Boulder County is one of the 64 counties of the U.S. state of Colorado of the United States. As of the 2010 census, the population was 294,567. The most populous municipality in the county and the county seat is Boulder.
According to the U.S. Census Bureau, the county has a total area of 740 square miles (1,900 km), of which 726 square miles (1,880 km) is land and 14 square miles (36 km) (1.9%) is water.
As of the census of 2000, there were 271,651 people, 114,680 households, and 68,808 families residing in the county. The population density was 392 people per square mile (151/km²). There were 119,900 housing units at an average density of 162 per square mile (62/km²). The racial makeup of the county was 88.54% White, 0.88% Black or African American, 0.61% Native American, 3.06% Asian, 0.06% Pacific Islander, 4.67% from other races, and 2.18% from two or more races. 10.46% of the population were Hispanic or Latino of any race.
There were 114,680 households out of which 30.70% had children under the age of 18 living with them, 48.90% were married couples living together, 7.70% had a female householder with no husband present, and 40.00% were non-families. 26.30% of all households were made up of individuals and 5.50% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 3.03.
In the county, the population was spread out with 22.90% under the age of 18, 13.40% from 18 to 24, 33.60% from 25 to 44, 22.30% from 45 to 64, and 7.80% who were 65 years of age or older. The median age was 33 years. For every 100 females, there were 102.20 males. For every 100 females age 18 and over, there were 101.70 males.
In 2014, the median income for a household in the county was $69,407, and the median income for a family was $94,938. Males had a median income of $65,489 versus $48,140 for females. About 7.0% of families and 14.6% of the population were below the poverty line, including 14.6% of those under age 18 and 5.9% of those age 65 or over.
In 2017 Bloomberg ranked the Boulder metropolitan area as the top “brain” area in the US.