Illinois Child Support Calculator

Illinois uses the Income Shares model. Illinois adopted the income shares model effective July 1, 2017, replacing the prior flat-percentage model. Combined net income of both parents is used to look up base support obligation from a schedule. Each parent pays a proportional share. Parenting-time adjustment applies when each parent has the child at least 146 overnights.

Illinois's guideline runs on combined net income (both parents). That distinction matters: entering gross income into a formula built on net income (or the reverse) produces a materially wrong figure. Most states count income from essentially any regular source -- wages, self-employment, bonuses, commissions, rental and investment income, and many benefits -- not just salary.

Not necessarily. Illinois's guideline weighs each parent's share of combined income alongside the parenting-time arrangement, so a significantly higher earner can still owe support even at an even split of time.

Yes. The figure produced by Illinois's Income Shares guideline is presumptive, not mandatory: a court can order a different amount when applying the guideline would be unjust or inappropriate for the specific case, and generally must state its reason on the record. Common grounds include extraordinary medical or educational expenses, a child's special needs, or costs the formula does not capture.

You have to file for a modification -- a change in your circumstances does not change what you owe on its own. Illinois, like most states, requires a substantial change in circumstances: a significant involuntary income change, a change in the parenting schedule, or a material change in the child's needs. Until a court or the state child support agency issues a new order, the existing amount stays fully enforceable and arrears keep accruing, and a modification is generally not applied retroactively before the date you filed -- so filing promptly matters. In Illinois, modifications are handled through IL Dept. of Healthcare and Family Services — Child Support.

Illinois publishes it directly: "IL HFS Income Shares — Estimator & Worksheets". This site's calculator is a fast estimate built on the same published guideline, not a replacement for the official form -- a court filing generally requires the state's own worksheet. The link is in the Illinois Resources section on this page.