Illiopolis, IL has a population of 822 and is the 16,418th largest city in the United States. The population density is 1,793 per sq mi which is 707% higher than the Illinois average and 1879% higher than the national average. The median age in Illiopolis is 39 which is approximately 5% higher than the Illinois average of 37. In Illiopolis, 67% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 86% of Illiopolis residents were born in Illinois, 13% were born out of state, 0% were born outside of the United States and 0% were foreign born.