Quick answer: Illinois uses the income shares model of 750 ILCS 5/505. Each parent’s income is converted to net income (by default with the Department of Healthcare and Family Services’ standardized gross to net conversion table), the two nets are added, and HFS’s Income Shares Schedule gives the basic child support obligation for the number of children. Each parent pays his or her percentage share; the parent with the minority of the parenting time pays. When each parent has the children at least 146 overnights a year (shared physical care), the basic obligation is multiplied by 1.5, each parent’s portion is multiplied by the percentage of time the children are with the other parent, and the larger amount pays the difference. Child care and health insurance premiums are prorated and added.
How the Illinois guideline works
- Net income. Standardized net income from the conversion table, or individualized net income. See how standardized net income works.
- Combined net income and the schedule. The schedule runs from $25 to $27,374.99 of combined monthly net income for one to six children.
- Shares. Each parent’s net income divided by the combined net income.
- Parenting time. The standard calculation, or the shared physical care formula at 146 overnights each. See shared physical care.
- Add-ons. Child care and health insurance premiums, prorated by income share, with credit to the parent who pays.
The full walk-through, with a worked example, is in how Illinois calculates child support.
Important: a change on January 1, 2027
Public Act 104-733 (Senate Bill 3524), signed July 31, 2026, takes effect January 1, 2027. It lowers the shared physical care threshold from 146 to 110 overnights (or overnight equivalents) and adds adjustments for 110 to 145 overnights, among other changes. This calculator applies the law in effect through December 31, 2026 and does not include the 2027 changes.
Child support across Illinois
The guideline applies in every Illinois circuit court, from Chicago, Aurora, Naperville, Peoria, Springfield or Rockford. HFS updates both the conversion table and the schedule each year; the figures here are the ones HFS published in 2026. The low-income presumptions, deviations and individualized net income rules are not included.
Frequently asked questions
How is child support calculated in Illinois?
Illinois uses the income shares model in 750 ILCS 5/505: each parent's net income is added, HFS's Income Shares Schedule gives the basic obligation for the number of children, and each parent pays a percentage share, with the parent who has the minority of the parenting time paying.
What is shared physical care in Illinois?
Under the law in effect through December 31, 2026, each parent has the children at least 146 overnights a year. The basic obligation is multiplied by 1.5, split by income share, multiplied by the other parent's time and offset.
What is standardized net income in Illinois?
Net income taken from HFS's gross to net conversion table, which applies standardized tax amounts and has separate columns for the parent with the majority of parenting time and the parent without it.
What is the top of the Illinois child support schedule?
$27,374.99 of combined monthly net income in the 2026 schedule. Above that the court decides additional support.
What changes in Illinois child support in 2027?
Public Act 104-733 takes effect January 1, 2027 and lowers the shared physical care threshold from 146 to 110 overnights or overnight equivalents, with adjustments for 110 to 145 overnights.