What Percentage Is Child Support in Illinois? IL Calculation Guide for 2025

Child Support in Illinois: No More Fixed Percentages

If you are looking for a simple percentage to calculate child support in Illinois, you will not find one. Since July 2017, Illinois has used the Income Shares model, which means there is no single fixed percentage that applies to every case. Instead, the court calculates support based on both parents' combined income, the number of children, and the specific parenting time arrangement.

This was a major change from the old system, which simply applied a flat percentage to the non-custodial parent's net income. The 2017 reform was designed to create a more equitable system that reflects the reality that both parents share the financial responsibility of raising children.

Under the current system, the effective "percentage" varies significantly depending on the family's circumstances. Two families with the same number of children but different income levels and custody arrangements could end up with very different support amounts and effective percentages.

How the Illinois Income Shares Model Works

The Income Shares model is based on the principle that children should receive the same proportion of parental income they would have received if the parents lived together. Here is how it works step by step:

Step 1: Determine Each Parent's Net Income

The court calculates each parent's net income by taking gross income (salary, wages, bonuses, commissions, self-employment income, and other sources) and subtracting allowable deductions. Allowable deductions in Illinois include:

• Federal and state income taxes
• Social Security and Medicare taxes
• Mandatory retirement contributions
• Health insurance premiums (especially for the child)
• Pre-existing child support or maintenance obligations
• Mandatory union dues

For more on how courts determine what money is available, see our article on disposable income for child support.

Step 2: Combine the Incomes

Both parents' net incomes are added together to get the combined adjusted net income. This combined figure is what the state uses to look up the total child support obligation.

Step 3: Look Up the Support Obligation

Illinois publishes an Income Shares Schedule — a table that shows the total monthly child support obligation based on the combined net income and the number of children. This table is updated periodically to reflect changes in the cost of raising children. The schedule covers combined monthly incomes ranging from very low levels up to high-income thresholds.

Step 4: Divide Proportionally

The total support obligation is divided between the parents in proportion to their share of the combined income. For example, if Parent A earns $6,000 per month and Parent B earns $4,000 per month, the combined income is $10,000. Parent A earns 60% and Parent B earns 40%. If the total support obligation for their two children is $1,500 per month, Parent A's share is $900 and Parent B's share is $600.

Step 5: Apply Parenting Time Adjustments

If the parents share parenting time, the court adjusts the amount to account for the expenses each parent directly incurs during their time with the children. This is where the 146-night rule comes into play, which we discuss in detail below.

Combined Income Table Examples

While the full Illinois Income Shares Schedule covers a wide range of income levels, here are illustrative examples of the approximate total monthly support obligations for different combined income levels:

These figures are approximate and subject to periodic updates. The parent with less parenting time typically pays their proportional share to the parent with more parenting time. Use our free child support calculator for an estimate tailored to your situation.

Shared Parenting Adjustments

Illinois law recognizes that when both parents spend significant time with the children, each parent directly incurs expenses for the children during their parenting time. The shared parenting adjustment ensures that the support calculation accounts for this.

The 146-Night Rule

The critical threshold in Illinois is 146 nights per year — approximately 40% of the year. If the parent who would otherwise be paying support has the children for 146 or more overnights annually, the shared parenting adjustment applies. This does not necessarily mean a 50/50 split; even 146 nights (roughly 2.8 nights per week) triggers the adjustment.

How the Adjustment Works

When the 146-night threshold is met, the court performs an additional calculation:

1. Calculate the basic obligation: Determine the standard Income Shares amount as described above.
2. Determine each parent's share of overnights: Calculate what percentage of the year each parent has the children overnight.
3. Calculate each parent's direct spending: Multiply the total support obligation by each parent's overnight percentage to estimate what each parent spends directly on the children during their time.
4. Compare and offset: The parent with the higher income obligation may still owe support, but the amount is reduced to account for their direct spending during parenting time.

This adjustment can significantly reduce the support obligation for parents who actively share parenting time.

The 2017 Reform: What Changed

Before July 1, 2017, Illinois used a Percentage of Income model that was straightforward but often unfair. Under the old system:

• The non-custodial parent paid a flat percentage of their net income
• Percentages were: 20% for 1 child, 28% for 2 children, 32% for 3, 40% for 4, 45% for 5, and 50% for 6 or more
• The custodial parent's income was not considered in the calculation
• Parenting time was not factored into the formula

This system had significant drawbacks. A non-custodial parent earning $40,000 per year could end up paying the same effective percentage as one earning $400,000, while the custodial parent's income was completely ignored. Additionally, a non-custodial parent who had the children nearly half the time was treated the same as one who saw them only on weekends.

Why Illinois Changed

The reform was driven by several factors:

• Fairness to both parents: The old system ignored the custodial parent's income entirely, creating inequitable outcomes
• Recognizing shared parenting: As shared custody became more common, the law needed to account for both parents' direct expenses
• National trend: Most states had already adopted the Income Shares model, and Illinois was an outlier
• Federal recommendations: Federal guidelines encouraged states to use models that consider both parents' incomes

What Happened to Existing Orders

The 2017 reform applied to new child support orders established after July 1, 2017. Existing orders were not automatically changed, but either parent can request a modification to have the order recalculated under the new Income Shares model. If you have an old order that has not been updated, you may want to explore whether a modification would benefit you.

Additional Expenses on Top of Base Support

In addition to the base support amount calculated through Income Shares, Illinois courts may order contributions to:

• Health insurance: The cost of providing health insurance for the child is typically allocated between the parents proportionally to their incomes
• Unreimbursed medical expenses: Co-pays, deductibles, dental care, vision care, and other medical costs not covered by insurance
• Childcare: Work-related daycare, before and after school care, and summer childcare
• Extracurricular activities: Reasonable costs for sports, lessons, and enrichment activities
• Education expenses: Tutoring, special needs education, and potentially private school if the child was previously enrolled

These add-on expenses are shared between the parents proportionally based on their respective shares of the combined income.

Example Calculation

Let us walk through a sample calculation to illustrate how the Illinois Income Shares model works in practice:

The situation:

• Parent A (primary residential parent): Net income of $4,000 per month
• Parent B (non-residential parent): Net income of $6,000 per month
• Two children
• Parent B has the children 80 nights per year (below the 146-night threshold)

Step 1: Combined net income = $4,000 + $6,000 = $10,000 per month

Step 2: From the Income Shares Schedule, the total support obligation for two children at $10,000 combined income is approximately $2,470 per month

Step 3: Parent A's share = 40% ($988). Parent B's share = 60% ($1,482)

Step 4: Since Parent B has the children less than 146 nights, no shared parenting adjustment applies

Result: Parent B pays $1,482 per month to Parent A

If the same parents shared parenting time at 146+ nights per year, the shared parenting adjustment would reduce Parent B's obligation to account for the expenses Parent B directly incurs during their parenting time.

High-Income Situations

For families with combined incomes above the Income Shares Schedule's maximum threshold, Illinois courts have discretion to determine an appropriate support amount. The court typically:

• Uses the schedule as a starting point
• Considers the children's actual needs
• Evaluates the standard of living the children would enjoy if the parents lived together
• May apply a formulaic approach to income above the threshold or exercise discretion based on the specific circumstances

Need help with your Illinois child support? Use our free child support calculator for an instant estimate, visit our Illinois state page for detailed information, or read our article on what child support is for a complete overview. You can also learn about what child support is supposed to cover.