How Child Support Amounts Are Calculated in Illinois

Illinois uses a formula based on both parents' income and custody time

Illinois child support is not a fixed dollar amount — it depends on how much each parent earns, how much time the child spends with each parent, and how many children need support. The state uses the Income Shares Model, which means the court calculates what percentage of combined parental income should go toward the child, then divides that obligation between parents based on their individual earnings and custody arrangement.

The calculation starts with net monthly income for each parent. This includes wages, salary, bonuses, self-employment income, rental income, and some benefits. It excludes things like child support already being paid for other children, spousal maintenance paid, and certain tax deductions. Once the court knows both parents' net income, it applies the state's percentage guidelines to determine the total support obligation, then splits it based on each parent's share of income and parenting time.

A parent who has the child more than 50 percent of the time may pay less support or receive support from the other parent, depending on the income difference. The parent with higher income and less parenting time typically pays more.

Key Takeaways

• Illinois calculates child support using combined parental income and the percentage of time each parent has the child, not a flat monthly amount.
• Net monthly income includes wages, self-employment earnings, and rental income, but excludes existing child support obligations and certain deductions.
• The state publishes income percentage guidelines that vary by number of children — the obligation is higher for three children than for one.
• A parent with primary custody and significantly lower income may receive support from the higher-earning parent, even if that parent has less parenting time.

The income percentage guidelines that set the base obligation

Illinois publishes a table showing what percentage of combined parental income should go toward child support for each number of children. These percentages are updated periodically and do not change based on individual family circumstances — they are the starting point for every calculation.

For example, if the combined net monthly income of both parents is $5,000 and there are two children, the state guideline percentage determines the total monthly obligation. That obligation is then divided between the parents based on their individual incomes. If one parent earns 60 percent of the combined income and the other earns 40 percent, the 60-percent earner typically pays 60 percent of the total obligation.

The percentages increase as the number of children increases. Support for one child takes a smaller percentage of income than support for four children. Courts can deviate from these percentages if they find the guideline amount would be unjust or inappropriate, but they must document their reason in writing.

How parenting time affects the amount each parent pays

The parent who spends less time with the child usually pays more support. Illinois measures parenting time in overnights per year. If one parent has the child 219 or fewer overnights annually (roughly 60 percent of the time with the other parent), that parent is considered the primary residential parent, and the other parent typically pays support.

If both parents have the child roughly equally — each with more than 146 overnights per year — the calculation shifts. The parent with higher income may still pay support to the lower-earning parent, but the amount is reduced because both parents are sharing the day-to-day costs. Some cases result in the higher-earning parent paying the lower-earning parent; others result in no support obligation if incomes are very close.

Parenting time is documented in the custody order or parenting plan. If the actual time differs significantly from what the order says, either parent can ask the court to recalculate support based on the real schedule.

Income sources included and excluded from the calculation

The court includes most forms of income: W-2 wages, self-employment net income, bonuses, commissions, rental income, investment income, and income from a business or partnership. It also includes certain benefits like unemployment compensation, workers' compensation, and disability payments.

The court excludes child support already being paid for other children, spousal maintenance (alimony) paid to an ex-spouse, and certain tax deductions. If a parent is voluntarily unemployed or underemployed — meaning they could earn more but choose not to — the court may impute income, or assign an income level based on their earning capacity, rather than what they actually report.

Self-employed parents must provide tax returns and business records so the court can determine actual net income. Income that fluctuates month to month is usually averaged over a 12-month period.

Deviations from the guideline amount and when courts use them

Illinois law lists 17 factors a court can consider when deciding whether to deviate from the guideline percentage. These include the financial resources of both parents, the standard of living the child would have had if the parents remained together, the non-monetary contributions each parent makes to the child's care, and the impact of custody arrangements on each parent's ability to work.

A court might order more than the guideline amount if one parent has substantial assets or income beyond what is counted as monthly income. It might order less if the paying parent has other children to support, has significant medical expenses, or if the guideline amount would leave them below the poverty line.

Any deviation must be explained in the court order. A parent who disagrees with the amount can appeal based on whether the court properly applied the law and the factors, though courts have broad discretion in these decisions.

How to find the current Illinois income percentage guidelines

The Illinois Supreme Court publishes the current child support guidelines on its website. The guidelines are updated every four years or when the legislature changes the law. The document shows the percentage of combined income for one child through six or more children.

You can also find the guidelines through the Illinois Department of Human Services, which administers child support enforcement. If you are working with an attorney, they will have the current table. Many online calculators use the Illinois formula, but they are only estimates — the actual amount depends on how the court interprets income, parenting time, and any deviations.

Frequently Asked Questions

Does Illinois have a minimum or maximum child support amount?

Illinois does not set a flat minimum, but courts rarely order less than $50 per month. There is no maximum — support can be very high if both parents earn substantial income. The guideline percentage applies to all income levels, so a parent earning $200,000 per year will have a much higher obligation than one earning $40,000.

What happens if one parent is unemployed or makes very little?

If a parent is unemployed through no fault of their own, the court may set support based on their actual income or a reduced amount. If the court believes a parent is intentionally unemployed or underemployed to avoid support, it can impute income based on their education, work history, and earning capacity. The parent can ask the court to recalculate if their situation changes.

Can child support be modified if income changes?

Yes. Either parent can ask the court to modify support if there is a substantial change in income, custody time, or other circumstances. A change of 20 percent or more in either parent's income usually qualifies. The modification takes effect from the date the request is filed, not retroactively to when the income changed.

How does having multiple children affect the support amount?

The guideline percentage increases with each additional child. Support for two children is a higher percentage of combined income than support for one child. The total obligation is then divided between parents based on their income shares, just as with one child.

Is child support the same as custody, or can they be separate?

Custody and support are separate decisions. A parent can have primary custody and still owe support if the other parent has significantly higher income. Conversely, a parent with less custody time might not owe support if they earn less than the primary parent. The court decides both, but they are calculated independently.