Waterloo is a city located in Monroe County Illinois. It is also the county seat of Monroe County. With a 2020 population of 10,794, it is the 208th largest city in Illinois and the 2940th largest city in the United States . Waterloo is currently growing at a rate of 1.01% annually and its population has increased by 10.02% since the most recent census, which recorded a population of 9,811 in 2010. Waterloo reached it's highest population of 10,794 in 2021. Spanning over 8 miles, Waterloo has a population density of 1,327 people per square mile.
The average household income in Waterloo is $80,986 with a poverty rate of 3.73%. The median rental costs in recent years comes to $830 per month, and the median house value is $192,100. The median age in Waterloo is 40.4 years, 41.2 years for males, and 39.7 years for females.