Illinois uses an “income shares” model to calculate child support, meaning both parents’ incomes are combined to estimate what the household would have spent on the child if the family were still together. The parent who spends less time with the child typically pays a share of that amount to the other parent. You can run a quick estimate using the free online tool from the Illinois Department of Healthcare and Family Services, but understanding how the formula works helps you know what to expect.
How the Income Shares Model Works
The basic idea is straightforward. The state starts with each parent’s net income, adds them together, and looks up the combined amount on a standardized table that estimates child-rearing costs for that income level and number of children. That table amount is called the basic child support obligation.
Each parent is then assigned a percentage of that obligation based on how much they contribute to the combined income. If one parent earns 60% of the total and the other earns 40%, costs are split along that same ratio. The parent with fewer overnights typically pays their percentage to the custodial parent, since the custodial parent is already spending directly on the child day to day.
Calculating Each Parent’s Net Income
Net income is your gross income minus certain allowed deductions. Gross income includes wages, salaries, bonuses, commissions, self-employment earnings, investment income, retirement distributions, and most other recurring sources of money. It does not include means-tested public benefits like SNAP or TANF.
From gross income, you subtract federal and state income taxes (based on the tax rate for a single filer), Social Security and Medicare taxes, mandatory retirement contributions required as a condition of employment, union dues, and existing child support or maintenance obligations for other relationships. What remains is your net income for child support purposes.
If a parent is voluntarily unemployed or underemployed, the court can “impute” income, meaning it assigns an earning capacity based on work history, education, and job availability rather than actual current earnings.
Looking Up the Basic Support Obligation
Illinois publishes a schedule of basic support obligations that pairs combined net income with the number of children. The table works like a tax bracket chart: you find the row matching the parents’ combined monthly net income and the column for the number of children, and the intersection gives you the presumed monthly cost of raising those children. The schedule is updated periodically by the Department of Healthcare and Family Services, and the current version is available on the HFS website.
Once you have that figure, each parent’s share is proportional to their income. For example, if the table says the basic obligation for one child at your combined income level is $1,200 per month, and you earn 55% of the combined income, your share would be $660.
Add-On Expenses: Healthcare, Childcare, and More
The basic obligation covers everyday costs like food, clothing, housing, and transportation. Certain additional expenses are split separately on top of that amount, using the same income percentages.
Health insurance premiums: The cost of adding the child to a parent’s health insurance plan is divided between both parents. A parent can be ordered to provide coverage as long as the premium is “affordable,” which Illinois defines as no more than 5% of that parent’s gross income. To be required to contribute toward health insurance costs at all, a parent must have a net income of at least 133% of the federal poverty guidelines.
Unreimbursed medical expenses: Costs not covered by insurance, such as copays, prescriptions, glasses, braces, or therapy, are also divided proportionally between the parents.
Childcare costs: Work-related or education-related daycare and childcare expenses are added on top of the basic obligation and split by each parent’s income share.
Education and extracurricular expenses: School fees, tutoring, sports, and similar costs may also be included, depending on the circumstances and what the court orders.
How Shared Parenting Time Changes the Math
If both parents each have the child for at least 146 overnights per year, Illinois treats that as “shared physical care,” and the calculation adjusts significantly. The logic is that both households are duplicating many of the same expenses (two bedrooms, two sets of groceries, two of everything), so the baseline number goes up.
In a shared care situation, the basic child support obligation is multiplied by 1.5. That 50% increase approximates the duplicated costs. Each parent’s share of this higher amount is then multiplied by the percentage of time the child spends with the other parent. The two resulting figures are offset against each other, and the parent who owes more pays the difference to the other parent.
Here is a simplified example. Suppose the basic obligation is $1,200 and the parents split time 60/40. The shared care obligation becomes $1,800 ($1,200 times 1.5). If Parent A earns 55% and Parent B earns 45%, Parent A’s share is $990 and Parent B’s share is $810. Parent A’s $990 is multiplied by Parent B’s time percentage (40%), giving $396. Parent B’s $810 is multiplied by Parent A’s time percentage (60%), giving $486. The difference is $90, paid by Parent B to Parent A. The exact outcome depends on the specific income split and overnight schedule.
If neither parent has a formal parenting plan showing 146 or more overnights, support is calculated under the standard formula, which assumes one parent has the majority of parenting time.
When the Court Can Deviate From the Guidelines
The guideline amount is presumed to be correct, but a court can order a different amount if applying the formula would be “inequitable, unjust, or inappropriate.” The judge must put the reasons in writing and state what the guideline amount would have been. Common reasons for a deviation include:
- Extraordinary medical expenses necessary to preserve the life or health of a parent or child
- Special needs costs for a child with medical, physical, or developmental needs that go beyond what the standard table accounts for
- Any other factor the court finds relevant after considering the child’s best interests
That third category is broad. Courts have used it to account for situations like significant travel expenses for parenting time, a child’s private school tuition, or an unusually high or low income that falls outside the range of the guidelines table.
Running Your Own Estimate
The Department of Healthcare and Family Services provides a free online child support estimator at its website. You enter each parent’s gross income, the number of children, the overnight schedule, health insurance premiums, childcare costs, and any other relevant add-ons. The tool applies the income shares formula and gives you an estimated monthly obligation.
To get the most accurate result, gather recent pay stubs or tax returns for both parents, know the annual number of overnights each parent has (or expects to have), and have figures ready for health insurance premiums and childcare costs. The estimator is helpful for planning, but the final support order can differ based on deductions, imputed income, or court-ordered deviations that the tool does not fully capture.
How Support Gets Finalized and Modified
Child support in Illinois is typically established through a court order during a divorce, parentage case, or standalone support proceeding. Both parents submit financial affidavits disclosing income, expenses, and assets. The court (or an administrative hearing if HFS is involved) applies the guidelines and issues an order.
Either parent can later request a modification if there has been a “substantial change in circumstances,” such as a significant income change, a job loss, a new medical condition, or a change in the parenting schedule. Illinois also reviews support orders every three years (or every 36 months) upon request to see if the existing amount differs from the current guidelines by a meaningful margin. If it does, the order can be adjusted without proving a separate change in circumstances.