According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. Callahan Divide is range of hills that extend twenty-six miles from west to southeast through and Callahan counties, with highest elevation 2411 feet, which together with other elevated areas in the two counties provide advantages in wind energy.
Per the 2010 United States Census, Callahan County had 13,544 people. Among non-Hispanics this includes 12,065 White, 135 Black, 56 Asian, 61 Native American, 7 Pacific Islander, 10 from some other race, and 185 from two or more races. The Hispanic or Latino population included 1,025 people. Of the 5,447 households 27.1% had children under the age of 18 living with them, 57.0% were married couplesliving together, 5.2% had a female householder with children and no husband present, and 28.5% were non-families. 24.7% of households were one person and 32.3% had someone who was 65 or older. The age distribution was 76.2% over the age of 18 and 18.2% 65 or older. The median age was 43.0 years. The gender ratio was 48.8% male and 51.2% female. Among 5,447 occupied households, 78.7% were owner-occupied and 21.3% were renter-occupied. At the 2000 census there were 12,905 people, 5,061 households, and 3,750 families in the county. The population density was 14 people per square mile. There were 5,925 housing units at an average density of 7 per square mile. The racial makeup of the county was 94.78% White, 0.22% Black or African American, 0.63% Native American, 0.26% Asian, 0.05% Pacific Islander, 2.70% from other races, and 1.35% from two or more races. 6.29% of the population were Hispanic or Latino of any race. Of the 5,061 households 31.90% had children under the age of 18 living with them, 61.60% were married couples living together, 9.30% had a female householder with no husband present, and 25.90% were non-families. 23.30% of households were one person and 12.20% were one person aged 65 or older. The average household size was 2.53 and the average family size was 2.97. The age distribution was 26.20% under the age of 18, 6.60% from 18 to 24, 24.90% from 25 to 44, 25.30% from 45 to 64, and 17.00% 65 or older. The median age was 40 years. For every 100 females, there were 94.40 males. For every 100 females age 18 and over, there were 90.90 males. The median household income was $32,463 and the median family income was $37,165. Males had a median income of $27,086 versus $19,720 for females. The per capita income for the county was $15,204. About 9.00% of families and 12.20% of the population were below the poverty line, including 14.80% of those under age 18 and 9.80% of those age 65 or over.
