Illinois Counties, Demographics & Geography
Complete official guide to all 102 counties in Illinois (IL). Explore 2024 US Census population estimates, county seats, FIPS codes, land areas, and interactive mapping tools powered by GeoMap Suite.
Illinois County Breakdown
Showing 9 of 9 documented counties and seats.
County Name | FIPS | County Seat | Population (Census) | Land Area | Quick Tool |
|---|---|---|---|---|---|
| Cook County | 17031 | Chicago | 5,275,541 | 945 sq mi | Lookup |
| DuPage County | 17043 | Wheaton | 932,877 | 327 sq mi | Lookup |
| Lake County | 17097 | Waukegan | 714,342 | 444 sq mi | Lookup |
| Will County | 17197 | Joliet | 696,355 | 837 sq mi | Lookup |
| Kane County | 17089 | Geneva | 516,522 | 520 sq mi | Lookup |
| McHenry County | 17111 | Woodstock | 310,229 | 603 sq mi | Lookup |
| Winnebago County | 17201 | Rockford | 285,350 | 514 sq mi | Lookup |
| Madison County | 17119 | Edwardsville | 265,859 | 716 sq mi | Lookup |
| Sangamon County | 17167 | Springfield | 196,343 | 868 sq mi | Lookup |
Geographic Structure of Illinois Counties
County Administration & Governance
In Illinois, county governments manage vital public infrastructure, tax assessments, law enforcement, judicial courts, election administration, and public records. The county seat (Chicago and other municipal headquarters) houses the primary courthouse, commissioners, and sheriff’s department.
All Illinois counties carry a unique 5-digit Federal Information Processing Standard (FIPS) code, where the first two digits (17) designate the state and the remaining three digits identify each specific jurisdiction.
Geographic & Demographic Highlights
- Most Populous County:Cook County (5,275,541 residents)
- Largest by Land Area:Cook County (945 sq mi)
- State Capital:Springfield
- Census Bureau Region:Midwest United States
Demographic calculations derive from the United States Census Bureau 2024 Population Estimates Program (PEP) and TIGER/Line geographic boundary files.