Arlington County is a county in the Commonwealth of Virginia, and is coterminous with the U.S. Census Bureau–census-designated place of Arlington, which is the second-largest principal city of the Washington metropolitan area. As a result, the county is often referred to in the region simply as “Arlington” or “Arlington, Virginia“. In 2016, the county’s population was estimated at 230,050, making it the sixth-largest county in Virginia, or the fourth-largest city if it were incorporated as such. It is the highest-income county in the United States by median family income, and has the highest concentration of singles in the region.
The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from Washington, D.C., of which it was briefly a part. With a land area of 26 square miles (67 km2), Arlington is the geographically smallest self-governing county in the United States, and by reason of state law regarding population density, has no incorporated towns within its borders. Due to the county’s proximity to downtown Washington, D.C., Arlington is home to many important installations for the capital region and US government, including the Pentagon, Reagan National Airport, and Arlington National Cemetery. Many schools and universities have campuses in Arlington, most prominently the Antonin Scalia Law School of George Mason University.
Arlington County is located in northeast Virginia and is surrounded by Fairfax County and the Falls Church to the southwest, the City of Alexandria to the southeast, and Washington, D.C. to the northeast directly across the Potomac River, which forms the county’s northern border. Other landforms also form county borders, particularly Minor’s Hill and Upton’s Hillon the west.
According to the U.S. Census Bureau, the county has a total area of 26.1 square miles (67.6 km2), of which 26.0 square miles (67.3 km2) is land and 0.1 square miles (0.3 km2) (0.4%) is water. It is the smallest county by area in Virginia and is the smallest self-governing county in the United States. About 4.6 square miles (11.9 km2) of the county is federal property. The county is roughly in the shape of a rectangle 4 miles (6.4 km) by 6 miles (9.7 km), with the small end slanting in a northwest-southeast direction. It has no incorporated areas. Its county seat is the census-designated place (CDP) of Arlington, which is coterminous with the boundaries of the county; however, the county courthouse and most government offices are located in the Courthouse neighborhood.
As of the 2010 census, there were:
- 207,627 people
- 98,050 households,
- and 41,607 families residing in Arlington.
The population density was 8,309 people per square mile (2,828/km²), the highest of any county in Virginia.
According to the US Census, the racial makeup of the county in 2012 was:
- 63.8% non-Hispanic White
- 8.9% Non-Hispanic Black or African American
- 0.8% Non-Hispanic Native American
- 9.9% Non-Hispanic Asian (2.0% Indian, 1.7% Chinese, 1.1% Filipino, 0.9% Korean, 0.7% Vietnamese, 2.7% Other Asian)
- 0.1% Pacific Islander
- 0.29% Non-Hispanic other races
- 3.0% Non-Hispanics reporting two or more race
- 15.4% of the population was Hispanic or Latino of any race (3.4% Salvadoran, 2.0% Bolivian, 1.7% Mexican, 1.5% Guatemalan, 0.8% Puerto Rican, 0.7% Peruvian, 0.6% Colombian)
- 28% of Arlington residents were foreign-born as of 2000.
- Demographics courtesy of U.S. Census Quickfacts
There were 86,352 households out of which 19.30% had children under the age of 18 living with them, 35.30% were married couples living together, 7.00% had a female householder with no husband present, and 54.50% were non-families. 40.80% of all households were made up of individuals and 7.30% had someone living alone who was 65 years of age or older. The average household size was 2.15 and the average family size was 2.96.
Families headed by single parents was the lowest in the DC area, under 6%, as estimated by the Census Bureau for the years 2006–2008. For the same years, the percentage of people estimated to be living alone was the third highest in the DC area, at 45%. In 2009, Arlington was highest in the Washington DC Metropolitan area for percentage of people who were single – 70.9%. 14.3% were married. 14.8% had families. In 2014 Arlington had the 2nd highest concentration of roommates after San Francisco among the 50 largest U.S. cities.
According to a 2007 estimate, the median income for a household in the county was $94,876, and the median income for a family was $127,179. Males had a median income of $51,011 versus $41,552 for females. The per capita income for the county was $37,706. About 5.00% of families and 7.80% of the population were below the poverty line, including 9.10% of those under age 18 and 7.00% of those age 65 or over.
The age distribution was 16.50% under 18, 10.40% from 18 to 24, 42.40% from 25 to 44, 21.30% from 45 to 64, and 9.40% who were 65 or older. The median age was 34 years. For every 100 females there were 101.50 males. For every 100 females age 18 and over, there were 100.70 males.
CNN Money ranked Arlington as the most educated city in 2006 with 35.7% of residents having held graduate degrees. Along with five other counties in Northern Virginia, Arlington ranked among the twenty American counties with the highest median household income in 2006. In 2009, the county was second in the nation (after nearby Loudoun County) for the percentage of people ages 25–34 earning over $100,000 annually (8.82% of the population).  In August 2011, CNN Money ranked Arlington seventh in the country in its listing of “Best Places for the Rich and Single.”
According to a 2016 study by Bankrate.com, Arlington is the best place to retire, with nearby Alexandria coming in at second place. Criteria of the study included cost of living, rates of violent and property crimes, walkability, health care quality, state and local tax rates, weather, local culture and well-being for senior citizens.