Frankfort, IL has a population of 18,415 and is the 2,368th largest city in the United States. The population density is 1,223 per sq mi which is 450% higher than the Illinois average and 1250% higher than the national average. The median age in Frankfort is 43 which is approximately 16% higher than the Illinois average of 37. In Frankfort, 72% of the population over 15 years of age are married, 94% speak English and 1% speak Spanish. 75% of Frankfort residents were born in Illinois, 19% were born out of state, 1% were born outside of the United States and 5% were foreign born.