sqmi (901 km ) 338sqmi (875km) 11sqmi (28km), 3.04% Population -( 2007 ) - Density 383,644 831/sqmi (321/km) Website: www.pwcgov.org Prince William County is a county located in the Washington Metropolitan Area . The estimated population in 2007 of the county was 383,644, a 36.6% increase since 2000. Its county seat