Illinois uses income shares to set child support, not a percentage of one parent's pay
Illinois child support is calculated using the income shares model, which means both parents' incomes are added together, and the court decides what portion each parent pays based on how much time the child spends with each one. This is different from some states that straightforward take a percentage of the paying parent's income. The calculation starts with the combined parental income, applies a percentage based on the number of children, then divides that obligation between parents according to their income split and custody arrangement.
The court uses the Illinois Child Support Guidelines, which are set by state law and updated periodically. These guidelines are not optional — a judge must follow them unless both parents agree to a different amount in writing, or the judge documents specific reasons why the guideline amount would be unjust or inappropriate for your situation.
Key Takeaways
- Illinois adds both parents' gross incomes together and applies a percentage based on the number of children, then divides the total obligation by income share and custody time.
- The guideline percentages range from 20 percent of combined income for one child to 50 percent for five or more children, but actual payments depend on each parent's share of that income.
- Custody time matters: the parent with more parenting time pays less, because the other parent's costs are higher.
- Gross income includes wages, self-employment earnings, bonuses, rental income, and some benefits, but the court can exclude certain items or cap income at a threshold set by state law.
- Either parent can ask the court to change the order if income changes significantly or circumstances shift.
What counts as income in the Illinois calculation
The court looks at gross income, which means income before taxes are taken out. This includes W-2 wages from a job, self-employment income, bonuses, commissions, rental income, interest and dividend income, Social Security benefits (in some cases), unemployment benefits, workers' compensation, and income from pensions or retirement accounts.
Some income is excluded or treated differently. Child support received for a different child, means-tested public benefits (like TANF or SNAP), and certain foster care or kinship care payments do not count. If a parent is voluntarily unemployed or underemployed — meaning they could earn more but choose not to — the court may assign them "imputed income" based on what they could reasonably earn. This prevents a parent from lowering their support obligation by quitting a job or taking a lower-paying position.
Illinois law also sets a cap on income used in the guideline calculation. As of the most recent update, income above a certain threshold (which changes periodically and varies slightly by rule) may not be included in the standard formula, though the court can still order support above the guideline amount if the parents' income exceeds that cap. Check the current threshold with the Illinois Department of Child Support Services or a family law attorney, since this number is adjusted.
The percentage applied based on number of children
Once the court adds both parents' gross incomes, it applies a percentage that depends on how many children need support:
| Number of Children | Percentage of Combined Income |
|---|---|
| 1 child | 20 percent |
| 2 children | 28 percent |
| 3 children | 32 percent |
| 4 children | 40 percent |
| 5 or more children | 50 percent |
These percentages are the starting point. If the combined income is $100,000 and there are two children, the total obligation is 28 percent of $100,000, or $28,000 per year. But that does not mean each parent pays half. The next step divides that obligation based on each parent's share of the combined income and how much time each parent spends with the child.
How custody time affects the calculation
The parent with more parenting time pays a smaller share of the total obligation, because they are already spending money directly on the child during their time. Illinois measures this as the percentage of overnights or days each parent has the child per year.
If one parent has the child 70 percent of the time and the other has 30 percent, the parent with 30 percent pays more support. The exact adjustment depends on the parenting time split and each parent's income. A parent with very little parenting time (say, 10 percent) may pay a larger share than their income percentage alone would suggest.
If parents have roughly equal parenting time (close to 50/50), the calculation still uses income share, but the obligation is divided more directly by income percentage. For example, if one parent earns 60 percent of the combined income and the other earns 40 percent, and they split parenting time equally, the higher-earning parent pays 60 percent of the guideline obligation.
How the court divides the obligation between parents
Here is a concrete example: Parent A earns $50,000 gross per year. Parent B earns $30,000 gross per year. Combined income is $80,000. They have one child, so the guideline percentage is 20 percent. The total obligation is $80,000 × 0.20 = $16,000 per year, or about $1,333 per month.
Parent A earns 62.5 percent of the combined income ($50,000 ÷ $80,000). Parent B earns 37.5 percent. If they split parenting time equally, Parent A would owe 62.5 percent of $1,333, which is about $833 per month. Parent B would owe 37.5 percent, which is about $500 per month. Usually, the parent with less parenting time pays the other parent the difference.
If Parent A has the child 80 percent of the time and Parent B has 20 percent, the calculation adjusts. Parent B's obligation increases because they have less parenting time and therefore fewer direct expenses. The exact adjustment is complex and depends on the specific parenting schedule, so courts use worksheets or software to calculate it precisely.
Adjustments and deviations from the guideline amount
The guideline calculation is the starting point, but the court can adjust it. If a parent pays child care costs, health insurance premiums for the child, or has significant debt related to the child's needs, those may be factored in. A parent may also request a deviation — a different amount than the guideline — if the guideline would be unjust or inappropriate.
Reasons for deviation include: one parent has significantly higher income than the other, the child has special needs or exceptional expenses, a parent has other children from different relationships, one parent has unusually high parenting time, or the parents have agreed to a different amount. The court must document its reasons in writing if it deviates from the guideline.
If a parent's income changes substantially — a job loss, a significant raise, a major change in parenting time — either parent can ask the court to modify the order. The court will recalculate using current income and circumstances.
Where to find the current guideline amounts and worksheets
The Illinois Department of Child Support Services publishes the current guideline percentages and income cap on its website. The state also provides child support calculation worksheets that show how the math works step by step. If you are involved in a child support case, your attorney or the court can provide these worksheets, or you can request them from the circuit court clerk in your county.
Many counties in Illinois have local child support enforcement offices that can explain how the guideline applies to your specific situation, though they cannot give legal information. If you are negotiating a support amount with the other parent or preparing for a court hearing, working with a family law attorney is the most reliable way to understand how the calculation applies to your income and parenting arrangement.
Frequently Asked Questions
Does child support change if one parent loses their job?
Yes. If a parent's income drops significantly, they can ask the court to modify the order. The court will recalculate using the new income. However, if the parent is voluntarily unemployed or underemployed, the court may still assign imputed income based on what they could earn, so a job loss alone does not automatically lower support.
What if the parents have equal parenting time?
The guideline percentage still applies to the combined income, but the obligation is divided by each parent's income share. If both parents earn the same amount and have equal time, they may owe equal support, or one may owe the other a smaller amount depending on the exact calculation. The court uses a worksheet to determine this.
Can parents agree to a different amount than the guideline?
Yes, but the agreement must be in writing and signed by both parents. The court must review it to make sure it is not unfair to the child. If both parents agree and the court approves, the amount can be different from what the guideline formula produces.
Does the guideline amount include health insurance and child care costs?
The guideline percentage is based on gross income and does not automatically include those costs. However, if one parent pays for the child's health insurance or child care, the court may adjust the obligation or order the other parent to reimburse a portion. These are separate from the base child support amount.
What happens if a parent's income is very high?
Illinois law sets an income cap for the guideline calculation. Income above that threshold may not be included in the standard formula, but the court can still order support above the guideline amount based on the child's needs and the parents' ability to pay. The court has discretion to decide how much of the high income goes toward support.