Douglas County is the seventh-most populous of the 64 counties of the U.S. state of Colorado. As of the 2010 census, the population was 285,465. The county seat is Castle Rock.
According to the U.S. Census Bureau, the county has a total area of 844 square miles (2,190 km), of which 841 square miles (2,180 km) is land and 2.6 square miles (6.7 km) (0.3%) is water.
As of the census of 2000, there were 175,766 people, 60,924 households, and 49,835 families residing in the county. The population density was 209 people per square mile (81/km²). There were 63,333 housing units at an average density of 75 per square mile (29/km²). The racial makeup of the county was 92.77% White, 2.51% Asian, 0.95% Black, 0.41% Native American, 0.06% Pacific Islander, 1.43% from other races, and 1.88% from two or more races. 5.06% of the population were Hispanic or Latino of any race.
There were 60,924 households out of which 47.2% had children under the age of 18 living with them, 73.8% were married couples living together, 5.7% had a female householder with no husband present, and 18.2% were non-families. 13.3% of all households were made up of individuals and 1.9% had someone living alone who was 65 years of age or older. The average household size was 2.88 and the average family size was 3.19.
In the county, the population was spread out with 31.6% under the age of 18, 4.8% from 18 to 24, 37.9% from 25 to 44, 21.6% from 45 to 64, and 4.2% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 99.7 males. For every 100 females age 18 and over, there were 97.4 males.
The median income for a household in the county was $82,929, and the median income for a family was $88,482 (these figures had risen to $93,819 and $102,767 respectively as of a 2007 estimate). Males had a median income of $60,729 versus $38,965 for females. The per capita income for the county was $34,848. About 1.6% of families and 2.1% of the population were below the poverty line, including 1.9% of those under age 18 and 3.7% of those age 65 or over.
Douglas County had the highest median household income of any Colorado county or statistical equivalent in 2000. In 2008, it ranked #8 in the United States in that category – it was one of two in the top 15 not in the vicinity of New York or Washington.