McHenry County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 308,760, making it the sixth-most populous county in Illinois. Its county seat is Woodstock.
According to the U.S. Census Bureau, the county has a total area of 611 square miles (1,580 km), of which 603 square miles (1,560 km) is land and 7.6 square miles (20 km) (1.3%) is water.
As of the 2010 census, there were 308,760 people, 109,199 households, and 82,288 families residing in the county. The population density was 511.9 inhabitants per square mile (197.6/km). There were 116,040 housing units at an average density of 192.4 per square mile (74.3/km). The racial makeup of the county was 90.1% white, 2.5% Asian, 1.1% black or African American, 0.3% American Indian, 4.3% from other races, and 1.7% from two or more races. Those of Hispanic or Latino origin made up 11.4% of the population. In terms of ancestry, 34.4% were of German heritage, 18.7% were of Irish ancestry, 14.2% Polish, 10.8% Italian, 7.8% English, and 3.7% of American heritage.
Of the 109,199 households, 40.1% had children under the age of 18 living with them, 62.3% were married couples living together, 8.9% had a female householder with no husband present, 24.6% were non-families, and 19.8% of all households were made up of individuals. The average household size was 2.81 and the average family size was 3.25. The median age was 38.0 years.
The median income for a household in the county was $76,482 and the median income for a family was $86,698. Males had a median income of $61,971 versus $42,125 for females. The per capita income for the county was $31,838. About 4.9% of families and 6.2% of the population were below the poverty line, including 8.3% of those under age 18 and 4.6% of those age 65 or over.