Piatt County is a county in Illinois. According to the 2010 United States Census, it had a population of 16,729. Its county seat is Monticello.
According to the US Census Bureau, the county has a total area of 439 square miles (1,140 km), of which 439 square miles (1,140 km) is land and 0.3 square miles (0.78 km) (0.06%) is water.
As of the 2010 United States Census, there were 16,729 people, 6,782 households, and 4,823 families residing in the county. The population density was 38.1 inhabitants per square mile (14.7/km). There were 7,269 housing units at an average density of 16.6 per square mile (6.4/km). The racial makeup of the county was 98.0% white, 0.3% black or African American, 0.3% Asian, 0.2% American Indian, 0.2% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 1.0% of the population. In terms of ancestry, 26.1% were German, 21.9% were American, 15.2% were English, and 13.4% were Irish.
Of the 6,782 households, 32.0% had children under the age of 18 living with them, 59.4% were married couples living together, 8.0% had a female householder with no husband present, 28.9% were non-families, and 24.5% of all households were made up of individuals. The average household size was 2.46 and the average family size was 2.92. The median age was 42.6 years.
The median income for a household in the county was $55,752 and the median income for a family was $65,850. Males had a median income of $50,425 versus $32,304 for females. The per capita income for the county was $26,492. About 5.1% of families and 7.6% of the population were below the poverty line, including 10.8% of those under age 18 and 4.5% of those age 65 or over.