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Free Closing Tool
Illinois Property Tax Proration Calculator
Calculate property tax prorations for Illinois residential real estate closings. Because Illinois taxes are paid in arrears, accurate proration ensures the seller credits the buyer fairly at closing.
Illinois property taxes are paid in arrears — meaning the taxes you pay in any given year cover the prior year. At a residential closing, the seller provides a credit to the buyer to cover the taxes that have accrued during the seller's ownership but haven't been billed yet. Getting this number right matters: an incorrect proration can cost either party hundreds or thousands of dollars.
Illinois has collected property taxes in arrears since the state's early days, a system rooted in the practical reality that local assessors needed time to value land and calculate tax rates before issuing bills. By the time the Illinois Constitution of 1870 formalized property tax procedures, the pay-in-arrears cycle was already standard practice. The result is a billing lag that persists today: taxes assessed for one year are not billed and collected until the following year (or later, in Cook County). That lag is precisely why proration at closing requires careful calculation — without it, a buyer would be stuck paying taxes for the seller's period of ownership.
How Illinois Tax Proration Works
Standard Tax Proration
Illinois Tax Proration Calculator
Calculate real estate tax prorations for Illinois properties (taxes paid in arrears)
Calculation Results
Tax proration breakdown for the closing
Enter tax information and closing date to calculate proration
About Illinois Tax Proration
Paid in arrears: Since Illinois property taxes are paid in arrears, tax proration ensures that the buyer is fairly compensated for the tax bills they will receive after the closing, for the period in time in which the seller still owned the property.
Two installments: The property tax bills are typically paid in two installments, usually due in the spring and fall of the following year. In many counties, the first installment is due around June 1st and the second around September 1st.
Assessed Value Changed
For new construction, reassessment, or appeal
Tax Proration — When the Assessed Value Has Changed
Use this calculator when the property has been reassessed, newly constructed, or the assessed value has changed
Cook County residential is 10%. Other counties may vary.
The state equalization factor varies by county and tax year. Check your county assessor's website for the current factor.
Your composite local tax rate can be found on your most recent tax bill or your county clerk's website.
Exemptions (Optional)
Enter dollar amounts from your tax bill. These reduce the total tax before proration. Leave blank or enter 0 if not applicable.
Calculation Results
Step-by-step tax computation and proration breakdown
Enter property details and closing date to calculate the tax proration
When to Use This Calculator
Changed assessed value: Use this calculator when the property's assessed value has changed and the most recent tax bill no longer reflects the expected taxes — for example, after new construction, a reassessment, a successful tax appeal, or when the property has been recently improved or subdivided.
How it works: This calculator computes the estimated annual tax from scratch using the property value, assessment level, state equalizer factor, and local tax rate — then prorates that amount to the closing date. The result is an estimate; actual taxes may differ based on exemptions, special assessments, and final equalization factors.
Frequently Asked Questions
Why are Illinois property taxes paid in arrears?
Illinois property taxes are billed and paid a year or more after the tax year they cover. For example, a homeowner pays their 2024 property taxes in 2025. This means that at closing, the seller owes taxes for the period they owned the property but those taxes haven't been billed yet. The proration credit estimates this amount so the buyer isn't stuck paying the seller's share.
What proration percentage should I use?
The proration percentage adjusts the most recent tax bill to account for expected increases. A common starting point is 105% (meaning a 5% estimated increase), but the actual percentage is negotiated between buyer and seller during attorney review. If taxes have been stable, 100% may be appropriate. If a reassessment or new construction has occurred, a higher percentage or the assessed value calculator may be more accurate.
What is the difference between the two calculators?
The Standard Tax Proration Calculator uses the most recent tax bill and adjusts it by a negotiated percentage — this is the method used in most Illinois residential closings. The Assessed Value Calculator computes the estimated tax from scratch using the property value, assessment level, state equalizer, and local tax rate. Use the second calculator when the most recent tax bill no longer reflects expected taxes, such as after new construction, a reassessment, or a successful tax appeal.
How does Cook County tax proration differ from other counties?
Cook County splits property tax bills into two installments. The first installment (due around March 1) is 55% of the prior year's total tax bill. The second installment (due around August 1) is the balance based on the current year's assessment, equalization, and tax rate. Other Illinois counties typically issue a single annual bill. Our calculators account for these differences.
Are these calculators a substitute for legal advice?
No. These tools provide estimates to help you understand how tax prorations work in Illinois real estate closings. The actual proration amount depends on the specific terms negotiated in the contract, applicable exemptions, and the most current tax data. Your closing attorney will verify the final proration on the settlement statement.