Ottawa, IL has a population of 18,707 and is the 2,333rd largest city in the United States. The population density is 1,483 per sq mi which is 551% higher than the Illinois average and 1537% higher than the national average. The median age in Ottawa is 40 which is approximately 8% higher than the Illinois average of 37. In Ottawa, 56% of the population over 15 years of age are married, 93% speak English and 5% speak Spanish. 82% of Ottawa residents were born in Illinois, 14% were born out of state, 0% were born outside of the United States and 4% were foreign born.