward later marriage," said James A. Weed, chief of marriage and family statistics at the Census Bureau. "It appears that men and women are returning to the pattern of later marriages popular in the early decades of the century."

This gradual upward trend in age at first marriage is most striking among women, who are marrying later than at any time since 1890, when the census first began asking about marital sta-tus.

In 1890, the median age at first marriage was 26.1 years for men and 22 years for women. These numbers remained relatively stable for the next half-century, then began to drop dramatically after World War II. The trend hit bottom in 1956, when the average bride was 20 and her husband just 22.5, then began to climb steadily to the current averages of 25.2 years for men and 22.5 years for women.

"We're almost back to where we were 90 years ago," Weed said, "but for very different reasons."

This trend, the report found, is accompanied by a narrowing gap between men and women in their ages at first marriage. It said "these changes suggest that the marriage pattern for women is becoming more like that for men as women pursue higher education and participate in the labor force before marriage."

More than half of American women over 16 are now working or looking for work, Weed noted, compared with 20 percent in 1890.

The report said the number of unmarried couples living together increased from 523,000 a dozen years ago to 1,863,000 in March, 1982, when the survey was taken.

Although the increase in unmarried couples is "phenomenal," the census also found an increase in married-couple households, from 44.7 million in 1970 to 49.6 million in 1982. "Thus," the study explained, "unmarried couples still represent less than 4 percent of all couples."

Weed added that growth in the number of unmarried couples has slowed in the last two years.

The number of children in single-parent households also grew rapidly between 1970 and 1982. The report said more than 13.7 million children under 18 years of age lived with one parent last year, compared with 8.2 million in 1970.

This increase "is all the more striking in view of the 10 percent decline in the total number of all children under 18 that occurred during the 1970 to 1982 period," the report said.

In nine out of 10 of these single-parent households the mother is the custodial parent, and she is most often divorced, the study found.

But the report noted a quintupling of the number of children living with a mother who never married, which rose from 527,000 in 1970 to 2.8 million in 1982. Children living with only their fathers still represented a small proportion of all children.

The census found that living arrangements of children varied significantly according to race. In 1982, about half of all black children under 18 lived with one parent while an additional 9 percent lived with someone other than a parent, such as a grandparent, aunt or foster parent.

Forty-two percent of black children lived with both parents, the study found, compared with 81 percent of white children living with both parents.