What Illinois Child Support Amounts Look Like for One Child

Illinois does not set a fixed dollar amount for child support — the court calculates it based on both parents' income, custody time, and other factors

Illinois uses a income shares model, which means the court looks at what both parents earn, figures out what percentage of their combined income should go toward the child, and then divides that amount between them based on how much time each parent spends with the child. For one child, the percentage of combined parental income that goes to child support is 20 percent. That 20 percent is then split: the parent who does not have primary custody pays their share, and the parent with primary custody may also owe their share depending on the custody arrangement.

The calculation starts with net income — what each parent actually takes home after taxes, not their gross salary. The court adds both parents' net incomes together, multiplies by 20 percent, and that is the total child support obligation. If one parent has the child 80 percent of the time and the other 20 percent, the parent with 20 percent custody pays a larger share of that 20 percent total.

There is a cap: in 2024, child support calculations only explore to combined parental income up to a certain threshold, which the state adjusts yearly. Income above that threshold may still be considered, but the court has discretion on how much of it to include. This means two families with very different incomes can end up with the same support amount if both exceed the cap.

Key Takeaways

• Illinois calculates child support as 20 percent of combined parental net income for one child, divided between parents based on custody time.
• The parent with less custody time typically pays the larger share, but both parents can owe support depending on the custody split.
• The calculation uses net income (take-home pay), not gross salary, and only applies to income up to a state-set cap that changes yearly.
• A judge can adjust the amount up or down based on factors like health insurance costs, childcare expenses, and whether either parent has other children to support.
• The actual dollar amount depends entirely on what both parents earn, so there is no single number that applies to all Illinois families.

How the 20 percent calculation works in practice

Say Parent A earns $3,000 per month net and Parent B earns $2,000 per month net. Combined, that is $5,000. Twenty percent of $5,000 is $1,000 — that is the total child support obligation for one child. If Parent A has the child 80 percent of the time and Parent B has the child 20 percent of the time, Parent B pays roughly $800 per month (their 20 percent share of the $1,000 total). Parent A, who has primary custody, may owe nothing or a small amount depending on the exact custody arrangement and whether the judge adjusts the calculation.

If the custody split is 50-50, the math changes. Both parents share the child equally, so they split the $1,000 obligation based on their income ratio. Parent A (earning $3,000) pays 60 percent of the $1,000, which is $600. Parent B (earning $2,000) pays 40 percent, which is $400. The parent with higher income pays more even in a 50-50 arrangement.

These are simplified examples. The actual calculation also subtracts certain expenses — childcare costs, health insurance premiums for the child, and sometimes other factors — before the 20 percent is applied. A judge can also deviate from the formula if circumstances warrant it.

Income cap and high-earning parents

Illinois sets an annual income cap for the standard child support calculation. In recent years, this cap has been in the range of $250,000 to $290,000 of combined parental income, but the exact figure changes each year and you should confirm the current year's cap with the court or a family law attorney. Income above the cap is not automatically included in the calculation.

If both parents earn well above the cap, the judge has the option to include some or all of the excess income, but is not required to. The judge considers factors like the child's standard of living, the parents' ability to pay, and what the child actually needs. This means two families where parents earn $400,000 and $500,000 combined might receive very different support orders depending on the judge's reasoning.

Adjustments the court can make

The 20 percent formula is a starting point, not a final answer. Illinois law allows judges to adjust the amount based on several factors. If one parent pays for the child's health insurance, that cost is usually subtracted from their income before the calculation. If one parent pays for childcare so the other can work, that expense is also typically deducted. If either parent has other children from other relationships, the judge may lower the support amount to account for those obligations.

A judge can also adjust if the formula would be unjust or inappropriate — for example, if one parent has a very low income and cannot afford the calculated amount, or if the child has special needs that require more support. The judge must document in writing why they are deviating from the formula, so the reasoning becomes part of the court record.

What happens if parents share custody equally

When both parents have the child roughly 50 percent of the time, child support still applies, but the amount is usually smaller than in a primary-custody arrangement. The court still calculates 20 percent of combined income, but divides it based on each parent's income share rather than custody time. The parent with higher income pays the other parent to equalize the child's standard of living between households.

If both parents earn exactly the same amount, the child support obligation may be zero or very small, because neither parent is earning significantly more than the other. However, if one parent earns substantially more, that parent pays support even in a 50-50 custody split.

Self-employment income and variable earnings

If either parent is self-employed or has income that varies month to month, the court typically averages their income over a period of time — often the past two years — to smooth out fluctuations. A parent who earned $30,000 one year and $50,000 the next would have an average of $40,000 used in the calculation. This prevents child support from swinging wildly when business income changes.

Self-employed parents must provide tax returns, profit-and-loss statements, and sometimes business records to prove their income. If a parent is hiding income or underreporting earnings, the other parent can request an investigation, and the court can impute income — meaning the judge assigns an income figure based on what the parent should be earning given their skills and work history.

Modifications and changes over time

A child support order is not permanent. If either parent's income changes significantly — a job loss, a major raise, a disability — either parent can ask the court to modify the amount. Illinois generally allows modification if there has been a substantial change in circumstances. A 10 percent or greater change in either parent's income is often considered substantial, but smaller changes can also may have access to depending on the situation.

The court will recalculate using the new income figures and issue a modified order. Until the modification is approved by a judge, the original amount remains due. Parents cannot straightforward stop paying or reduce payments on their own, even if they believe the amount is unfair.

Frequently Asked Questions

Does Illinois have a minimum child support amount?

Illinois does not set a flat minimum, but the calculation itself produces a floor: 20 percent of combined parental income for one child. If both parents have very low income, the amount will be low. However, a judge can order a parent to pay more than the formula suggests if the parent has the ability to pay and the child has significant needs.

What if one parent does not have a job or reports zero income?

The court does not accept zero income at face value. If a parent is unemployed or underemployed, the judge can impute income based on their education, work history, and earning potential. A parent with a college degree who claims to be unemployed may be assigned an income based on what someone with that education typically earns in the area. The parent can argue they are genuinely unable to work, but they must provide evidence.

Can child support be waived if parents agree?

Parents can agree to a child support amount that differs from the formula, including an amount lower than the calculation suggests, but the court must approve it. The judge will review the agreement to make sure it is not unfair to the child. If the agreed amount is significantly below what the formula produces, the judge may reject it or require the parents to explain why the lower amount is appropriate.

Does child support end at age 18 in Illinois?

Child support typically ends when the child turns 18, but if the child is still in high school, support continues until the child graduates or turns 19, whichever comes first. Support does not continue into college unless the parents agree to it or the court orders it as part of the original judgment.

How often does Illinois update the income cap?

Illinois adjusts the income cap annually, usually in January, based on changes in the cost of living. You should check the current year's cap with the Illinois Department of Human Services or a family law attorney, as it changes and affects how high-earning parents' support is calculated.