If Illinois Split in Two, These 5 Counties Change Everything

If Illinois ever actually split in two, these five counties could completely change the map.

And the reason is money.

DuPage.

Lake.

Will.

Kane.

McHenry.

Together, those five suburban counties currently produce more economic output than the other 96 Illinois counties outside Cook County combined.

Five counties.

Versus ninety-six.

That is where the Illinois split discussion starts looking very different from the usual picture of Chicago on one side and rural Illinois on the other.

Part One Was About the Border. This Is About the Money.

In the first Illinois Split story, I looked at where a new state line could actually fall and what that could mean for Frankfort, Mokena, New Lenox, Tinley Park, Orland Park and the rest of the southwest suburbs.

This part asks a different question.

If Illinois ever seriously tried to divide itself, where would the economic power actually be?

And once you look at the numbers, the answer is not as simple as Chicago versus Downstate.

Five Counties Versus 96

The latest available county-level GDP figures from the U.S. Bureau of Economic Analysis are for 2024.

Add together the economies of the five traditional Chicago collar counties:

• DuPage County
• Lake County
• Will County
• Kane County
• McHenry County

Combined, they produced approximately $316 billion in current-dollar economic output.

Now take every Illinois county except Cook and those five.

That leaves 96 counties.

Combined, those 96 produced approximately $286 billion.

That does not make the other 96 counties unimportant.

It does show how misleading the phrase “Chicago versus Downstate” can be.

Economically, a huge chunk of the non-Cook side is still tied to the Chicago metropolitan area.

The New State's Economic Center Would Be in the Suburbs

Under the Cook-versus-101-county version of the proposal, the 101 counties outside Cook currently produce roughly $602 billion in annual economic output.

The five collar counties account for slightly more than half of that.

Approximately 52.5%.

So if you drew the line according to the current proposal and froze today's economy in place, the economic center of the hypothetical new state would not be deep in Southern Illinois.

It would be concentrated heavily in the suburbs surrounding Chicago.

That is a very different picture than most people probably have in their heads when they hear about Illinois splitting apart.

The Income Tax Numbers Point in the Same Direction

GDP measures economic production. It is not the same thing as tax revenue.

But Illinois individual income-tax data tells a similar story.

According to the Illinois Department of Revenue's final Tax Year 2023 county report, residents of DuPage, Lake, Will, Kane and McHenry counties generated approximately $7.5 billion in Illinois individual income-tax liability.

The other 96 non-Cook counties combined generated approximately $5.7 billion.

Again, that is not total state revenue.

It is specifically Illinois individual income-tax liability reported by county.

But the pattern matters.

Whether you look at economic output or resident income taxes, the collar counties carry enormous weight on the non-Cook side of the equation.

Will County Makes This Local

This is not just a DuPage County story.

Will County alone produced roughly $50 billion in economic output in 2024.

And Will County also contains infrastructure that helps move goods through the entire Chicago region and across the country.

I-55.

I-57.

I-80.

I-355.

Major freight rail lines.

Massive intermodal facilities around Joliet and Elwood.

The Will County Center for Economic Development says the county contains five Class I railroads and three major intermodal facilities.

Its BNSF and Union Pacific intermodal operations handle millions of shipping containers annually.

That matters because the Chicago economy and the suburban economy are not two separate machines sitting next to each other.

They are connected pieces of the same regional system.

This Is Where the Simple Map Starts Falling Apart

Politically, it is easy to draw Cook County in one color and the other 101 counties in another.

Economically, the line is much harder to see.

People live in one county and work in another.

Freight moves across county lines.

Businesses serve the entire region.

Transit crosses county boundaries.

Water systems cross county boundaries.

Supply chains do not stop at the Cook County line.

So while a future state border could theoretically be drawn there, the economy underneath it would still be deeply connected.

Could the 101 Counties Survive Without Cook?

This is where we have to be careful.

The existing numbers tell us something important, but they do not tell us the future.

The 101 counties outside Cook currently contain more than 7.5 million people and generate roughly $602 billion in economic output.

That is a substantial economy by any normal measure.

But that does not automatically mean a new state would be financially stronger, weaker, richer or poorer.

There is no complete financial plan for two functioning states.

A real separation would have to sort through questions involving:

• state debt,
• pension obligations,
• state assets,
• tax rates,
• state agencies,
• universities,
• highways,
• public employees,
• school funding, and
• the final state boundary itself.

Economic output today cannot tell us exactly what businesses, taxpayers or governments would do afterward.

Cook County Is Still an Economic Giant

None of this means Cook County suddenly becomes irrelevant.

Far from it.

Cook County alone produced approximately $546 billion in economic output in 2024.

That is close to half of the entire Illinois economy.

So this is not a story about Chicago being weak.

It is a story about the collar counties being much stronger than the usual political map suggests.

And that creates a very different kind of split.

Maybe the 33 Counties Aren't the Most Important Counties

Dozens of Illinois counties have already voted in favor of advisory questions exploring separation from Cook County.

Those votes are what pushed this issue into the public conversation.

But economically, those may not be the counties that matter most if the idea ever became serious.

The real question may be what happens if these five counties ever have to pick a side.

DuPage.

Lake.

Will.

Kane.

McHenry.

Because without them, the economic math changes dramatically.

The Bottom Line

The Illinois split debate is usually presented as a fight between Chicago and Downstate Illinois.

The numbers show something much more complicated.

Five collar counties currently produce more economic output than the other 96 non-Cook counties combined.

They also generate more Illinois individual income-tax liability than those 96 counties combined.

And under the current Cook-versus-101-county proposal, those five counties would make up more than half of the non-Cook side's existing economy.

So if Illinois ever seriously tried to divide itself, the biggest fight might not actually be over Chicago.

It might be over who gets the suburbs.

Sources and Data

• Illinois General Assembly: HJR 16
• U.S. Bureau of Economic Analysis: GDP by County
• Illinois Department of Revenue: Tax Year 2023 Individual Income Tax Statistics by County
• U.S. Census Bureau: Illinois Population Estimates
• Will County Center for Economic Development: Transportation and Freight Assets
• Chicago Metropolitan Agency for Planning: Regional Freight System Assessment