Chatham, IL has a population of 12,569 and is the 3,296th largest city in the United States. The population density is 1,799 per sq mi which is 689% higher than the Illinois average and 1886% higher than the national average. The median age in Chatham is 35 which is approximately 7% lower than the Illinois average of 37. In Chatham, 68% of the population over 15 years of age are married, 96% speak English and 2% speak Spanish. 79% of Chatham residents were born in Illinois, 18% were born out of state, 0% were born outside of the United States and 3% were foreign born.