How Illinois Calculates Child Support
Illinois uses a combined income approach for child support based on both parents' net monthly incomes. The governing law is the Illinois Marriage and Dissolution of Marriage Act, specifically 750 ILCS 5/505. Illinois significantly updated its child support guidelines in 2017, moving from a simple percentage of the paying parent's income to a model that weighs both parents' financial situations together.
The key distinction in Illinois is that calculations use net income rather than gross income. Net income in Illinois means income after subtracting federal taxes, Illinois state income tax (a flat 4.95 percent in 2026), Social Security, Medicare, mandatory union dues, and any prior court-ordered support or maintenance obligations you are currently paying. Two parents with similar gross incomes can have meaningfully different net incomes depending on their tax situations and existing obligations.
The Illinois Child Support Formula
Illinois follows four steps.
Step one is calculating each parent's net monthly income using the deductions listed above. Step two is adding both net incomes to produce the combined net monthly income. Step three is looking up the Basic Child Support Obligation in Illinois's support schedule using the combined net income and number of children. Step four is calculating each parent's percentage of the combined net income and applying that percentage to determine their individual share of the obligation.
Here is a practical example. Parent A has a net monthly income of $3,200 after all deductions. Parent B has a net monthly income of $1,800. Combined net income is $5,000. Parent A's income share is 64 percent. If Illinois's schedule sets the Basic Child Support Obligation at $1,100 for one child at $5,000 combined net income, Parent A's obligation is $704 per month. Add-ons for healthcare and childcare are then allocated proportionally on top of that amount.
Calculating Net Income in Illinois
Because Illinois uses net income, the first step requires working through your deductions carefully.
Start with total gross monthly income from all sources. Subtract federal income tax based on your actual withholding and filing status. Subtract Illinois state income tax at the flat rate of 4.95 percent. Subtract Social Security at 6.2 percent of wages up to $176,100 annually in 2026. Subtract Medicare at 1.45 percent of all wages. Subtract mandatory union dues if applicable. Subtract any court-ordered child support or maintenance you are currently paying to someone outside of this case.
The result is your Illinois net income for child support purposes. This figure is typically lower than many people expect because every mandatory obligation comes out first.
What Counts as Income in Illinois
Illinois courts include wages, salaries, commissions, bonuses, overtime, self-employment income, rental income, pension and retirement distributions, Social Security benefits, SSDI payments, unemployment compensation, and workers' compensation.
Courts can impute income to a parent who is voluntarily unemployed or underemployed. Illinois courts evaluate historical earnings, job skills, and the local employment market when setting an imputed income level. A parent who chooses to earn less cannot reduce their child support obligation by doing so.
Illinois excludes needs-based public assistance and child support received for children from other relationships from the income calculation.
Step-by-Step: How to Use This Calculator
Step 1. Calculate your net monthly income. Work through each deduction one at a time: federal tax, Illinois state tax at 4.95 percent, Social Security, Medicare, union dues, and prior court-ordered obligations. Do not enter gross income or take-home pay. Illinois requires the specific net income figure after the deductions listed above.
Step 2. Calculate the other parent's net monthly income using the same process. Use your best estimate if you do not know their exact deductions.
Step 3. Enter the number of children covered by this order.
Step 4. Enter your parenting time percentage. Count overnights per year and divide by 365.
Step 5. Add healthcare costs. Enter the monthly children's health insurance premium.
Step 6. Add childcare costs. Enter monthly work-related childcare expenses.
Step 7. Verify the net income figures in the results before accepting the final number. Net income errors are the most common source of inaccurate Illinois estimates.
Parenting Time Adjustments in Illinois
Illinois applies a parenting time adjustment when the paying parent has significant time with the children. The adjustment reflects the direct costs each parent bears during their time.
As parenting time approaches 50 percent, both parents' obligations are calculated and offset. The parent with the higher net obligation pays the difference. At equal parenting time with equal net incomes, the two obligations cancel and no payment flows in either direction.
Illinois courts also apply a shared parenting formula when parenting time is substantial but not exactly equal. The specific credit depends on the actual number of overnights per year.
Add-On Expenses in Illinois
Healthcare premiums and work-related childcare costs are added to the base obligation and allocated proportionally by net income share. Courts may also include extraordinary medical expenses, educational costs, and extracurricular expenses when circumstances warrant.
Reading Your Results
The results display shows each parent's net monthly income, the combined net income, the Basic Child Support Obligation from Illinois's schedule, income share percentages, any parenting time adjustment, add-on allocations, and the final monthly obligation.
The most critical verification step is confirming your net income inputs. Illinois's formula is sensitive to those figures. A $300 difference in net monthly income can shift the final obligation by $50 to $150 per month at typical income levels.
After You Get Your Estimate
Illinois courts follow the 750 ILCS 5/505 guidelines in all standard cases. Deviation is allowed when the guideline amount would be inappropriate. Courts can consider financial hardship, extraordinary expenses, high-income situations, or other relevant factors when a deviation is requested.
Modification in Illinois requires a showing of a substantial change in circumstances. A commonly applied benchmark is a 20 percent or more change in the paying parent's gross income. A change in the child's needs or a significant shift in parenting time are also grounds for modification. Either parent can file when circumstances have genuinely changed.
A licensed Illinois family law attorney can help you verify your net income calculation and interpret your results. Many offer a free initial consultation.