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Updated for 2026 guidelines

Illinois Child Support Arrears Interest Calculator

Illinois puts a real price on missed support. Every unpaid installment turns into a judgment by operation of law, and judgments earn 9 percent simple interest. This page runs that math and explains who actually adds the interest to a balance.

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Estimate an Illinois arrears balance with interest

Arrears owed

The unpaid support amount itself, before any interest.

Accrual period
How do you want to enter the period?

Estimated arrears with interest: $5,450.00

Interest of $450.00 at 9% simple per year (735 ILCS 5/12-109(b)).

How this estimate was computed
Line itemAmount
Arrears principal735 ILCS 5/12-109(b) (child support judgments bear interest; monthly computation at 1/12 of the Sec. 2-1303 statutory rate on the unpaid balance) (Illinois General Assembly (ilga.gov))$5,000.00
Simple interest at 9% per year for 12 month(s) (735 ILCS 5/12-109(b))Every judgment arising by operation of law from a child support order bears simple interest at the 9% judgment rate (735 ILCS 5/2-1303), commencing 30 days from the effective date of each such judgment. HFS/DCSS does not routinely compute interest administratively; obligees may seek it judicially.735 ILCS 5/12-109(b) (child support judgments bear interest; monthly computation at 1/12 of the Sec. 2-1303 statutory rate on the unpaid balance) (Illinois General Assembly (ilga.gov))$450.00
Arrears principal plus interest735 ILCS 5/12-109(b) (child support judgments bear interest; monthly computation at 1/12 of the Sec. 2-1303 statutory rate on the unpaid balance) (Illinois General Assembly (ilga.gov))$5,450.00

Assumptions

  • Accrual period expressed as 12 month(s); partial months prorate.

Estimate only. The court or state agency calculation controls.

Illinois arrears interest under 735 ILCS 5/12-109(b); rules last reviewed 2026-08-06.

How arrears interest works in Illinois

When an Illinois support payment comes due and goes unpaid, it does not just sit as a debt. Under 735 ILCS 5/12-109(b), each unpaid installment becomes a judgment by operation of law, with no court hearing needed, and every such judgment bears interest starting 30 days after it arises.

The rate comes from the general judgment statute, 735 ILCS 5/2-1303: 9 percent per year. Support orders entered or modified since 2006 must say on their face that unpaid amounts accrue simple interest. Simple means the interest grows on the missed payments themselves, not on earlier interest, so a $10,000 balance left alone for a year adds $900, not more.

Here is the practical catch. HFS and its Division of Child Support Services do not routinely compute or collect this interest on the administrative ledger. The statutory right exists, but turning it into money usually means asking a court to reduce the interest to a sum as part of enforcement. Parents are sometimes surprised that the state balance and the with-interest balance are different numbers.

The principal itself is untouchable in reverse. Illinois law does not allow retroactive modification of support that already came due, so a court can change what accrues going forward but cannot erase installments that are already judgments.

Interest rate
9% per year, simple, under 735 ILCS 5/12-109(b) and 5/2-1303source
When it starts
30 days after each unpaid installment becomes a judgmentsource
Who computes it
Not HFS routinely; interest is typically pursued through the courtsource
Compounding
None; the statute requires simple interest on support judgmentssource

How to use this estimate

Enter the arrears principal and how long it has gone unpaid. The tool applies 9 percent simple interest over that period, which is the statutory ceiling for a balance that has sat still the whole time.

Real ledgers are messier than one number. Each missed installment is its own judgment with its own 30-day start date. A court computing interest installment by installment will land near this figure for a balance that sat untouched, and below it when payments trickled in along the way.

If you are owed the money, this number is useful leverage: it shows what a judicial interest determination could add to the state ledger balance. If you owe it, the same number shows why waiting is expensive.

What this estimate includes

  • The 9 percent statutory rate from 735 ILCS 5/2-1303, applied as simple interest
  • Proration for partial years, matching the simple-interest arithmetic
  • The statutory citations a court would rely on to add interest to a balance

What it leaves out

  • Installment-by-installment accrual with separate 30-day start dates
  • Credit for partial payments and enforcement collections along the way
  • The HFS administrative ledger balance, which usually excludes interest
  • Interest a court has already reduced to judgment in your case

Illinois arrears FAQs

Does unpaid child support earn interest in Illinois?

Yes. Each unpaid installment becomes a judgment by operation of law, and under 735 ILCS 5/12-109(b) it bears interest starting 30 days later at the 9 percent judgment rate from 735 ILCS 5/2-1303. The interest is simple, not compound.

Why does my state ledger not show any interest?

Because HFS does not routinely compute interest administratively. The agency tracks the principal: the payments charged and the payments made. The 9 percent interest is a statutory right the person owed support can pursue, usually by asking the court to determine and add it during enforcement.

Is the 9 percent compounded?

No. Section 505(b) requires orders to state that unpaid amounts accrue simple interest, so the rate applies to the missed payments only. A $5,000 judgment balance grows by $450 a year, every year, without interest on interest.

Can old Illinois arrears be forgiven or reduced?

Not by the court going backward. Illinois bars retroactive modification, so installments that already came due stay owed. The parent who is owed the money can voluntarily settle or waive amounts owed to them, and debt assigned to the state has its own separate programs, but no judge can rewrite the past-due balance.

When does the interest clock start?

Thirty days after each installment becomes a judgment, which happens by operation of law when the payment goes unpaid. That means a balance built over years has many start dates, one per missed payment, which is why a precise court calculation works installment by installment.

Does this interest rule apply to maintenance arrears too?

Section 12-109(b) speaks to judgments arising from orders for support. Maintenance arrears reduced to judgment bear judgment interest under the general 9 percent rule as well, but the automatic judgment-by-operation-of-law mechanics are written for support orders, so treat maintenance arrears as a question for counsel.

Official sources

Official sources last verified: .

Reviewed by SupportDecode Editorial Team Page content reviewed Sources Methodology Update log

Changelog: page first published with the 9 percent simple judgment interest rule and the HFS interest policy nuance. Material changes are dated in the update log.