Washington's population loss slowed last year to about 2,000 persons, the smallest decline in more than a decade, according to a new U.S. Census Bureau report.

The bureau's estimate of the city population in mid-1982 was 631,000, compared with 633,000 a year earlier and 638,333 counted in the 1980 census.

From 1970 to 1980 the population of the District fell about 119,000, or 15.7 percent. Losses of 10,000 to 19,000 occurred every year of the decade.

Demographers suggested that the sharp slowdown in the population decline since 1980 may stem from a "bottoming out" of the exodus of middle-income black families to the suburbs. They said the depressed housing market and the difficulty in obtaining mortgages also may have deterred many people from moving.

Donald Starsinic, the census official in charge of preparing the new estimates, said, "There's been such a heavy loss of black families with children from the District to the suburbs that it tends to bottom out . . . There's a limit to the number of families left that can make the move or are willing to do it."

The new report also shows slight population growth in Maryland, up 6,000 from 1981 to 4,265,000, and a more substantial increase in Virginia--up 66,000 to 5,491,000. Both changes are consistent with the trends in the two states during the previous decade.

Gan Ahuja, the District government's chief demographer, said the recession, marked by high interest rates since 1980, may also have "kept many families where they are."

"People aren't moving because the housing market has been bad," Ahuja said. "It's hard to buy a new house. They can't afford it."

Ahuja also noted that births in the District have leveled off and said improvements in the public school system may be keeping more families in the District. Since 1979, the annual loss of students, most of them black, from the city schools has been cut by more than half, from 7,688 to 3,320. In Prince George's County, which had the area's largest increase in blacks during the 1970s, the number of black students in the public school system has dropped slightly for the past two years.

The new population estimates are based on several complex formulas using birth and death statistics, school enrollment reports, data from income tax returns, employment and Medicare records, and immigration reports.

Although the estimates give no breakdown by race, Ahuja said preliminary data developed by the city government show virtually no change since 1980 in the District's racial composition. In 1980, blacks made up slightly more than 70 percent of the city's population, about the same proportion as a decade earlier.

This is sharply different from the pattern of the 1950s and 1960s, when the District's black population almost doubled and the number of whites dropped by more than half.

According to Census Bureau estimates, the last time Washington's population grew was in 1963, when it climbed to 798,000. It held steady in 1967, but fell substantially every year since.

Overall, the population of the United States grew last year by 2,186,000, the Census Bureau said, to 231.5 million. The growth rate of about 1 percent a year was about the same as in the 1970s.

States in the West and South continued to account for virtually all the growth.

In the past two years, five states have lost population. Four are in the Midwest, which has been hard hit by the recession in the automobile and steel industries--Michigan (down 1.7 percent), Indiana (0.4 percent), Iowa (0.3 percent) and Ohio (0.1 percent). The fifth, West Virginia (0.1 percent), has been harmed by a down-turn in coal, Starsinic said.

The states with the fastest growth were Nevada (up 10 percent), Alaska (8.9 percent), Texas (7.4 percent), Florida (6.9 percent), Wyoming (6.8 percent) and Utah (6.3 percent). Starsinic said no 1982 estimates have been issued yet for individual cities or counties. District of Columbia figures are released with those for the 50 states.