How is Child Support Calculated in Illinois?

 Posted on March 31, 2026 in Family Law

When parties going through a divorce have children, the issue of child support will inevitably arise. How child support is calculated is different in each state. Illinois uses an "income shares" model, which takes into account the income of both parents. This makes the calculation of child support more equitable for both spouses than it was previously.

Anyone facing child support orders will want to understand how they factor into their finances in 2026. Our DuPage County child support lawyers can help you understand how the numbers are determined and what to expect after orders have been made.

How Does Illinois Calculate Child Support?

Illinois moved away from a simple percentage-based formula about ten years ago. The income shares model now used is outlined under 750 ILCS 5/505. The idea is that a child should receive the same level of financial support they would have received if both parents were still living together.

The court adds both parents’ incomes together to get a combined net income. That number is then used to find the basic child support obligation based on the Illinois "schedule of support." Each parent is responsible for their share of that obligation based on the percentage of the combined income they earn.

What Counts as Income?

Illinois uses an "income shares" model. Courts begin with each parent’s gross income (all income from all sources) and then convert it to net income using statutory tax calculations.

Income may include:

• Wages, salary, tips, overtime 
• Self-employment income 
• Bonuses and commissions 
• Social Security (retirement/disability) 
• Unemployment and disability benefits 
• Rental and investment income 
• Retirement or pension distributions 
• Other recurring financial benefits or support 

Excluded income includes:

• Means-tested public assistance (e.g., SSI, SNAP) 

If a parent is voluntarily unemployed or working less than they are able to, the court may assign them an income based on what they could reasonably earn. This is called imputed income, and it prevents parents from reducing their support obligation by simply choosing not to work.

How Does Parenting Time Affect Child Support in Illinois?

The amount of parenting time each parent has can affect the final child support number. Illinois law recognizes that when a parent spends more time with a child, they are already spending more money on that child's day-to-day needs.

When one parent has the child for more than 146 overnights per year – which works out to about 40 percent of the year – an adjustment to the base support amount may apply. The court will also look at the actual parenting time schedule alongside other factors before making changes to the standard calculation.

This is one of the reasons parenting time arrangements and child support are closely connected. How a parenting time schedule is structured can have a financial impact on both parents.

What Other Costs Can Be Added to Illinois Child Support?

Illinois courts can also order parents to share additional expenses on top of the basic obligation. These may include:

• Health insurance premiums for the child
• Unreimbursed medical and dental expenses
• Childcare costs while the custodial parent works or attends school
• Educational expenses
• Extracurricular activities

These are sometimes called "add-on" expenses. They are typically divided between parents in proportion to their incomes, just like the base support amount.

Can an Illinois Judge Order Different Amounts From the Child Support Guidelines?

Illinois courts generally follow the income shares calculation, but a judge has the authority to order a different amount if following the guidelines would be unfair or inappropriate given the circumstances of the case. If a judge does deviate from the standard calculation, they are required to put their reasons for doing so in writing.

Factors that might lead to a deviation include a child's special medical or educational needs, an unusually high or low income for one parent, or an arrangement where parenting time is divided in an unusual way.

When Can Child Support Orders Be Modified?

Child support orders are not permanent. Either parent can ask the court to review and modify the order if there has been a substantial change in circumstances. Common reasons for a modification include:

• A significant increase or decrease in either parent's income
• A change in the child's needs, such as a new medical condition
• A major change in the parenting time schedule
• A parent losing their job or becoming disabled

In cases involving the Department of Healthcare and Family Services, a support order may be reviewed after 36 months and modified without showing a substantial change if applying the guidelines results in at least a 20% difference from the current order.

Penalties for Not Paying Child Support

Failure to pay child support as ordered can result in a court finding of contempt. If a parent is found guilty of contempt, that parent can be forced to pay back-owed support, with interest, as well as the attorney's fees of the parent who had to seek enforcement of the support order. In more serious cases, the court can order incarceration.

Call a Wheaton, IL Child Support Lawyer Today

Child support calculations can get complicated, especially when self-employment income, shared parenting time, or add-on expenses are involved. Having a lawyer to help walk you through the ins and outs of child support makes the process substantially easier.

At Mirabella, Kincaid, Frederick & Mirabella, LLC, our DuPage County family law attorneys will take the time to make sure you have a complete understanding of your child support obligations. Contact MKFM Law at 630-665-7300 to schedule your initial attorney meeting. Our child support attorneys represent family law clients throughout the western suburbs from our offices located in Kane, DuPage, and DeKalb Counties.