Arlington has the lowest fertility rate, the smallest average family size, and the highest percentage of households made up of unrelated persons of any Washington area jurisdiction, according to new analyses of the 1980 Census by county and regional officials.

In addition, it ranks among the area leaders in the numbers of single-person households, unmarried residents, working women and foreign-born residents, according to the analyses.

"As the Washington area has led the nation in such trends , Arlington has led the Washington area," said demographer George Grier, who, along with Robert Wheeler, the county's planning chief, and John McClain, of the Metropolitan Washington Council of Governments, presented the findings to a group of Arlington citizens last Wednesday.

What it all adds up to, explained Grier, is a "revolutionary" change in Arlington's makeup that is likely to continue throughout the rest of the century.

"The main thing we see is that Arlington County is changing in ways different than other communities," said Wheeler, "and private and government services will have to change to meet the needs."

For example, he said, there will be far more elderly people and fewer children in coming years, diminishing the need for child day-care facilities and schools, while there will be an increasing need for nursing homes and transportation for the elderly.

Much of the change is the result of both the continued decline in the county's number of children and a surge in the amount of housing stock tailored to one-person households, they said.

"The average household size continues to fall more rapidly in Arlington and without any sign of bottoming out," said Wheeler. For the first time, he said, "Arlington has really lost families."

Between 1970 and 1980, Arlington County lost about 9,500 families, as its population dwindled by 21,685 people to 152,599.

The "baby bust" -- the drop in the number of births -- was more evident in Arlington than anywhere else in the Washington area. Arlington's fertility rate was 166 live births for every 1,000 women between the ages of 15 and 44. Alexandria, which had the second lowest, had a rate of 180. The national rate is 309.

The Arlington fertility rate "must be one of the lowest . . . on record" anywhere in the nation, said Grier.

Grier said that, although the Washington area as a whole led the nation with 33 percent of its population never having been married (the national average was 26 percent), 36 percent of Arlington's residents fell into that category.

According to the 1980 census, a clear majority (55 percent) of Arlington's population is single. That means that they have never been married or they are separated, divorced or widowed.

Also, during the '70s, Arlington became more racially and ethnically diverse, according to the census. By 1980, the county had lost about 10 percent of its white residents, who 10 years ago made up 92.6 percent of the community's population. Meanwhile, the numbers of other racial minorities, including blacks, Asians, Pacific Islanders and Hispanics, increased by 11.8 percent.

The greatest growth was among Asians and Pacific Islanders, whose numbers swelled from 2,879 to 12,450 during the 1970s. Most of them are Indochinese refugees, and Grier estimated that today one of every 19 Arlington residents is from Southeast Asia.

Regarding working women, Grier said that, while half of all American women work, in Arlington, 63 percent of all women held jobs.

Arlington also had the fifth highest per capita income in the nation: $12,564, Grier said.