Rock Falls, IL has a population of 9,104 and is the 4,222nd largest city in the United States. The population density is 2,287 per sq mi which is 929% higher than the Illinois average and 2425% higher than the national average. The median age in Rock Falls is 40 which is approximately 7% higher than the Illinois average of 37. In Rock Falls, 54% of the population over 15 years of age are married, 92% speak English and 8% speak Spanish. 83% of Rock Falls residents were born in Illinois, 13% were born out of state, 0% were born outside of the United States and 3% were foreign born.