The equalization factor, and why your assessment changed without anyone visiting
The state multiplier adjusts a whole county at once. It is not a reassessment, and it is not appealable.
Every year the Illinois Department of Revenue publishes a number for each county called the equalisation factor, or more commonly the multiplier. In most counties it is close to 1.0 and nobody notices. In Cook County it was 3.0355 for 2024, and it is doing an enormous amount of work.
What problem it solves
Illinois law says property should be assessed at one third of fair cash value. In practice counties drift — assessments lag a rising market, or an office is more conservative than its neighbour — and Cook County deliberately assesses residential property at 10% under its own ordinance.
That creates a real problem. Many taxing bodies, particularly school districts, cross county lines. If County A assesses at 30% of value and County B at 33%, a district straddling both would be taxing identical houses on different bases, and state aid formulas that compare counties would be comparing different things.
So the Department calculates, for each county, the factor that would bring its aggregate assessments to the statutory third, and applies it. The result is the equalised assessed value, and that — not your assessed value — is what your tax rate is applied to.
How your bill actually assembles
| Step | What happens | Example (Cook) |
|---|---|---|
| 1 | Assessor sets assessed value | $30,000 |
| 2 | × state equalisation factor | × 3.0355 |
| 3 | = equalised assessed value | $91,065 |
| 4 | − exemptions | − $10,000 homestead |
| 5 | = taxable value | $81,065 |
| 6 | × composite tax rate | × 7.5% |
| 7 | = your bill | ≈ $6,080 |
Note where the exemption lands — after equalisation. That is why an Illinois homestead exemption expressed in assessed dollars is worth more than it first looks.
Why your assessment changed without anyone visiting
This is the question the multiplier most often answers. Your assessed value can sit unchanged while your equalised assessed value moves, because the Department revised the county's factor. Nobody looked at your house. Nobody reassessed your township. A statewide calculation about aggregate county behaviour moved the number your rate is applied to.
It works both ways. A rising multiplier raises everyone's equalised value together; a falling one lowers it. Because it applies uniformly across the county, it does not change your position relative to your neighbours at all.
What you can and cannot appeal
You cannot appeal the multiplier. It is set by the state, applies to an entire county, and no individual property circumstance bears on it. An appeal that argues the multiplier is unfair will go nowhere, because the office hearing it has no authority over it.
You can appeal the assessed value it is applied to, which is the only lever you have — and because the multiplier is large in Cook, the lever is unusually effective there. Each dollar you remove from your assessed value removes $3.04 from your equalised value.
Where to find yours
The Illinois Department of Revenue publishes final and tentative multipliers by county each year, and your county clerk publishes them alongside the rate calculations. Your tax bill will typically show the equalised assessed value directly, which lets you work backwards: divide it by your assessed value and you have the factor that was applied.
If those two numbers do not divide to something close to the published county factor, that is worth a phone call — it usually means an exemption or a rate area you were not expecting, and occasionally it means an error.