Eureka, IL has a population of 5,827 and is the 5,859th largest city in the United States. The population density is 2,190 per sq mi which is 885% higher than the Illinois average and 2317% higher than the national average. The median age in Eureka is 39 which is approximately 3% higher than the Illinois average of 37. In Eureka, 59% of the population over 15 years of age are married, 98% speak English and 2% speak Spanish. 82% of Eureka residents were born in Illinois, 16% were born out of state, 1% were born outside of the United States and 1% were foreign born.