The 2015 Census estimates put the population at 12,859,995, which was a decrease from the 2014 numbers. Illinois is one of only 7 states that has a negative growth rate in 2016. At -0.17% per year, it is ahead of only West Virginia. Between 2000 and 2010, the population of Illinois grew by a mere 3.3%, compared to the national average US population growth of 9.7%.
The 2018 population estimates Illinois is the sixth largest state in the USA based on population.
Illinois Surface Area and Population Density
Most of the Illinois population is heavily concentrated in the North East of the state in the metropolitan area of Chicago. Chicago, which is home to 2,720,546 people, remains the largest city in Illinois. This city dwarfs the state's other cities, including the capital of the state. The next largest is Aurora, with 200,661 people.
This marks one of Illinois' defining features -- although it has a large urban population, it is spread through a number of smaller cities. Including Chicago, there are twelve cities (including the city of Springfield, the state's capital) with more than 75,000 people.
Many of these smaller cities make up a part of the Chicago Metropolitan Area (prosaically also known as Chicagoland), which holds between 8.3 and 9.8 million people, depending on how you classify its boundaries.
Cook County is the largest county in Illinois, housing 5,238,216 people. The next largest county is neighboring Dupage, with 933,736 people. Almost half of the Illinois population can be found in just those two counties.
Illinois Gender and Religion Statistics
The median age in Illinois is 37.4 years of age, with a slight gender difference over the state- 50.9% female and 49.1% male.
In religious terms, Illinois shows its preferences with a 71% preference for Christian based faiths, a 6% affiliation with non-Christian based faiths, and 22% of the population being unaffiliated with any particular religion.
Illinois Boundary, Census, and Statehood History
Illinois was included in the Northwest Territory established in 1787 and then in Indiana Territory (1800). Illinois Territory was established in 1809, and included virtually all of present-day Wisconsin and portions of Minnesota and the Upper Peninsula of Michigan. Illinois was admitted as a State on December 3, 1818 with essentially its present boundaries.
In 1790 the Northwest Territory had no census coverage. The population shown for 1800 is the total of two counties and one community in present-day Illinois, at that time part of Indiana Territory. Knox County, Indiana Territory, also included some population in what is now Illinois. In 1810, the returns for the two counties of Illinois Territory included some settlements in present-day Wisconsin. The northern part of the State was not fully covered by the census until 1830.