As of the census of 2000, there were 969,749 people, 350,714 households, and 250,409 families residing in the county. The population density was 2,455 people per square mile (948/km²). There were 359,411 housing units at an average density of 910 per square mile (351/km²). The racial makeup of the county was:
11.03% of the population were Hispanic or Latino of any race. Like many of the most affluent areas of the U.S. in the 21st century, Fairfax County is home to people from diverse backgrounds, including significant numbers of people of Korean, Indian, Pakistani, Vietnamese and Jewish ancestry.
In 2000 there are 350,714 households, of which 36.30% have children under the age of 18 living with them, 59.40% were married couples living together, 8.60% had a female householder with no husband present, and 28.60% were non-families. 21.40% of all households were made up of individuals and 4.80% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.20.
In the county, the population was spread out with 25.40% under the age of 18, 7.50% from 18 to 24, 33.90% from 25 to 44, 25.30% from 45 to 64, and 7.90% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 98.60 males. For every 100 females age 18 and over, there were 96.20 males.
The median income for a household in the county was $81,050, and the median income for a family was $92,146; in a 2007 estimate, these figures rose to $102,460 and $120,804, respectively. Males had a median income of $60,503 versus $41,802 for females. The per capita income for the county was $36,888. About 3.00% of families and 4.50% of the population were below the poverty line, including 5.20% of those under age 18 and 4.00% of those age 65 or over. A more recent report from the 2007 American Community Survey indicated that poverty in Fairfax County, Virginia had risen to 4.9%.
Judged by household median income, Fairfax County is among the highest-income counties in the country, and was first on that list for many years. However, in the 2000 census it was overtaken by Douglas County, Colorado. According to U.S. Census Bureau estimates for 2005, it had the second-highest median household income behind neighboring Loudoun County, at $94,610. In 2007, Fairfax County reclaimed its position as the richest county in America, in addition to becoming the first jurisdiction in American history to have a median household income in excess of $100,000. In 2008, Loudoun County reclaimed the first position, with Fairfax County a close second (although the U.S. Census Bureau notes that the difference is statistically insignificant).
Several major highways run through Fairfax County, including the Capital Beltway (Interstate 495), Interstate 66, Interstate 95, and Interstate 395. The American Legion Bridge connects Fairfax to Montgomery County, Maryland. The George Washington Memorial Parkway, Dulles Toll Road, and Fairfax County Parkway are also major arteries. Other notable roads include Braddock Road, Old Keene Mill Road, Little River Turnpike, State Routes 7, 28, and 123, and US Routes 1, 29, and 50.
The county is in the Washington D.C. metro area, the nation's third most congested area.
Northern Virginia, including Fairfax County, is the third worst congested traffic area in the nation, in terms of percentage of congested roadways and time spent in traffic. Of the lane miles in the region, 44 percent are rated “F” or worst for congestion. Northern Virginia residents spend an average of 46 hours a year stuck in traffic.
Washington Dulles International Airport lies partly within Fairfax County and provides most air service to the county. Fairfax is also served by two other airports in the Washington area, Ronald Reagan Washington National Airport and Baltimore-Washington International Thurgood Marshall Airport.
Manassas Regional Airport, in neighboring Prince William County, is also used for regional cargo and private jet service.
Fairfax County contracts its bus service called the Fairfax Connector to Veolia Transportation. It is also served by WMATA's metrobus service. Fairfax County is served by the Washington Metro trains. The Orange, Blue, Yellow and the planned Silver lines all serve Fairfax County. In addition, VRE (Virginia Railway Express) provides commuter rail service with stations in Lorton and Franconia-Springfield.
The county maintains many miles of bike trails running through parks, adjacent to roads and through towns such as Vienna and Herndon. The Washington & Old Dominion Railroad Trail runs through Fairfax County, offering one of the region's best, and safest, routes for recreational walking and biking. In addition, nine miles (14 km) of the Mount Vernon Trail runs through Fairfax County along the Potomac River.
However, compared to other regions of the Washington area, Fairfax County has a dearth of designated bike lanes for cyclists wishing to commute in the region. On May 16, 2008, Bike-to-Work Day, the Fairfax County Department of Transportation released the first countywide bicycle route map.
The Fairfax Cross County Trail runs from Great Falls National Park in the northern end of the county to Occoquan Regional Park in the southern end. Consisting of mostly dirt paths and short asphalt sections, the trail is used mostly by recreational mountain bikers, hikers, and horse riders.