Child Support in Illinois: How It's Calculated
Illinois uses the Percentage of Income Model (Income Shares hybrid post-2017) to determine child support obligations. This guide walks through how the formula works, what factors Illinois courts consider, and how to modify or enforce an order — with links to official Illinois resources.
Percentage of Income Model (Income Shares hybrid post-2017) in Illinois
Illinois revised its child support law in 2017 and now uses a hybrid approach that applies an Income Shares calculation when both parents share significant time. For primary custody arrangements, Illinois historically used a percentage of the paying parent's net income. The 2017 law introduced a more nuanced shared parenting calculation.
Under the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/505), the baseline percentages for primary custody are: 20% for 1 child, 28% for 2, 32% for 3, 40% for 4, 45% for 5, and 50% for 6 or more children based on net income. For shared parenting (each parent has 146+ overnights), Illinois uses an Income Shares calculation that considers both parents' incomes and the time allocation.
Key Factors Illinois Courts Consider
- Paying parent's net income (or both parents' income in shared custody situations)
- Number of children
- Number of overnights each parent has — 146+ per year triggers the shared parenting formula
- Child's health insurance costs
- Work-related childcare costs
- Extraordinary expenses such as educational needs
- Support obligations for other children
Official Illinois Resources
Get an accurate number from an official source
Child support calculations are complex and fact-specific. For an accurate figure, use Illinois's official worksheet or consult a licensed family law attorney. This guide explains the framework — it does not produce a binding calculation.
Step-by-Step: How Illinois's Formula Works
Here is how Illinois applies the Percentage of Income Model (Income Shares hybrid post-2017) to calculate a child support obligation, step by step:
1. Determine the paying parent's income
Illinois applies its formula to the paying (non-custodial) parent's income. Under the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/505), the baseline percentages for primary custody are: 20% for 1 child, 28% for 2, 32% for 3, 40% for 4, 45% for 5, and 50% for 6 or more children based on net income.
2. Apply the state's percentage to income
A fixed percentage is applied based on the number of children. Under the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/505), the baseline percentages for primary custody are: 20% for 1 child, 28% for 2, 32% for 3, 40% for 4, 45% for 5, and 50% for 6 or more children based on net income. For shared parenting (each parent has 146+ overnights), Illinois uses an Income Shares calculation that considers both parents' incomes and the time allocation.
3. Add health insurance and childcare if applicable
In Illinois, health insurance premiums for the child and work-related childcare costs may be added to the base support amount or ordered separately, depending on the specific statute.
4. Apply parenting time adjustments
If the paying parent exercises significant parenting time, Illinois may reduce the obligation. Paying parent's net income (or both parents' income in shared custody situations)
5. Apply income caps and deviation factors
Illinois may cap the income subject to the percentage, and courts can deviate from the guideline amount with written findings that the formula result would be unjust or inappropriate.
Modifying Child Support in Illinois
In Illinois, a parent can seek modification by showing a substantial change in circumstances since the last order — such as a significant income change, a change in custody or parenting time, or a change in the child's needs. Illinois also permits a review every 3 years as a matter of right. The modification is filed with the circuit court that issued the original order. Support modifications are prospective only — any reduction applies from the date the petition was filed and served.
How Illinois Enforces Child Support Orders
Illinois enforces child support through the Division of Child Support Services (DCSS), part of the Department of Healthcare and Family Services. Tools include income withholding orders, tax refund interception, license suspension (driver's, professional, and recreational), credit bureau reporting, property liens, and contempt of court. Illinois also uses the State Disbursement Unit (SDU) to process and track all child support payments.
Document Everything for Your Illinois Case
Illinois courts rely on documented records — parenting time logs, expense receipts, and payment histories. ReplyCalmly helps you track all of it and generate organized, exportable reports.
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