Child support in Illinois.
What the state’s own guideline says, where the number comes from, and the one figure that moves it more than anything you will argue about.
See a different stateIllinois
Illinois switched from a straight percentage of the paying parent's income to income shares on July 1, 2017, so anything you read describing "20% for one child" in Illinois is out of date. Today both parents' net incomes are combined, the state's published schedule says what a family at that income normally spends on that number of children, and each parent owes their proportional share. The parent with less parenting time pays their share to the other.
How Illinois works out the number
Authority: 750 ILCS 5/505, with the Schedule of Basic Child Support Obligations, the Gross to Net Income Conversion Table and the Addendum published by the Illinois Department of Healthcare and Family Services; current tables effective March 20, 2026Yes, and Illinois uses a bright line: "if each parent exercises 146 or more overnights per year with the child, the basic child support obligation is multiplied by 1.5" before being divided, and then each parent's share is further adjusted for the time the child spends with them. Below 146 overnights for one parent, no shared-care adjustment applies.
Use the state’s own calculator, not ours
Illinois publishes this itself, free. It is called Support Obligation Worksheet, and Shared Physical Care Support Obligation Worksheet, both published by Illinois HFS. We are not going to build a competing calculator and ask you to trust it — this is the one a judge will recognize.
Open the Illinois calculator →What goes into the calculation
What counts as income
"Gross income" means "the total of all income from all sources," leaving out means-tested public benefits and child support received for other children, and including maintenance that is taxable to the recipient. Self-employment income, business income and overtime all count. "Net income" is gross income minus either a "standardized tax amount" or an "individualized tax amount," minus allowed adjustments. The standardized figure assumes "federal and state income taxes for a single person claiming the standard tax deduction, one personal exemption" plus Social Security and Medicare.
When a judge can treat you as earning more
If a parent "is voluntarily unemployed or underemployed, child support shall be calculated based on a determination of potential income," weighing employment history, job skills, education, age, health and "record of seeking work." If there is not enough work history to go on, there is a rebuttable presumption that potential income equals "75% of the most recent United States Department of Health and Human Services Federal Poverty Guidelines for a family of one person."
If you do not earn much
For a paying parent at or below 75% of the federal poverty guidelines, the minimum order is "$40 per month, per child" capped at "$120 per month" total. A $0 order is allowed for a parent with no income, a parent receiving only means-tested assistance, or a parent unable to work because of "medically proven disability, incarceration, or institutionalization."
Childcare and health insurance
The court may order each parent to contribute to "reasonable child care expenses" on top of basic support. Health insurance premiums for the children are also allocated, and the cost of the child's coverage cannot exceed "5% of the providing parent's gross income" to be considered reasonable.
Private school, special needs, activities
The court may order contributions to "reasonable school and extracurricular activity expenses" and to unreimbursed medical and dental costs, separately from the basic support number.
When a judge can depart from the number
A judge may deviate if applying the guidelines "would be inequitable, unjust, or inappropriate," and must put the reasons in written findings.
The single biggest trap in Illinois is old information. Plenty of what you will find online still describes the pre-2017 flat-percentage system, which no longer exists. The second thing worth knowing is the 146-overnight cliff: at 145 overnights you get no shared-care adjustment, and at 146 the whole calculation changes. That is roughly two overnights a week plus alternating holidays, and it is worth counting carefully before you agree to a schedule.
Later on
When it ends
Support ends "no earlier than the date on which the child covered by the order will attain the age of 18." If the child has not graduated high school by 18, it continues to "the earlier of… high school graduation or… age 19."
College and support after 18
Illinois is one of the states where a court can order parents to help pay for college. The authority is 750 ILCS 5/513, which lets a court order contribution to the educational expenses of a non-minor child, including college. This is a separate proceeding from basic child support and comes with its own conditions and caps.
Changing the amount later
The general rule is "a substantial change in circumstances." In cases where a parent is receiving child support services from the Department of Healthcare and Family Services, and "at least 36 months have elapsed since the order for child support was entered or last modified," a modification can be based on "an inconsistency of at least 20%, but no less than $10 per month" between the current order and the guideline amount. A court can also modify without any change in circumstances "upon a showing of a need to provide for the health care needs of the child."
How far back a change can reach
A modification reaches back only to the date notice of the motion was given: support "may be modified only as to installments accruing subsequent to due notice by the moving party of the filing of the motion for modification." Missed months before that are not recoverable.
What we could not confirm
- We could not open the official text of 750 ILCS 5/513, so we cannot give you the age cutoff or the cap on court-ordered college contribution. Illinois does tie the cap to the cost of attending the University of Illinois at Urbana-Champaign, but confirm the current numbers with the statute or your attorney before you plan around them. We also could not confirm Illinois' enforcement tools from an official source.
- 1 official page we tried to read refused automated access. We did not try to get around the block, so some detail above comes from a different official source rather than the one we would have preferred.
Compare all fifty states
The overnight thresholds, the ending ages and the college rules, side by side.
The rest of what we have written about Illinois. Every figure is quoted from Illinois’s own statute or its courts’ own published schedule, and carries the date we last checked it.