Major highways

Adjacent counties

National protected areas

• Deerlodge National Forest (part)
• Helena National Forest (part)

2020 census

As of the 2020 census, there were 12,085 people living in the county.^[13]

2010 census

As of the 2010 census, there were 11,406 people, 4,512 households, and 3,301 families residing in the county. The population density was 6.9 inhabitants per square mile (2.7/km²). There were 5,055 housing units at an average density of 3.1 per square mile (1.2/km²). The racial makeup of the county was 95.5% white, 1.4% American Indian, 0.4% Asian, 0.1% black or African American, 0.4% from other races, and 2.2% from two or more races. Those of Hispanic or Latino origin made up 2.0% of the population. In terms of ancestry, 31.3% were German, 19.1% were Irish, 17.2% were English, 10.8% were Norwegian, and 4.2% were American.

Of the 4,512 households, 28.8% had children under the age of 18 living with them, 63.7% were married couples living together, 5.4% had a female householder with no husband present, 26.8% were non-families, and 22.6% of all households were made up of individuals. The average household size was 2.48 and the average family size was 2.90. The median age was 46.2 years.

The median income for a household in the county was $56,695 and the median income for a family was $67,195. Males had a median income of $50,978 versus $34,148 for females. The per capita income for the county was $26,437. About 2.7% of families and 12.8% of the population were below the poverty line, including 5.1% of those under age 18 and 8.6% of those age 65 or over.

Towns

• Boulder (county seat)
• Whitehall

Census-designated places

Unincorporated communities

Former communities

• Comet
• Wickes