As of the census of 2000, there were 280,813 people, 94,570 households, and 72,724 families residing in the county. The population density was 831 people per square mile (321/km²). There were 98,052 housing units at an average density of 290 per square mile (112/km²). The racial makeup of the county was 68.93% White, 18.76% Black or African American, 0.39% Native American, 3.81% Asian, 0.13% Pacific Islander, 4.35% from other races, and 3.62% from two or more races. 9.74% of the population were Hispanic or Latino of any race.
By 2005 non-Hispanic whites were 54.2% of Prince William County's population. 19.4% of the population was African-American. 0.5% was Native American. 6.4% of the population was Asian American. The growth of the Asian population was, numerically and as a percentage of the total population in this subgroup, dwarfed by the growth of the Latino population, which made up 18.0% of the county's total population by 2005.
There were 94,570 households, out of which 44.20% had children under the age of 18 living with them, 61.30% were married couples living together, 11.20% had a female householder with no husband present, and 23.10% were non-families. 17.10% of all households were made up of individuals, and 3.00% had someone living alone who was 65 years of age or older. The average household size was 2.94, and the average family size was 3.32.
In the county, the population distribution included 30.40% under the age of 18, 8.80% from 18 to 24, 35.20% from 25 to 44, 20.80% from 45 to 64, and 4.80% 65 or older. The median age was 32 years. For every 100 females there were 99.50 males. For every 100 females age 18 and over, there were 97.40 males.
The median income for a household in the county was $65,960, and the median income for a family was $71,622. Males had a median income of $45,595, compared to $34,286 for females. The per capita income for the county was $25,641. About 3.30% of families and 4.40% of the population were below the poverty line, including 5.60% of those under age 18 and 4.70% of those age 65 or over.
Potomac and Rappahannock Transportation Commission (PRTC) is the public transportation system in Prince William County, Virginia. Services provided by PRTC include OmniRide, OmniLink, and OmniMatch.
Virginia Railway Express (VRE) is a commuter rail service that connects the Northern Virginia area with Washington, DC.