Boone County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 54,165. Its county seat is Belvidere.
According to the U.S. Census Bureau, the county has a total area of 282 square miles (730 km), of which 281 square miles (730 km) is land and 1.3 square miles (3.4 km) (0.5%) is water.
As of the 2010 United States Census, there were 54,165 people, 18,505 households, and 14,273 families residing in the county. The population density was 193.0 inhabitants per square mile (74.5/km). There were 19,970 housing units at an average density of 71.1 per square mile (27.5/km). The racial makeup of the county was 84.4% white, 2.0% black or African American, 1.3% Asian, 0.4% American Indian, 9.3% from other races, and 2.6% from two or more races. Those of Hispanic or Latino origin made up 20.2% of the population. In terms of ancestry, 25.6% were German, 13.7% were Irish, 9.2% were English, 8.6% were American, 7.7% were Italian, 7.1% were Swedish, and 5.0% were Polish.
Of the 18,505 households, 41.5% had children under the age of 18 living with them, 61.6% were married couples living together, 10.2% had a female householder with no husband present, 22.9% were non-families, and 18.9% of all households were made up of individuals. The average household size was 2.91 and the average family size was 3.32. The median age was 36.8 years.
The median income for a household in the county was $61,210 and the median income for a family was $69,380. Males had a median income of $53,581 versus $34,651 for females. The per capita income for the county was $26,105. About 7.9% of families and 10.4% of the population were below the poverty line, including 15.3% of those under age 18 and 5.0% of those age 65 or over.