Cook County, Illinois

Cook County is a county in Illinois, United States, covering Chicago and its inner and middle suburbs on the southwestern shore of Lake Michigan. It had a population of 5,275,541 at the 2020 census, an estimated 5,194,625 by 2025, and covers 945 square miles (2,448 km2) of land centred near 41°48′N 87°53′W. Only Los Angeles County, California has more residents, and roughly 40 percent of all Illinois residents live here.

Chicago is the county seat and by far the largest city; the postal code is 60602 at the county building in the Loop. The county holds 136 municipalities wholly or partly within its borders — 23 cities, one town and 112 villages — plus 29 suburban townships, and it runs on Central Time. Its FIPS code is 17031.

Towns & Villages in Cook County

The 2020 census counted 136 municipalities lying wholly or partly inside Cook County. Population figures below are citywide 2020 census counts; an asterisk (*) marks the 28 municipalities that straddle a county line, so part of that figure lives in a neighbouring county. Cook County has no census designated places — almost all of its territory is incorporated.

The straddlers matter for reading the numbers. Elgin (mostly in Kane County), Buffalo Grove and Deerfield (mostly in Lake County), Bartlett, Bensenville, Burr Ridge, Hanover Park, Hinsdale, Oak Brook, Roselle and Woodridge (mostly in DuPage County), and Frankfort, Homer Glen and University Park (mostly in Will County) each keep only a slice of their population inside Cook. Chicago itself extends a small sliver west into DuPage County at O’Hare. A handful of unincorporated pockets survive between the suburbs — Central Stickney, Indian Hill, La Grange Highlands, Nottingham Park, Sag Bridge and Sutton among them — with no municipal government of their own.

Townships

Suburban Cook County is divided into 29 civil townships, which handle property assessment, general assistance and some road work. Chicago abolished its townships in 1902 and Evanston’s was dissolved as well, so both cities sit outside the township system. Township populations below are from the 2010 count, the last one Wikipedia publishes for these units.

ZIP Codes

Cook County spans 215 ZIP codes, far too many to list one by one, running from 60004 (Arlington Heights) to 60827 (Riverdale). The suburbs use the 600xx to 605xx ranges: the northwest suburbs sit mostly in 600xx–601xx (60004 Arlington Heights, 60056 Mount Prospect, 60067 Palatine, 60077 Skokie), the North Shore in 602xx (60201–60203 Evanston, 60091 Wilmette is the exception in 600xx), the west suburbs in 603xx (60301–60304 Oak Park, 60302 being the main delivery code), the south suburbs in 604xx (60402 Berwyn, 60411 Chicago Heights, 60452 Oak Forest, 60462 Orland Park), and the near-southwest corridor in 605xx (60501 Summit, 60513 Brookfield, 60525 La Grange). Chicago’s codes carry the 606 prefix, 60601 through 60661 for street delivery plus box-only codes. A short 607xx–608xx band covers enclaves ringing the city: 60706 Harwood Heights, 60707 Elmwood Park, 60712 Lincolnwood, 60714 Niles, 60803 Alsip, 60804 Cicero, 60805 Evergreen Park and 60827 Riverdale.

Geography

The county occupies a flat lake plain left by glacial Lake Chicago, rising gently toward low moraines in the northwest and the Palos hills in the southwest. Its total area is 1,635 square miles, but 690 square miles of that — 42.2 percent — is Lake Michigan water, leaving 945 square miles of land. The Chicago River, the Calumet River and the Des Plaines River drain the county, with the Cal-Sag Channel and the Chicago Sanitary and Ship Canal carrying river flow away from the lake. The Forest Preserve District of Cook County, created in 1915, holds a green belt of woods and sloughs along the river valleys.

Lake County lies to the north, McHenry County to the northwest, Kane County to the west, DuPage County to the west, Will County to the south, and Lake County, Indiana to the southeast, with water borders on Lake Michigan facing Porter County, Indiana and Berrien County, Michigan. Lake County, Kane County, DuPage County, Will County, Lake County, Indiana, Porter County and Berrien County all touch Cook County by land or across the lake. Cook is one of the few US counties to border two same-named counties in different states — Lake County on each side of the state line.

Demographics

Cook County recorded 5,275,541 residents at the 2020 census, up 1.6 percent on 2010; the Census Bureau’s 2025 estimate is 5,194,625, a slight decline since. Chicago holds 2,746,388 of those residents, about 52 percent of the county. Cicero, at 85,268, is the largest municipality entirely inside the county; Evanston (78,110) and Arlington Heights (77,676) follow close behind. The median age was 37.4 years, with 20.9 percent of residents under 18 and 15.1 percent aged 65 or older. Density works out to roughly 5,580 people per square mile of land, the highest of any Illinois county by a wide margin.

Economy & Local Life

The county’s economy is the core of the Chicago metropolitan economy. The Loop is the second-largest central business district in the United States, anchored by finance and derivatives trading at CME Group and by corporate headquarters including United Airlines and McDonald’s. O’Hare International Airport, on the county’s northwest side, is among the world’s busiest airports and drives a broad logistics and hotel economy through Rosemont, Des Plaines and Elk Grove Village, whose industrial park is one of the largest in North America. Health care is a major employer countywide: the county’s own Cook County Health system, built around John H. Stroger, Jr. Hospital, is the third-largest public health system in the nation. Suburban manufacturing and warehousing cluster along the I-290 and I-294 corridors in Cicero, Bedford Park, McCook and Franklin Park. On the lakefront, the Chicago Air and Water Show each August draws some of the largest single-event crowds in the county’s calendar.

History

The Illinois General Assembly created Cook County on 15 January 1831 out of Putnam County, the 54th county organised in Illinois. It was named for Daniel Pope Cook, an early Illinois congressman and the first attorney general in the state’s history. Fort Dearborn, built in 1803 at the mouth of the Chicago River, was the county’s first permanent US settlement point; Chicago incorporated as a city in 1837. The county took its final shape in 1839 when DuPage County was carved off its western side.

Growth after that was explosive: 10,201 residents in 1840, 1.19 million by 1890 and 2.4 million by 1910, as railroads, the stockyards and steel drew migrants from Europe and, during the Great Migration, Black families from the South. The Great Chicago Fire of 1871 destroyed the city’s core but not its growth; the 1919 Chicago race riot, one of the deadliest episodes of racial violence in US history, left 38 dead and shaped the county’s segregated housing patterns for decades. By 1960 the county held more than five million people, a level it has roughly kept since.

Getting There

Cook County is the hub of the Midwest interstate network: I-90, I-94, I-290, I-294, I-355, I-55, I-57 and I-80 all cross it, converging on Chicago. Metra commuter rail runs eleven lines through the county and the CTA covers Chicago and the inner suburbs.

• Milwaukee — about 90 miles north
• Rockford — about 90 miles northwest
• Indianapolis — about 185 miles southeast
• Springfield — about 200 miles southwest

O’Hare International Airport and Midway International Airport are both inside the county, in Chicago itself. Amtrak’s national network converges on Chicago Union Station, the largest rail hub between the coasts, with additional suburban Amtrak stops at Glenview and Summit.

Frequently Asked Questions

Where is Cook County located?
In northeastern Illinois on Lake Michigan, containing Chicago and its inner suburbs, bordered by Lake, McHenry, Kane, DuPage and Will counties and by Lake County, Indiana.

What is the population of Cook County?
5,275,541 at the 2020 census, with a 2025 estimate of 5,194,625. It is the second-most populous county in the United States after Los Angeles County.

How big is Cook County?
1,635 square miles in total, of which 945 square miles (2,448 km2) are land; the remaining 42.2 percent is Lake Michigan water.

What is the county seat of Cook County?
Chicago, which has been the seat since the county was organised in 1831 and holds about 52 percent of its population.

How many towns and cities are in Cook County?
136 municipalities lie wholly or partly in the county: 23 cities, 112 villages and one town, Cicero. About 28 of them straddle into neighbouring counties.

What ZIP codes does Cook County use?
215 codes from 60004 to 60827. Chicago uses the 606 prefix; the suburbs use 600xx through 605xx plus a 607xx–608xx band of near-in enclaves.

Does Cook County have townships?
Yes — 29 of them, but only outside Chicago and Evanston. Chicago abolished its townships in 1902, though the assessor still uses old township lines for property assessment.

More counties in this state: Towns and villages in Illinois.