Belvidere, IL has a population of 25,300 and is the 1,704th largest city in the United States. The population density is 2,112 per sq mi which is 850% higher than the Illinois average and 2231% higher than the national average. The median age in Belvidere is 35 which is approximately 6% lower than the Illinois average of 37. In Belvidere, 54% of the population over 15 years of age are married, 74% speak English and 23% speak Spanish. 67% of Belvidere residents were born in Illinois, 19% were born out of state, 0% were born outside of the United States and 14% were foreign born.