Frankfort Square, IL has a population of 9,116 and is the 4,219th largest city in the United States. The population density is 3,558 per sq mi which is 1501% higher than the Illinois average and 3827% higher than the national average. The median age in Frankfort Square is 39 which is approximately 5% higher than the Illinois average of 37. In Frankfort Square, 61% of the population over 15 years of age are married, 91% speak English and 4% speak Spanish. 82% of Frankfort Square residents were born in Illinois, 10% were born out of state, 2% were born outside of the United States and 6% were foreign born.