Illinois uses income shares to set child support, not a percentage of your paycheck

Illinois child support is calculated using the income shares model. This means the court adds both parents' incomes together, finds what percentage of that combined income goes to child support based on the number of children, then divides that amount between the parents based on how much each one earns. It is not a flat percentage of one parent's income.

The calculation starts with net income — what you actually take home after taxes, Social Security, Medicare, and court-ordered support for other children. The court then applies a percentage based on how many children need support. For one child, the guideline is roughly 20 percent of combined net income. For two children, roughly 28 percent. For three children, roughly 32 percent. For four children, roughly 40 percent. For five or more children, roughly 45 percent. These percentages are set by Illinois law and do not change based on individual circumstances.

Once the court knows the total support amount, it splits that between parents based on their income shares. If one parent earns 70 percent of the combined income and the other earns 30 percent, the higher earner typically pays 70 percent of the total support obligation. The lower earner may also owe support if they have custody less than half the time.

Key Takeaways

  • Illinois adds both parents' net incomes together, applies a percentage based on the number of children, then divides the total by income share rather than using a simple percentage of one parent's pay.
  • Net income includes wages, self-employment income, rental income, and certain benefits, but excludes taxes, existing child support obligations, and some court-ordered payments.
  • The percentage guideline ranges from 20 percent of combined net income for one child to 45 percent for five or more children, and these percentages are set by state law.
  • A judge can order more or less than the guideline amount if circumstances like high income, low income, or shared custody make the guideline unjust.
  • Child support in Illinois continues until the child turns 18, or 19 if still in high school, unless the child is emancipated or adopted.

What counts as income for child support purposes

Income for child support includes W-2 wages, self-employment earnings, rental income, investment income, unemployment benefits, workers' compensation, disability benefits, and certain retirement distributions. It also includes bonuses, commissions, overtime, and tips if they are regular and recurring.

Income does not include public assistance like TANF or SNAP, Supplemental Security Income (SSI), or the first $30,000 of child support received from another case. If you are self-employed, the court uses your net business income after legitimate business expenses, but it may add back certain deductions like depreciation or entertainment expenses that the court views as discretionary.

If you are unemployed or underemployed, the court may impute income — meaning it assigns you an income level based on your work history, education, or job market rates in your area, even if you are not currently earning that much. This prevents a parent from deliberately reducing income to lower their support obligation. However, the court must find that the underemployment is voluntary before imputing income.

How custody time affects the calculation

The percentage of time each parent has custody matters because it changes how much support the lower-earning parent owes. If both parents have the children roughly equally (within a few percentage points), the support obligation may be reduced or eliminated for the lower earner, depending on the income gap.

Illinois law defines parenting time as the number of overnights per year. If one parent has the children 183 or more overnights per year (roughly half), that parent is considered to have significant parenting time. The more overnights a parent has, the less the other parent typically owes, because the first parent is covering more of the day-to-day costs.

If one parent has the children 90 percent of the time and the other has them 10 percent, the support obligation is much higher than if the split is 60-40. The court uses actual parenting time from the custody order or, if there is no formal order, from what the parents actually practice. If parenting time changes significantly after the order is entered, either parent can ask the court to modify the support amount.

When a judge can order more or less than the guideline amount

Illinois law says the guideline percentage is a rebuttable presumption. This means it is the starting point, but a judge can order a different amount if the guideline would be unjust or inappropriate given the facts of the case.

Reasons a judge might order more than the guideline include: one parent has very high income (over $240,000 per year, though this threshold changes), the children have special needs or medical expenses, one parent is hiding income, or the children attend private school. Reasons a judge might order less include: one parent has very low income and cannot meet basic living expenses, the parents have nearly equal custody and similar incomes, or one parent is already paying substantial support for other children.

When a judge deviates from the guideline, they must write down their reasons in the court order. This matters because if support is later modified, the judge needs to know why the original amount was different from the guideline.

How the court determines net income from your pay stub

The court starts with your gross income — the total before any deductions. Then it subtracts mandatory deductions: federal income tax withholding, Social Security tax, Medicare tax, state income tax, and local taxes if applicable. It also subtracts health insurance premiums you pay for the children and any existing child support or spousal support you are already ordered to pay.

The court does not subtract voluntary deductions like 401(k) contributions, union dues, or life insurance, though some judges will consider these if they are substantial and necessary. If you are self-employed, you report your net business income (revenue minus legitimate business expenses) and then subtract the self-employment tax you actually pay.

You will need to provide recent pay stubs, tax returns, and a financial affidavit (a sworn statement of your income and expenses) to the court. If your income varies — for example, you work on commission or have seasonal work — the court may average your income over the past two years to smooth out the ups and downs.

How child support changes if income or custody changes

Child support is not permanent at the amount ordered. Either parent can ask the court to modify it if there is a substantial change in circumstances. In Illinois, a substantial change usually means a change of 10 percent or more in either parent's net income, or a significant change in parenting time.

If you lose your job, get a significant raise, or your custody arrangement changes, you can file a motion to modify. You will need to show the court your current income (through recent pay stubs and tax returns) and explain what changed. The court will recalculate using the same income shares method and set a new amount if the change is substantial enough.

Child support continues until the child turns 18, or 19 if the child is still enrolled in high school full-time. If a child is adopted, becomes emancipated, or dies, the support obligation for that child ends. Support for remaining children continues unless the total obligation is also modified.

What happens if you do not pay or if you disagree with the amount

If you do not pay child support, the other parent can ask the court to enforce it. The court can garnish your wages, intercept your tax refunds, suspend your driver's license, or hold you in contempt of court. Contempt can result in jail time if you have the ability to pay but refuse.

If you believe the amount ordered is wrong — either because the judge made a math error or because you have new information about income or custody — you can file a motion to modify or a motion to reconsider. You must do this in the same court that issued the original order. If you simply stop paying because you disagree, you will owe back support plus interest and court costs.

If you cannot afford the current amount because your income has dropped, file a modification motion as soon as possible. The court can only change the amount going forward from the date you file, not retroactively, so waiting makes the debt larger.

Frequently Asked Questions

Does Illinois count overtime or bonuses as income for child support?

Yes, if they are regular and recurring. One-time bonuses may not count, but if you receive a bonus every year or overtime most weeks, the court will include it. You will need to show proof through pay stubs or tax returns that the income is consistent, not just occasional.

What if I am self-employed or have irregular income?

The court typically averages your income over the past two years using your tax returns. If your income is highly variable, the judge may use a lower figure to account for lean months, or may set the support amount with the understanding that it will be reviewed annually. Provide complete tax returns and business records to show the court your actual earnings.

Can child support be modified if my ex gets a better job?

Yes. If the other parent's income increases significantly, you can file a motion to modify and ask the court to recalculate. The same applies if your income drops. You must show the court documentation of the income change and file the motion in the court that issued the original order.

Does shared custody mean we split the child support 50-50?

Not necessarily. Even with equal custody time, the parent who earns more income typically still pays support to the parent who earns less. The calculation accounts for both custody time and income. If both parents have equal custody and equal income, support may be minimal or zero, but the income difference usually means one parent still owes something.

What if the other parent is hiding income?

You can ask the court to order financial discovery — meaning the other parent must produce tax returns, bank statements, and business records. If you believe income is being hidden, you can request that the court impute income based on their education, work history, or lifestyle. Bring evidence like property purchases, vehicle registrations, or business filings to support your claim.