To calculate property tax in Illinois, you run your home through a four-step formula: take the fair market value, convert it to an assessed value, multiply by the state equalization factor, subtract any exemptions you qualify for, and multiply what’s left by your local aggregate tax rate. The arithmetic is simple. What makes it feel complicated is that each number comes from a different government body, and Cook County plays by its own rules at the start.
The Four Numbers You Need
Before you touch the formula, pull these from your most recent tax bill or your county assessor’s office:
- Fair market value. The estimated price your property would sell for. Your county assessor sets this and prints it on your Notice of Assessed Value.
- Assessment level. The percentage of market value that becomes taxable. In every county except Cook, it’s 33⅓ percent.
- Equalization factor. A county-wide multiplier the Illinois Department of Revenue publishes each year.
- Aggregate tax rate. The combined rate from every taxing body that covers your parcel: school district, municipality, park district, library district, fire protection district, and any others. It’s on your bill.
With those four in hand, the rest is multiplication and subtraction.
Step 1: Convert Market Value to Assessed Value
Multiply your fair market value by the statutory assessment level. Outside Cook County, that level is 33⅓ percent of fair cash value.1Justia. Illinois Code 35 ILCS 200 Title 3 A $300,000 home produces an assessed value of $100,000.
Cook County uses a classification system. Residential property (Class 2) is assessed at 10 percent of estimated market value; commercial property is assessed at 25 percent.2Cook County Assessor’s Office. Class 2 Residential Questions The same $300,000 home in Cook County has an assessed value of just $30,000. That gap gets closed in the next step.
Illinois law requires a full physical revaluation at least once every four years, the quadrennial cycle. Some counties split the work into four districts and reassess one quarter each year on a rolling basis. Between reassessments, individual properties can still be adjusted when something changes on them.
Step 2: Multiply by the Equalization Factor
Multiply your assessed value by the county’s equalization factor to get your Equalized Assessed Value, or EAV. The multiplier exists so that properties across the state end up valued on a comparable basis relative to the 33⅓ percent standard.3Illinois Department of Revenue. Cook County Final Multiplier Announced
In a downstate county with a multiplier of 1.0200, a $100,000 assessed value becomes an EAV of $102,000. The multiplier sits near 1.0 because the county’s assessments were already close to the 33⅓ percent target.
Cook County’s multiplier is much larger because it starts from a 10 percent assessment level. The final 2024 Cook County multiplier was 3.0355, so the $30,000 assessed value on that $300,000 home becomes an EAV of roughly $91,065.4Illinois Department of Revenue. 2024 Cook County Final Multiplier Announced After equalization, the two homes end up at comparable taxable values even though they started from very different assessment percentages.
Step 3: Subtract Your Exemptions
Exemptions are dollar reductions to your EAV, not percentage discounts, so each one lowers the base your tax is calculated on. Add up everything you qualify for, then subtract it from your EAV.
Keep going with the downstate example: an EAV of $102,000 minus a $6,000 General Homestead Exemption leaves a net taxable value of $96,000.
Which exemptions apply depends on who lives in the home and how it’s used. The common ones are summarized below so you can plug the right numbers into this step.
General Homestead Exemption
Any owner-occupied primary residence qualifies. The maximum EAV reduction is $10,000 in Cook County, $8,000 in counties bordering Cook County, and $6,000 in all other counties.5Illinois General Assembly. Illinois Code 35 ILCS 200/15-175 Many counties apply it automatically; confirm with your assessor that you’re getting it.
Senior Citizens Homestead Exemption
Homeowners 65 or older who occupy the home as their principal residence receive an additional reduction of up to $8,000 in Cook County and contiguous counties, or $5,000 elsewhere.6Illinois Department of Revenue. Property Tax Relief – Homestead Exemptions, PTELL, and Senior Citizens Real Estate Tax Deferral Program Some counties require only one application; others, including Cook, require annual filing.
Senior Citizens Assessment Freeze
This locks your EAV at the level it was in the year you first qualified. Your tax bill can still rise if the rate rises, but the base value stays frozen. You must be 65 or older, own and occupy the home as your principal residence, and meet the household income limit. For the 2026 tax year, that limit rises to $75,000 under Public Act 104-0452.7Effingham County, Illinois. 2026 Property Tax Senior Freeze Exemption Increases Maximum Household Income Eligibility Requirements to $75,000
Homestead Exemption for Persons With Disabilities
Homeowners with a documented disability who occupy the property as their primary residence receive a $2,000 annual EAV reduction. File Form PTAX-343 with proof of disability through your chief county assessment office.6Illinois Department of Revenue. Property Tax Relief – Homestead Exemptions, PTELL, and Senior Citizens Real Estate Tax Deferral Program
Homestead Improvement Exemption
Remodel, add a room, or rebuild after a catastrophic event, and the added value is partially shielded. The exemption covers up to $75,000 in fair cash value added by the improvement (which translates to $25,000 in assessed value at the 33⅓ percent level) and lasts four years from the date the work is completed and occupied.6Illinois Department of Revenue. Property Tax Relief – Homestead Exemptions, PTELL, and Senior Citizens Real Estate Tax Deferral Program Homeowners miss this one often, especially after a kitchen or bathroom overhaul.
Standard Homestead Exemption for Veterans With Disabilities
EAV reductions scale with the veteran’s service-connected disability rating as certified by the U.S. Department of Veterans Affairs:8Illinois General Assembly. Illinois Code 35 ILCS 200/15-169
- 30% to 49% disability: $2,500 annual exemption
- 50% to 69% disability: $5,000 annual exemption
- 70% or higher disability: the first $250,000 of EAV is completely exempt
A veteran at the top tier with an EAV below $250,000 effectively pays no property tax.
Returning Veterans Homestead Exemption
Veterans returning from active duty in an armed conflict receive a $5,000 EAV reduction for the year they return and the following year.6Illinois Department of Revenue. Property Tax Relief – Homestead Exemptions, PTELL, and Senior Citizens Real Estate Tax Deferral Program Veterans who make accessibility-related improvements to the residence also receive protection from increased assessments for seven years after the work is completed.9Illinois Department of Revenue. Property Tax Relief Information for Veterans and Persons with Disabilities
Step 4: Multiply by the Aggregate Tax Rate
Your annual tax equals the net taxable value multiplied by the aggregate tax rate for your tax code area. That rate is the sum of individual levies from every taxing district serving your parcel. One address can sit inside a school district, community college district, park district, library district, fire protection district, and municipality, each adding its slice.
Finishing the example: $96,000 net taxable value × 8.5 percent aggregate rate = $8,160 for the year. Your rate is printed on your bill or available from the county clerk.
How the Rate Gets Set
Each taxing district tells the county clerk the total dollars it needs (its levy). The clerk divides that levy by the total EAV inside the district to produce a rate. When values across the district rise, the rate can fall even if the levy is unchanged, because the same dollars spread over a larger base. The reverse happens too.
Why the Tax Cap Doesn’t Cap Your Bill
Most non-home-rule taxing districts are subject to the Property Tax Extension Limitation Law, often called PTELL or the “tax cap.” PTELL limits the annual increase in a district’s total tax extension to the lesser of 5 percent or the prior year’s increase in the Consumer Price Index.10Illinois Department of Revenue. What Is the Property Tax Extension Limitation Law It does not cap your individual bill. If your EAV rose faster than your neighbors’, your share of the capped levy grows. PTELL slows district-wide revenue growth, not individual assessments.
Putting the Full Calculation Together
For a $300,000 downstate home with the General Homestead Exemption and an 8.5 percent aggregate rate:
- Fair market value: $300,000
- × 33⅓% assessment level = $100,000 assessed value
- × 1.0200 equalization factor = $102,000 EAV
- − $6,000 General Homestead Exemption = $96,000 net taxable value
- × 8.5% aggregate tax rate = $8,160 annual property tax
Swap in your own four numbers and your own exemptions. The sequence doesn’t change.