Child Support Percentage in Illinois: New Formula and Deviations

Illinois no longer uses a single child support percentage. Since July 1, 2017, the child support percentage in Illinois has been replaced by the Income Shares Model, which combines both parents’ net incomes, looks up a basic support amount on a state-published schedule, and assigns each parent their proportional share.¹ So if you’re searching for a flat percentage, the honest answer is that Illinois doesn’t work that way anymore.

The Old Percentages and Why They Changed

Before July 2017, Illinois calculated child support as a flat percentage of the non-custodial parent’s net income: 20% for one child, 28% for two, 32% for three, and higher percentages for larger families. That formula ignored the other parent’s earnings entirely. Public Act 99-0764 replaced it with the Income Shares Model, which is the approach most states already used.¹

Under the new system, both incomes count. A parent who earns 60% of the combined household income pays 60% of the scheduled support obligation. The parent with less parenting time typically writes the check to the parent with more time.

How the Calculation Actually Works

The math runs in a fixed order.

First, each parent’s gross income is converted to net income. Most cases use a standardized method that applies ordinary tax calculations, along with deductions for FICA, mandatory retirement contributions, required union dues, health insurance premiums for the parent, and any prior court-ordered support obligations. Parents with unusual tax situations can ask a judge to use an individualized calculation instead.² The Illinois Department of Healthcare and Family Services publishes an annual Gross to Net Income Conversion Table; the current version took effect March 5, 2025.¹

Second, the two net incomes are added together to get a combined adjusted net income.

Third, that combined figure and the number of children are matched against the Income Shares Schedule of Basic Obligations. The schedule shows what families at each income level typically spend raising their children, drawn from national economic data.³

Fourth, each parent’s percentage of the combined income determines their share of the scheduled obligation.

A quick example. Parent A nets $4,000 a month and Parent B nets $2,500, for a combined $6,500. Whatever number the schedule shows for two children at that income level, Parent A owes roughly 62% of it and Parent B owes roughly 38%. If Parent B has primary parenting time, Parent A pays their share to Parent B.²

What Counts as Income

Illinois defines gross income broadly: wages, salary, bonuses, commissions, self-employment income, rental income, and investment returns all count. Social Security disability and retirement benefits paid on behalf of a child count as the disabled or retired parent’s income, with a credit for what goes directly to the other parent for the child.⁴

Means-tested benefits like TANF, SSI, and SNAP are excluded, as are child support payments, survivor benefits, and foster care payments received for other children.⁴

If a parent is voluntarily unemployed or deliberately underemployed, the court can calculate support based on earning capacity rather than actual earnings. Judges look at work history, education, skills, age, health, and any real barriers to employment.

Shared Parenting Changes the Number

The straightforward proportional split assumes one parent has primary time. When each parent has the child for 146 or more overnights a year, roughly 40% of the time, Illinois treats it as shared parenting and the calculation shifts.⁴

The basic obligation from the schedule is multiplied by 1.5 to account for the duplicate costs of running two households for the child. Each parent’s income share of that adjusted amount is then reduced by the time the child spends with the other parent. The two figures are offset, and whoever owes more pays the difference.⁴

Healthcare and Childcare Are Separate

The scheduled obligation covers ordinary living costs. Healthcare and work-related childcare sit on top of it.

The court can order one or both parents to carry health insurance through an employer, union, or group plan. The premium attributable to the child is split by income share, and so are unreimbursed medical expenses.⁴

Work-related childcare, including daycare, before- and after-school programs, and camps when school is out, is also divided by income share. The court factors in the federal child care tax credit when calculating the net cost, and a parent whose childcare expenses change has to notify the other parent within 14 days.⁴

When Judges Deviate from the Guideline Number

The Income Shares result is a presumed amount, not a hard cap or floor. A judge can order a different figure, but must explain in writing why the guideline would produce an unjust result. Recognized reasons include:⁴

The child’s financial needs, including special medical, educational, or developmental needs
Each parent’s overall financial resources, including assets and debts
The standard of living the child would have enjoyed if the parents were together
The child’s physical and emotional condition, and educational needs beyond what the guidelines address

Deviations are most common in cases involving disability, unusually high medical costs, or very high or very low household income. A judge won’t move off the guideline number just because a parent thinks it feels high.

How Long Support Lasts, and the College Wrinkle

Every Illinois order includes a specific termination date. The default is the child’s 18th birthday. If the child turns 18 before finishing high school, support runs through graduation. If the child turns 19 and is still in high school, support ends on the 19th birthday. Support can also end earlier through emancipation, marriage, or military service.

Illinois is one of a smaller group of states where a court can order parents to contribute to college or vocational education costs after the child turns 18. Under Section 513 of the Illinois Marriage and Dissolution of Marriage Act, the court can allocate tuition, room and board, and related costs between the parents, considering each parent’s resources, the child’s academic performance, and the standard of living the child would have had if the parents were together. This catches parents off guard, because in most states support simply ends at 18 or high school graduation.

Changing an Existing Order

Child support orders can be modified. Either parent can petition, but the petitioner needs either a substantial change in circumstances or an order that’s at least three years old.²

Typical grounds are a significant income change for either parent, a change in the child’s needs, or a parenting schedule shift that crosses the 146-overnight threshold. The 2017 switch to Income Shares, by itself, is not a reason to modify an older order.²

1
HFS Illinois Department of Healthcare and Family Services. Income Shares
2
Illinois.gov. Income Shares FAQs
3
Illinois.gov. 2025 Schedule of Basic Obligations
4
Illinois General Assembly. Illinois Code 750 ILCS 5/505 – Child Support, Contempt, Penalties