Eureka is a city in Olio Township, Woodford County, Illinois, United States. The population was 5,295 at the 2010 census. It is the county seat of Woodford County. Eureka is part of the Peoria, Illinois Metropolitan Statistical Area.
Eureka is located at 40°42′56″N 89°16′31″W / 40.71556°N 89.27528°W / 40.71556; -89.27528 (40.715620, -89.275220).
According to the 2010 census, Eureka has a total area of 3.068 square miles (7.95 km), of which 3.02 square miles (7.82 km) (or 98.44%) is land and 0.048 square miles (0.12 km) (or 1.56%) is water.
As of the 2000 United States Census, there were 4,871 people, 1,754 households, and 1,169 families residing in the city. The population density was 1,810.5 people per square mile (699.1/km²). There were 1,831 housing units at an average density of 680.5 per square mile (262.8/km²). The racial makeup of the city was 97.97% White, 0.57% African American, 0.37% Native American, 0.33% Asian, 0.16% from other races, and 0.60% from two or more races. Hispanic or Latino of any race were 1.03% of the population.
There were 1,754 households out of which 31.7% had children under the age of 18 living with them, 55.9% were married couples living together, 8.3% had a female householder with no husband present, and 33.3% were non-families. 30.7% of all households were made up of individuals and 17.3% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 3.12.
In the city, the population was spread out with 24.3% under the age of 18, 14.5% from 18 to 24, 23.1% from 25 to 44, 16.9% from 45 to 64, and 21.1% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 85.2 males.
The median income for a household in the city was $44,744, and the median income for a family was $53,590. Males had a median income of $44,816 versus $22,692 for females. The per capita income for the city was $20,460. About 0.9% of families and 2.4% of the population were below the poverty line, including 1.8% of those under age 18 and 2.1% of those age 65 or over.