Decatur, IL has a population of 74,063 and is the 481st largest city in the United States. The population density is 1,749 per sq mi which is 687% higher than the Illinois average and 1830% higher than the national average. The median age in Decatur is 39 which is approximately 3% higher than the Illinois average of 37. In Decatur, 49% of the population over 15 years of age are married, 96% speak English and 2% speak Spanish. 80% of Decatur residents were born in Illinois, 17% were born out of state, 1% were born outside of the United States and 2% were foreign born.