Clay County (county code CY) is a county located in the U.S. state of Kansas. As of the 2010 census, the county population was 8,535. Its county seat and most populous city is Clay Center.
According to the U.S. Census Bureau, the county has a total area of 656 square miles (1,700 km), of which 645 square miles (1,670 km) is land and 10 square miles (26 km) (1.6%) is water.
As of the 2000 census, there were 8,822 people, 3,617 households, and 2,517 families residing in the county. The population density was 14 people per square mile (5/km²). There were 4,084 housing units at an average density of 6 per square mile (2/km²). The racial makeup of the county was 97.72% White, 0.57% Black or African American, 0.41% Native American, 0.15% Asian, 0.26% from other races, and 0.90% from two or more races. Hispanic or Latino of any race were 0.83% of the population.
There were 3,617 households out of which 30.50% had children under the age of 18 living with them, 59.90% were married couples living together, 6.10% had a female householder with no husband present, and 30.40% were non-families. 27.70% of all households were made up of individuals and 15.40% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 2.91.
In the county, the population was spread out with 24.90% under the age of 18, 6.70% from 18 to 24, 23.90% from 25 to 44, 23.70% from 45 to 64, and 20.80% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 95.60 males.
The median income for a household in the county was $33,965, and the median income for a family was $41,103. Males had a median income of $28,817 versus $17,760 for females. The per capita income for the county was $17,939. About 6.80% of families and 10.10% of the population were below the poverty line, including 14.60% of those under age 18 and 8.60% of those age 65 or over.