Grundy County is a county in the U.S. state of Illinois. According to the 2010 census, it has a population of 50,063. Its county seat is Morris.
According to the U.S. Census Bureau, the county has a total area of 430 square miles (1,100 km), of which 418 square miles (1,080 km) is land and 12 square miles (31 km) (2.9%) is water.
As of the 2010 United States Census, there were 50,063 people, 18,546 households, and 13,431 families residing in the county. The population density was 119.8 inhabitants per square mile (46.3/km). There were 19,996 housing units at an average density of 47.8 per square mile (18.5/km). The racial makeup of the county was 93.7% white, 1.2% black or African American, 0.7% Asian, 0.2% American Indian, 2.7% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 8.2% of the population. In terms of ancestry, 28.3% were German, 23.0% were Irish, 12.7% were Italian, 9.4% were Polish, 8.6% were English, 7.1% were Norwegian, and 3.0% were American.
Of the 18,546 households, 38.6% had children under the age of 18 living with them, 57.8% were married couples living together, 9.7% had a female householder with no husband present, 27.6% were non-families, and 22.5% of all households were made up of individuals. The average household size was 2.69 and the average family size was 3.16. The median age was 36.1 years.
The median income for a household in the county was $64,297 and the median income for a family was $75,000. Males had a median income of $58,491 versus $36,592 for females. The per capita income for the county was $27,895. About 5.2% of families and 6.9% of the population were below the poverty line, including 8.0% of those under age 18 and 6.5% of those age 65 or over.