How to calculate closing date prorations in Illinois

Quick takeaways

• In Illinois real estate closings, property taxes are typically prorated using the “last ascertainable tax bill” approach—meaning you prorate based on the most recent tax information available before closing.
• Illinois law requires that before closing, the parties prorate “taxes, assessments, expenses and other costs” for the relevant period. See 35 ILCS 200/21-15.
• DocketMath’s closing-date-prorations calculator computes prorated shares for the buyer and seller using dates and the tax amount you provide.
• The output is a time-based allocation that depends on the closing date and the tax coverage period you select.
• Note on rule scope: No claim-type-specific sub-rule was found for Illinois in the materials reviewed. So use the general/default period approach (i.e., prorate the standard coverage window tied to your agreement and the tax period).

Inputs you need

To calculate Illinois closing date prorations in DocketMath, gather these inputs first. The calculator’s output changes based on each one—especially the closing date and the tax period you’re prorating.

Required inputs (typical)

• Closing date (Illinois closing date): the date ownership/tax responsibility shifts under your purchase agreement.
• Proration start date: usually the start of the tax period you’re prorating (often tied to the assessment/tax year timeline).
• Proration end date: typically the end of the tax period (or the day before responsibility shifts—depending on how your agreement defines the boundary).
• Tax amount to prorate (the “last ascertainable tax bill”): the most recent bill or tax figure available before closing.
• Buyer share vs. seller share convention: many closings treat the closing date as belonging to either the buyer or the seller depending on contract language. DocketMath follows its configured convention—confirm it matches your agreement language (or adjust your date inputs so the day-count matches your contract expectation).

Data sources you’ll use

• Your lender/closing statement assumptions (if already provided).
• The most recent property tax bill or tax amount you can determine before closing.
• A reference for Illinois tax timing and arrears context (prorations can be based on what’s known even if the final bill is still pending).

For practical context on how Illinois closings handle timing and arrears, see:
https://www.chicagoclosings.com/across-illinois-real-estate-taxes-paid-arrears/

How the calculation works

DocketMath’s closing-date-prorations calculator performs a time allocation:

1. Determine the proration window

   • You supply a start and end date that define the period covered by the tax amount you’re prorating.
2. Compute total days in the period

   • DocketMath calculates the number of days between the proration start and proration end (according to the calculator’s day-count method).
3. Compute days attributable to the buyer (or seller)

   • The calculator counts days up to the closing date based on its convention (or the one you enforce through your date inputs).
4. Apply the “last ascertainable tax bill” figure

   • Illinois practice commonly uses the most recent tax bill figure available before closing as the base amount to prorate.
5. Output prorated amounts

   • You get buyer prorations and seller prorations (and, if enabled, totals and per-day rates).

Illinois rule: prorate “taxes, assessments, expenses and other costs”

Illinois law provides the backbone for prorating transaction costs connected to taxes:

• 35 ILCS 200/21-15 (emphasis added):
  “Before the closing of any real estate transaction, the buyer and seller shall prorate taxes, assessments, expenses and other costs …”

In practice, that means you treat these costs as attributable to a period of time around closing, rather than assigning the full amount to only one side.

“Last ascertainable tax bill” (default approach)

Because Illinois closings frequently deal with timing mismatches (tax amounts may not be finalized when the deal closes), the calculator needs a single tax amount to divide across the proration window.

Use this approach when:

• current-year taxes are not finalized, or
• you only have the prior or latest determinable bill amount available before closing.

From a transaction-practice standpoint, Illinois closings often address timing by prorating using a known figure and then reconciling later if required by the purchase agreement. For background on arrears and tax timing, see the Illinois context reference:
https://www.chicagoclosings.com/across-illinois-real-estate-taxes-paid-arrears/

Sanity-check the relationship (no legal advice)

A common way to verify you entered inputs correctly:

• Tax per day ≈ (Last ascertainable tax bill) ÷ (Total days in proration period)
• Buyer prorated amount ≈ Tax per day × (Buyer-attributable days)

If buyer’s share should increase when closing happens later (more days allocated to buyer), your dates should reflect that expectation.

Start here (calculator)

Use: /tools/closing-date-prorations

Common pitfalls

These issues commonly cause Illinois closing date proration results that don’t match the settlement statement.

Date boundary errors

• Closing date misinterpretation: Some agreements treat the closing date as belonging to the seller; others treat it as belonging to the buyer. If the calculator’s day boundary convention doesn’t match your agreement, totals may not reconcile.
• Wrong proration start/end: Selecting a full tax year window vs. the partial agreement window produces a plausible number—just not the expected one.

Wrong tax base figure

• Not using “last ascertainable” information: If you input a later-adjusted or finalized amount rather than what was determinable before closing, your proration can be off.
• Confusing assessments and taxes: Although both relate to property cost allocation, the statute references “taxes, assessments, expenses and other costs.” Make sure your base number matches what you’re intending to prorate under your agreement.

Reconciling expectations after closing

• Assuming taxes are “settled” at closing: Property tax liability mechanics often mean the bill is due later. Proration numbers may still require post-closing reconciliation under the contract terms.
• Forgetting proratable items beyond taxes: The statute references more than taxes. If your closing agreement prorates additional recurring charges, you may need additional computations beyond property tax alone.

Sources and references

• 35 ILCS 200/21-15 — Illinois statutory requirement that buyer and seller prorate “taxes, assessments, expenses and other costs” before closing.
• Transaction-practice context reference (Illinois tax timing/arrears background):
  https://www.chicagoclosings.com/across-illinois-real-estate-taxes-paid-arrears/
• DocketMath proration tool: /tools/closing-date-prorations

Next steps

1. Identify the most recent property tax bill amount you can determine before closing—this is the figure you’ll enter as the “last ascertainable tax bill.”
2. Confirm the proration start and end dates that correspond to the tax period covered by that bill (and how your agreement defines the closing-day boundary).
3. Enter into DocketMath:

   • closing date
   • proration window start date
   • proration window end date
   • the tax base amount
4. Run the calculator and compare buyer/seller outputs to the numbers in your draft settlement statement.
5. If it still doesn’t match, revisit only two things first:

   • whether the closing-day inclusion belongs to buyer or seller under the agreement, and
   • whether the date window matches the tax bill period you used.

Start here: /tools/closing-date-prorations

Related reading

• How to calculate closing date prorations in California — Full how-to guide with jurisdiction-specific rules
• How to calculate closing date prorations in Florida — Full how-to guide with jurisdiction-specific rules
• How to calculate closing date prorations in New York — Full how-to guide with jurisdiction-specific rules