Rochester, IL has a population of 3,687 and is the 7,807th largest city in the United States. The population density is 1,516 per sq mi which is 582% higher than the Illinois average and 1573% higher than the national average. The median age in Rochester is 40 which is approximately 7% higher than the Illinois average of 37. In Rochester, 61% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 75% of Rochester residents were born in Illinois, 24% were born out of state, 1% were born outside of the United States and 1% were foreign born.