What Is Chicago?

The UN changed its ruler.

Until 2025 the UN counted each city the way its own country did. For the United States that was the Census Bureau's urbanized area, the built-up footprint drawn from the 2010 census, which the UN's 2018 edition carried forward with its growth model to about 8.9 million. The 2025 edition applies one rule to the whole planet, and Chicago comes out at , ranked in the world. Jakarta, measured the same way, is first.

Same city, same decade. Only the definition moved. Five official Chicagos follow, each with its own rule and its own author. For the Chicagos before the statisticians, read What Was Chicago?

Step one: ignore every border.

The rule starts from a grid of one-kilometre squares laid over the whole planet, with an estimate of how many people live in each square. City limits, county lines and zip codes play no part.

The grid is drawn in an equal-area world projection, which is why it sits at a slight tilt over Chicago. Hover a square to read its population.

Step two: keep only the dense squares.

A square counts as dense if it holds at least 1,500 people per square kilometre of land. Water does not count toward the area, so a lakefront square with a sliver of shore can pass on a few hundred residents. Everything else goes dark.

Step three: dense squares must touch side to side.

Dense squares that share an edge belong to the same cluster. Squares that only meet at a corner do not. Watch the connection spread from the Loop and stop wherever the chain breaks.

If corners counted, the Loop's cluster would pick up another people.

Step four: a cluster needs 50,000 people.

There are dense clusters in the region. Most are a few squares around a town centre. The bar rises to 50,000 and everything under it drops out. clusters survive. They are the seeds of the urban centres.

Step five: fill the small holes.

Parks, rail yards, cemeteries and factory land leave gaps inside a cluster. Any enclosed gap smaller than 15 square kilometres is filled in. Nothing larger is.

holes in the region, squares in all.

Step six: smooth the edges by majority vote.

Each square just outside a centre looks at its 3-by-3 neighbourhood. Water squares abstain. If at least half of the voters are inside, the square joins. The vote repeats until nothing changes: rounds for the Chicago region.

The result: square kilometres, people, and thirteen neighbours.

Every break in the chain of dense squares became a border. Hammond, Arlington Heights, Elmhurst and Schaumburg are each their own city to the UN. Replaying the rule on the public grid reproduces the UN's boundary exactly, all squares.

Two years earlier the same mapmakers drew Chicago twice as big. One clause explains it.

The grid is built by the European Commission's Joint Research Centre, and its own 2023 release used the same six steps with one extra clause: a square also counts as urban if at least half of its land is built up, even when few people live there. That clause admits airports, rail yards, factories, malls and freight terminals, and those squares sit exactly in the gaps between Chicago and its neighbours.

Watch the squares with fewer than 1,500 people per square kilometre that the 2023 rule still counted, most of them through that clause, bridge the gaps: fourteen centres become one, square kilometres and million people. The 2025 grid made for the UN dropped the clause and counts people only. Neither is a mistake; they answer different questions.

The city limits: a record, not a rule.

The oldest of the official Chicagos is not computed at all. It is the sum of every annexation ordinance the City Council has passed since 1837, each one petitioned by landowners or approved by the voters of the area joining, and recorded with the county. The Census Bureau collects the result every year and publishes it; the file drawn here is the 2020 edition.

Nobody can recompute this line. Anyone can download it. That is what makes it the easiest Chicago to agree on and the hardest to defend as a measure of the city.

The urban area: houses, not people.

The Census Bureau's Chicago is built from housing. Blocks with at least 1,275 homes per square mile seed it; blocks with at least 425 join if they are within a short hop or a road jump; enclaves are filled and airports added by rule; anything under 2,000 homes or 5,000 people is dropped. The 2020 criteria were published in the Federal Register in March 2022.

Reproducible in principle from public block counts, but the special rules run to many pages, so this page draws the published boundary rather than replaying it.

The metro: whole counties, tied by commuting.

The Office of Management and Budget's Chicago is a list of counties. A county with an urban area of 50,000 or more is a central county; a neighbouring county joins when a quarter of its workers commute into the core, or a quarter of its jobs are held by people who live there. Counties join whole, so the line runs along county borders through farmland.

Reproducible from the public county-to-county commuting table. The 2023 delineation, the first under the 2020 standards, gives Chicago-Naperville-Elgin fourteen counties in three states.

Five Chicagos, five authors, three kinds of rule.

Two are computed from grids and can be replayed by anyone with the public data: the UN's 2025 centre, which this page reproduces exactly, and the Joint Research Centre's 2023 version with its built-up clause. One is computed from block counts by a published but intricate procedure: the Census Bureau's urban area. One is computed from commuting flows at the county level: OMB's metro. And one is not computed at all: the city limits, a legal record.

None of them is wrong. Each answers a different question, and each has a different author: a city council, a UN agency, a European research centre, a statistical bureau, a budget office.

Toggle them on and off in the panel.

In zip codes.

For a survey the question becomes which zip codes are in. Counting a zip when at least half its residents live inside a boundary gives zips for the city, for the UN's Chicago, for the older rule, for the urban area and for the metro.

Hover or tap a zip to see where it lands. Each zip takes the colour of every boundary you have switched on that includes it, so the overlaps show through.