Assumption, IL has a population of 1,228 and is the 13,892nd largest city in the United States. The population density is 1,396 per sq mi which is 528% higher than the Illinois average and 1441% higher than the national average. The median age in Assumption is 39 which is approximately 4% higher than the Illinois average of 37. In Assumption, 60% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 88% of Assumption residents were born in Illinois, 11% were born out of state, 0% were born outside of the United States and 1% were foreign born.