Washtenaw County (/ˈwɒʃtɪnɔː/) is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 344,791. The county seat is Ann Arbor. The county was authorized by legislation in 1822, and was organized as a county in 1826.
According to the U.S. Census Bureau, the county has a total area of 722 square miles (1,870 km), of which 706 square miles (1,830 km) is land and 16 square miles (41 km) (2.3%) is water.
As of the 2010 United States Census, there were 344,791 people residing in the county. 74.5% were White, 12.7% Black or African American, 7.9% Asian, 0.3% Native American, 1.2% of some other race and 3.4% of two or more races. 4.0% were Hispanic or Latino (of any race). 16.0% were of German, 7.6% English, 7.5% Irish, 6.3% American and 5.0% Polish ancestry.
Washtenaw has the highest proportion of Asian-American residents of any Michigan county.
As of the 2000 census of 2000, 17.4% of county residents were of German ancestry; 9.0% cited English, 8.4% Irish, 5.3% Polish and 5.0% American ancestry. According to Census 2000, 87.1% spoke only English at home; 2.7% spoke Spanish and 1.7% Chinese or Mandarin.
There were 125,327 households out of which 29.20% had children under the age of 18 living with them, 46.40% were married couples living together, 9.30% had a female householder with no husband present, and 41.20% were non-families. 29.50% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 2.41 and the average family size was 3.02.
In the county, the population was spread out with 22.10% under the age of 18, 17.10% from 18 to 24, 32.10% from 25 to 44, 20.60% from 45 to 64, and 8.10% who were 65 years of age or older. The median age was 31 years. For every 100 females, there were 98.90 males. For every 100 females age 18 and over, there were 97.10 males.
The median income for a household in the county was $51,990, and the median income for a family was $70,393 (these figures had risen to $59,887 and $80,779 respectively as of a 2007 estimate.) Males had a median income of $49,304 versus $33,598 for females. The per capita income for the county was $27,173. About 5.10% of families and 11.10% of the population were below the poverty line, including 8.60% of those under age 18 and 5.80% of those age 65 or over.