Cook County had the highest population of 102 counties in 2025, at 5.19 million. Hardin County had the lowest, at 3,588. The median was 24,344 and the figure for Illinois was 12.72 million. Source: the U.S. Census Bureau.
Resident population. Spain and the EU: 1 January, with Spanish figures from the official municipal register (Padrón). United States: Census Bureau population estimates for 1 July. Other countries and the World: World Bank mid-year estimates, which count all residents regardless of legal status or citizenship.
For Denmark, Statistics Denmark counts the registered resident population on 1 January. Greenland and the Faroe Islands are excluded. The annual series selects the first-quarter reference date from FOLK1A. The World Bank series exists for every country, so use it (through compare) to rank countries on one basis.
Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. Equal values share a rank.
How to cite
U.S. Census Bureau. Annual estimates of the resident population for the United States, states and counties, 1 July (Population Estimates Program). https://www2.census.gov/programs-surveys/popest/datasets/2020-2025/. Retrieved 2026-10-08.
Use this data in your AI agent
Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:
Rank Counties in Illinois by population using official data, and cite the source.
rank_places {"indicator":"population","level":"province","within":"Illinois","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=population&level=province&within=Illinois&limit=10
Cook County, with 5.19 million in 2025, according to the U.S. Census Bureau.
Which county has the lowest population?
Hardin County, with 3,588 in 2025.
Where does this ranking come from?
Every value comes from one source and one period, so the places are comparable: U.S. Census Bureau, Annual estimates of the resident population for the United States, states and counties, 1 July (Population Estimates Program). Retrieved 2026-10-08.
