COUNTY · IL
Cook County, Illinois
Cook County is a county in Illinois with FIPS code 17031 and an estimated population of 5,087,072 (Census 2023), 1st largest of 102 in the state.
County facts
- Name
- Cook County
- State
- Illinois
- Type
- county
- FIPS code
- 17031
- State FIPS
- 17
- Population (2023)
- 5,087,072
- Change since 2020
- -3.4%
- State rank
- 1 of 102
Population is the Census Bureau’s Vintage 2023 estimate.
Major cities in Cook County
Other counties in Illinois
Questions
What is the FIPS code for Cook County?
Cook County, Illinois has the five-digit FIPS code 17031: 17 for the state and 031 for the county.
What state is Cook County in?
Cook County is in Illinois.
How do I know if an address is in this county?
Use the address lookup. ZIP codes and city names can cross county lines, so an address is the reliable check.
Sources: U.S. Census Bureau geographic reference files; Census Bureau Population Estimates, Vintage 2023. Last reviewed 2026-10-27.