Boulder County, Colorado

Boulder County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 330,758. The most populous municipality in the county and the county seat is Boulder. Boulder County comprises the Boulder, Colorado Metropolitan Statistical Area, which is included in the Denver–Aurora, Colorado Combined Statistical Area.

Boulder County was one of the original 17 counties created by the Territory of Colorado on November 1, 1861. The county was named for Boulder City and Boulder Creek, so named because of the abundance of boulders in the creek which hampered early gold prospecting efforts. Boulder County retains essentially the same borders as in 1861, although a 27.5 square miles (71.2 km²) of its southeastern corner and its approximate population of 40,000 became part of the City and County of Broomfield in 2001.

Before the arrival of the first US settlers, the area was occupied by Native Americans led by Chief Niwot on the plains and seasonally by Utes in the mountains to the west. The first European American settlers were gold prospectors led by Captain Thomas Aikins. His group of about twenty settled at the mouth of Boulder Creek Canyon on October 17, 1858. Chief Niwot told them not to stay and it is said they promised to move into the mountains to prospect in the spring. However, in February 1859, they founded the town of Boulder. At about the same time, they also founded the first Gold Mining town in what would become Colorado, Gold Hill which is about 10 miles west from Boulder. Gold Hill was founded because of the placer gold discovered there. The area was the site of the first commercial scale placer mine in Colorado, producing over 5,500 ounces of gold in the first year of operations (1859).

In late December 2021, the Marshall Fire raged through the parched lands near Boulder, Colorado. Marshall Fire is the most destructive in Colorado's history. The fire impacted City of Superior, City of Louisville and unincorporated Boulder County areas. 991 homes were destroyed with an additional 127 damaged. Over 13,000 people in Superior and 21,000 in Louisville were ultimately evacuated while the fire was spreading due to unusual 100 mile per hour winds. Additionally, one person died and another is missing and presumed dead. The cause of the fire has not been officially announced, pending an investigation. However, an incident report filed by a ranger with Boulder Open Space and Mountain Parks identified two ignition points for the fire. The first ignition point was a shed that began to burn at approximately 11:30AM MST, December 30, 2021. The second ignition point was upwind from the first, and started around noon of the same day on "western side of the Marshall Mesa trailhead".

According to the U.S. Census Bureau, the county has a total area of 740 square miles (1,900 km²), of which 726 square miles (1,880 km²) is land and 14 square miles (36 km²) (1.9%) is water.

Rocky Mountain National Park is in Boulder County, Larimer County, and Grand County. Longs Peak, the park's highest summit at 4,345 meters (14,255 feet) elevation, is located in Boulder County.

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 people/km²). There were 119,900 housing units at an average density of 162 per square mile (63/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.