According to theUnited States Census Bureau, the county has a total area of 4,350 square miles (11,300 km2), of which 4,340 square miles (11,200 km2) is land and 11 square miles (28 km2) (0.2%) is water.[5] When established in 1886, Fergus County,Montana Territory, was a massive, undivided entity of 7,524 square miles. The county was centered around Lewistown, with 209 townships in a rapidly developing, expansive agricultural and mining region, prior to later divisions intoMusselshell (part 1911),Judith Basin (part 1920),Golden Valley (4 October 1920 part from Musselshell), andPetroleum (1925) counties. To organize and administer old Fergus County, districts were created. These districts, rather than the townships, were used to enumerate the population in the 1900 Federal Census.[6]
As of the2020 census, the county had a population of 11,446. Of the residents, 20.7% were under the age of 18 and 26.3% were 65 years of age or older; the median age was 47.5 years. For every 100 females there were 102.4 males, and for every 100 females age 18 and over there were 99.6 males. 52.6% of residents lived in urban areas and 47.4% lived in rural areas.[12][13]
There were 5,032 households in the county, of which 23.8% had children under the age of 18 living with them and 23.9% had a female householder with no spouse or partner present. About 34.8% of all households were made up of individuals and 17.9% had someone living alone who was 65 years of age or older.[12]
There were 6,013 housing units, of which 16.3% were vacant. Among occupied housing units, 72.2% were owner-occupied and 27.8% were renter-occupied. The homeowner vacancy rate was 2.7% and the rental vacancy rate was 10.2%.[12]
As of the2010 census, there were 11,586 people, 5,099 households, and 3,202 families living in the county. The population density was 2.7 inhabitants per square mile (1.0/km2). There were 5,836 housing units at an average density of 1.3 per square mile (0.50/km2). The racial makeup of the county was 96.6% white, 1.2% American Indian, 0.2% black or African American, 0.2% Asian, 0.2% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 1.5% of the population. In terms of ancestry, 30.8% wereGerman, 15.9% wereEnglish, 14.7% wereNorwegian, 12.8% wereIrish, and 4.2% wereAmerican.
Of the 5,099 households, 23.8% had children under the age of 18 living with them, 52.9% were married couples living together, 6.5% had a female householder with no husband present, 37.2% were non-families, and 32.6% of all households were made up of individuals. The average household size was 2.18 and the average family size was 2.75. The median age was 47.8 years.
The median income for a household in the county was $37,607 and the median income for a family was $48,623. Males had a median income of $35,110 versus $21,225 for females. The per capita income for the county was $22,295. About 12.5% of families and 14.7% of the population were below thepoverty line, including 24.2% of those under age 18 and 9.3% of those age 65 or over.