Quincy is a city located in Adams County Illinois. It is also the county seat of Adams County. With a 2020 population of 39,751, it is the 42nd largest city in Illinois and the 991st largest city in the United States . Quincy is currently declining at a rate of -0.25% annually and its population has decreased by -2.17% since the most recent census, which recorded a population of 40,633 in 2010. Quincy reached it's highest population of 45,288 in 1970. Spanning over 16 miles, Quincy has a population density of 2,530 people per square mile.
The average household income in Quincy is $65,733 with a poverty rate of 15.16%. The median rental costs in recent years comes to $710 per month, and the median house value is $116,300. The median age in Quincy is 40.7 years, 39.2 years for males, and 42.1 years for females.