How is child support calculated in Illinois?

Using the Income Shares Guideline Model since July 1, 2017, replacing the old percentage-of-noncustodial-parent-income model. Both parents' net incomes are combined, the basic obligation is read from the HFS Schedule of Basic Child Support Obligations, and each parent's percentage share of that combined income determines their portion.

Does shared parenting change the calculation?

Yes. HFS maintains a separate worksheet for cases where parents have shared physical care, using the standardized tax amount to determine net income and adjusting the obligation for the shared-care arrangement.

How do I request a modification in Illinois?

The standard is a substantial change in circumstances, and to qualify for a modification, the new calculated amount generally must differ from the existing order by at least 20%, or by at least $10 per month.

How is child support enforced in Illinois?

Income withholding for support is the primary method, automatically deducted from an employed paying parent's wages with no opt-out. Illinois HFS also uses tax refund interception, driver's license suspension, and contempt of court proceedings for continued nonpayment.

Illinois, United States

Illinois Child Support: A Parent's Guide

Q: What are the specific calculation steps?

The court determines each parent's monthly net income, adds both incomes together, selects the corresponding amount from the schedule of basic child support obligations, and calculates each parent's percentage share of that basic obligation.

How child support works in Illinois, in plain English: the income shares guidelines, modification, and HFS enforcement.

Legal sources reviewed: August 10, 2026

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In short: Illinois has used the Income Shares Guideline Model since 2017, combining both parents' net incomes against a state schedule and apportioning by each parent's share. A modification generally requires a 20% (or $10/month) difference from the existing order, and enforcement runs through automatic income withholding administered by the Department of Healthcare and Family Services (HFS).

This is an educational overview written for parents, not lawyers. It isn't legal advice. For anything beyond general understanding, talk to a licensed Illinois family law attorney.

What Illinois generally provides

Under 750 ILCS 5/505, effective July 1, 2017 (Public Act 99-0764), Illinois replaced its older percentage-of-noncustodial-parent-income model with the Income Shares Guideline Model: both parents' net incomes are combined and matched against the HFS Schedule of Basic Child Support Obligations.

Important terminology

Income Shares Guideline Model: Illinois's current calculation method, adopted in 2017, replacing the earlier flat-percentage approach.
HFS: the Illinois Department of Healthcare and Family Services, which maintains the schedule, worksheets, and administers the Division of Child Support Services (DCSS).
Shared physical care worksheet: a separate HFS worksheet used when parents share physical care of the children.

How the calculation works

Four defined steps

The court must determine each parent's monthly net income, add the parents' monthly net incomes together, select the corresponding amount from the schedule of basic child support obligations, and calculate each parent's percentage share of that basic obligation.

A separate path for shared parenting

Where parents have shared physical care, HFS maintains a dedicated worksheet using the standardized tax amount to determine net income, reflecting that shared-care arrangements change the underlying cost allocation compared to a primary-custody setup.

Requesting or defending a modification

Illinois requires a substantial change in circumstances to modify support, and as a practical threshold, the newly calculated amount generally needs to differ from the existing order by at least 20%, or by at least $10 per month, to qualify. HFS's Division of Child Support Services (DCSS) administers modification requests alongside establishment and enforcement. See the child support calculator and guideline finder for jurisdiction-specific calculation sources and guidance.

How enforcement works

Income withholding for support is Illinois's primary enforcement method: payments are deducted directly from an employed paying parent's wages, automatically, with no opt-out available to the paying parent. Beyond wage withholding, Illinois uses interception of tax refunds, driver's license suspension, and contempt of court proceedings for continued nonpayment. HFS can be reached at 800-447-4278 for questions about withholding and related enforcement matters.

Common questions

Is a small income change enough to modify support? Generally not on its own, since Illinois applies a 20% (or $10/month) threshold before a recalculated amount qualifies as a modification, though other substantial changes (job loss, a new child, disability) can still support a request even if the resulting dollar difference is close to that line.

Does the income shares model apply the same way to shared-care families? No, HFS provides a distinct worksheet for shared physical care that adjusts the standard calculation, so don't assume the primary-custody worksheet applies if your family's schedule is closer to equal.

What if my co-parent isn't paying? Contact HFS's Division of Child Support Services to pursue income withholding, tax refund interception, or license suspension.

Practical considerations

Because Illinois's 2017 shift to income shares changed the underlying formula from the older percentage model, don't rely on outdated information or an old calculation if your order predates the switch, confirm the current figures with HFS or a family law attorney. For the custody and allocation-of-parental-responsibilities rules that often accompany a support case, see the Illinois custody guide.

Official Illinois resources

FAQ

Questions, answered

This information is provided for educational purposes only and is not legal advice. Family law varies by individual circumstances and may change over time. Consult a qualified attorney or appropriate official resource for advice about your situation.