Grand Detour, IL has a population of 489 and is the 19,678th largest city in the United States. The population density is 418 per sq mi which is 88% higher than the Illinois average and 361% higher than the national average. The median age in Grand Detour is 51 which is approximately 37% higher than the Illinois average of 37. In Grand Detour, 73% of the population over 15 years of age are married, 98% speak English and 2% speak Spanish. 73% of Grand Detour residents were born in Illinois, 27% were born out of state, 0% were born outside of the United States and 0% were foreign born.