Illinois child support guidelines
Illinois uses an income-shares model under 750 ILCS 5/505 — support is based on both parents’ net incomes. This page explains the rules and links the official HFS Estimator; it doesn’t compute your amount.
How the Illinois formula works
Since the July 1, 2017 switch from a flat percentage of the paying parent’s income, the guideline works in steps:
- Convert gross to net — both parents’ incomes are standardized using the state’s Gross-to-Net Income Conversion Table;
- Combine the net incomes and find the basic obligation on the Schedule of Basic Child Support Obligations for that income and the number of children;
- Allocate proportionally by each parent’s share of the combined net income;
- Adjust for overnights (shared parenting), health insurance, and childcare.
Because it draws on two statutory tables, Illinois provides the official HFS Child Support Estimator rather than a single percentage.
The 146-overnight shared-parenting rule
Illinois has a bright-line parenting-time threshold. When each parent has the child for 146 or more overnights a year (about 40% of the year), the case is “shared parenting” and the math changes: the basic obligation is multiplied by 1.5 before it’s divided between the parents, on the theory that both homes now carry real day-to-day costs. Drop below 146 overnights for one parent and the standard sole-parenting calculation applies — so a handful of nights around that line can meaningfully change the result.
Modifying an order, and how long support lasts
Under 750 ILCS 5/510, a parent can modify on a substantial change in circumstances — or, as a shortcut, whenever the order differs from the current guideline by at least 20% (and no less than $10 a month) without having to prove a change. Support in Illinois generally continues until the child turns 18 (or finishes high school, if later); college contribution is handled separately under the statute.
Use the official calculator
The Illinois Department of Healthcare and Family Services publishes the official Child Support Estimator (linked above), with a guided version for parents and a direct-entry version for professionals. Use it for an estimate; this page only explains the model.
For a real case
The guideline amount is presumed correct but can be adjusted when appropriate. For advice on your situation, consult an Illinois family-law attorney.
Reviewed by Aqil Abbas Khan, Founder & Editor of ToolsNexus
This page is general information, not legal advice. Child support in Illinois is set by Illinois’s official guidelines, and the amount in any real case is determined by the court or the state child-support agency using the official worksheet — not by this page. Guideline figures, schedules, and adjustments change over time and depend on facts specific to your situation (each parent’s income, parenting time, health-insurance and childcare costs, and other factors a court may consider). Use the official Illinois calculator for an actual estimate, and consult a licensed family-law attorney in Illinois for advice about your circumstances. ToolsNexus is not a law firm and does not provide legal representation.
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Frequently asked questions
- How is child support calculated in Illinois?
- Since July 1, 2017 Illinois has used an income-shares model under 750 ILCS 5/505. It converts both parents' gross incomes to net using the state's standardized table, combines them, finds the basic obligation on the Schedule of Basic Child Support Obligations, and allocates it by each parent's share of the combined net income.
- What goes into the Illinois formula?
- Both parents' incomes (converted to net using the Gross-to-Net Income Conversion Table), the number of children, who has the majority of parenting time, the number of overnights (which can trigger the shared-parenting calculation), health insurance premiums, and childcare costs.
- How does shared parenting change support in Illinois?
- When each parent has the child for 146 or more overnights a year, Illinois applies the shared-care formula: the basic obligation is multiplied by 1.5 before being divided between the parents, reflecting that both households carry significant costs. Below 146 overnights for one parent, the standard sole-parenting calculation applies.
- When can an Illinois order be modified?
- Under 750 ILCS 5/510 a parent can modify on a substantial change in circumstances (such as a job loss or major income change). There is also a shortcut: without proving a substantial change, an order can be modified if it differs from the current guideline result by at least 20% (and no less than $10 a month).
- Where do I get an official estimate?
- Use the Illinois Child Support Estimator linked on this page, published by the Department of Healthcare and Family Services. This page explains the model but does not compute an amount, and it is not legal advice.