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The United States Census Bureau found that the population of Illinois was 12,812,508 in the 2020 United States census, moving from the fifth-largest state to the sixth-largest state (losing out to Pennsylvania). Illinois' population slightly declined in 2020 from the 2010 United States census by just over 18,000 residents and the overall population was quite higher than recent census estimates.^[83]

Illinois is the most populous state in the Midwest region. Chicago, the third-most populous city in the United States, is the center of the Chicago metropolitan area or Chicagoland, as this area is nicknamed. Although the Chicago metropolitan area comprises only 9% of the land area of the state, it contains 65% of the state's residents, with 21.4% of Illinois' population living in the city of Chicago itself as of 2020.^[84] The losses of population anticipated from the 2020 census results do not arise from the Chicago metro area; rather the declines are from the Downstate counties.^[85] As of the 2020 census, the state's geographic mean center of population is located at 41° 18′ 43″N 88° 22 23″W in Grundy County, about six miles northwest of Coal City.^[86]

Illinois is the most racially and ethnically diverse state in the Midwest. By several metrics, including racial and ethnic background, religious affiliation, and percentage of rural and urban divide, Illinois is the most representative of the larger demography of the United States