Child Support Percentage in Illinois: How It’s Calculated Now

There is no single child support percentage in Illinois. The flat-percentage system ended on July 1, 2017, and the state now uses the Income Shares Model, which combines both parents’ net incomes, looks up a basic support obligation on a published schedule, and splits that obligation between the parents in proportion to what each one earns.¹ So your share depends on your income, the other parent’s income, how many children you have, and how parenting time is divided.

The Old Percentages Are Gone

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 amounts for larger families. The other parent’s income was not part of the formula. Public Act 99-0764 replaced that system with the Income Shares Model, which is now used by most states.¹

If you still see “20% for one child” quoted somewhere, it is out of date. That number does not control any order entered after the 2017 change, and the switch alone is not a reason to reopen an older order.

How the Current Calculation Works

The method has four steps:

Figure each parent’s net income.
Add the two net incomes together to get combined net income.
Look up the basic support obligation for that combined income and that number of children on the Illinois Schedule of Basic Obligations.
Multiply the obligation by each parent’s share of the combined income. The parent with less parenting time pays their share to the other parent.

A simple example: Parent A nets $4,000 a month, Parent B nets $2,500. Combined net income is $6,500. The court looks up $6,500 for the right number of children on the schedule and finds the basic obligation. Parent A earns about 62% of the combined income, so Parent A is responsible for about 62% of that number; Parent B is responsible for the remaining 38%.²

The Illinois Department of Healthcare and Family Services publishes the schedule and updates it annually. The current version took effect March 5, 2025.³

What Counts as Income

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

Some things are excluded: means-tested benefits like TANF, SSI, and SNAP; child support received for other children; survivor benefits; and foster care payments.⁴

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

Getting from Gross to Net

The schedule runs on net income. Most cases use the standardized conversion method, which applies ordinary tax calculations; parents with unusual tax situations can ask a judge for an individualized calculation.² Under the standardized method, these deductions reduce gross to net:

Federal and state income taxes.
Social Security and Medicare (the employee’s share of FICA).
Mandatory retirement contributions required by the employer, not voluntary 401(k) deferrals.
Union dues, if required as a condition of employment.
Health insurance premiums for the parent’s own coverage.
Prior court-ordered support for other children or a former spouse.

HFS publishes a Gross to Net Income Conversion Table each year to standardize the math. The current version took effect March 5, 2025.¹

Shared Parenting Changes the Math

If each parent has the child for 146 or more overnights per year, roughly 40% of the time, the case is treated as shared parenting. In that situation, 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 share is then calculated based on income percentage and adjusted by the time the child spends with the other parent. The two amounts are offset, and whichever parent owes more pays the difference.⁴

Below the 146-overnight threshold, the standard calculation applies and the parent with less time pays their full income share to the other parent.

Health Insurance and Childcare Are Added Separately

The basic obligation covers ordinary day-to-day expenses. Health and childcare costs sit on top.

The court can order one or both parents to carry health insurance for the child through an employer, union, or group plan. The premium cost attributable to the child is divided between the parents by their income shares, and unreimbursed medical expenses are split the same way.⁴

Work-related childcare, including daycare, before- and after-school care, and camps when school is not in session, is also divided by income share. The court factors in the federal child care tax credit when figuring the net cost, and the parent paying for childcare must notify the other parent within 14 days if the expense changes.⁴

When a Judge Can Deviate from the Guideline

The Income Shares calculation produces a presumed amount, but a judge can enter a different number if the guideline would be unjust. The deviation and the reasons for it must be stated in writing. Factors that support deviation include:⁴

The child’s financial needs, including special medical, educational, or developmental needs.
Each parent’s financial resources, including assets and debts beyond income.
The standard of living the child would have had if the parents stayed together.
The child’s physical and emotional condition and educational needs.

Deviations tend to show up in cases involving disabilities, unusually high medical costs, or incomes at the extremes. A judge will not deviate because a parent dislikes the number; there has to be a documented reason tied to the child’s best interests.

How Long Support Lasts

Every Illinois order includes a specific termination date. The default is the child’s 18th birthday. If the child will turn 18 before finishing high school, the order runs to graduation; if the child is still in high school at 19, support ends on the 19th birthday regardless. Support can end earlier if the child is emancipated, marries, or joins the military.

One point that catches many parents by surprise: Illinois is one of the states where a court can order parents to contribute to a child’s college or vocational education costs after 18. Under Section 513 of the Illinois Marriage and Dissolution of Marriage Act, a judge can allocate tuition, room and board, and related expenses between the parents based on their financial resources, the child’s academic performance, and the family’s standard of living. This is separate from ordinary child support and continues past the age most people assume ends all financial obligation.

Modifying an Existing Order

Child support can be changed, but not on request alone. Illinois requires either a substantial change in circumstances or that the current order was entered at least three years ago. Common grounds include a significant change in either parent’s income from job loss, a raise, or disability; a change in the child’s needs; or a shift in the parenting schedule that crosses the 146-overnight shared parenting line. The adoption of the Income Shares Model in 2017 is not by itself grounds 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