Box Elder County is part of the Brigham City Micropolitan Statistical Area as well as the Salt Lake City–Ogden–Clearfield Combined Statistical Area.
As of the census of 2000, there were 42,745 people, 13,144 households, and 10,804 families residing in the county. The population density was 8 people per square mile (3/km²). There were 14,209 housing units at an average density of 2 per square mile (1/km²). The racial makeup of the county was 92.87% White, 0.17% Black or African American, 0.88% Native American, 0.96% Asian, 0.08% Pacific Islander, 3.45% from other races, and 1.60% from two or more races. 6.53% of the population were Hispanic or Latino of any race. The top 5 Ethnic groups in Box Elder County are:
There were 13,144 households out of which 47.10% had children under the age of 18 living with them, 71.00% were married couples living together, 7.90% had a female householder with no husband present, and 17.80% were non-families. Of the 13,144 households, 281 are unmarried partner households: 247 heterosexual, 22 same-sex male, and 12 same-sex female. 16.00% of all households were made up of individuals and 7.40% had someone living alone who was 65 years of age or older. The average household size was 3.22 and the average family size was 3.63.
In the county, the population was spread out with 36.10% under the age of 18, 10.50% from 18 to 24, 25.40% from 25 to 44, 17.70% from 45 to 64, and 10.40% who were 65 years of age or older. The median age was 28 years. For every 100 females there were 101.70 males. For every 100 females age 18 and over, there were 98.90 males.
The median income for a household in the county was $44,630, and the median income for a family was $49,421. Males had a median income of $38,814 versus $22,435 for females. The per capita income for the county was $15,625. About 5.80% of families and 7.10% of the population were below the poverty line, including 8.30% of those under age 18 and 5.30% of those age 65 or over