Illinois Child Support Calculator

How Illinois calculates child support

Until 2017 Illinois set support as a flat percentage of the paying parent's net income. That changed on July 1, 2017, when Public Act 99-0764 replaced it with the income shares model now found in 750 ILCS 5/505. If you have read that support is "20% of net income for one child," you are reading advice that is years out of date.

Under income shares, the court works through four steps:

1. Determine each parent's monthly net income and add them together.
2. Look that combined figure up in the Illinois HFS Schedule of Basic Child Support Obligations for the number of children. That gives the basic support obligation, roughly what two parents at that income level would spend on the children together.
3. Work out each parent's percentage share of the combined income.
4. The parent with less parenting time pays their percentage share of the basic obligation.

The schedule is published by the Illinois Department of Healthcare and Family Services and is reissued every year. This calculator uses the edition effective March 20, 2026, with the full 520-band table built in, so the figure comes from an exact lookup rather than an approximation.

What counts as “net income”

This is where most online estimates go wrong. Illinois does not mean your take-home pay after every deduction your employer makes. Under 750 ILCS 5/505, net income is your gross income minus either:

• the standardized tax amount, meaning federal and state income tax calculated for a single person claiming the standard deduction, one personal exemption and the applicable dependency exemptions, plus Social Security and Medicare; or
• the individualized tax amount, if the court uses your actual tax figures instead,

minus any further adjustments the statute allows. HFS publishes a gross-to-net conversion table alongside the support schedule for exactly this purpose.

One adjustment catches people out often enough to be worth stating plainly. If you are already supporting children from another relationship, the court deducts either the support you actually pay under an existing order, or 75% of what the guideline would be for those children if there is no order, whichever is less.

The calculator asks for net monthly income. If you only know your gross pay, your figure will come out high.

A worked example: majority parenting time

Two children. Parent A takes home $5,000 a month net, Parent B takes home $3,000. Parent A has the children 90 overnights a year, Parent B has them the other 275.

1. Combined monthly net income: $8,000.
2. The 2026 schedule gives a basic obligation for two children at $8,000 of $2,137 a month.
3. Parent A earns 62.5% of the combined income; Parent B earns 37.5%.
4. Parent A has the children fewer nights, so Parent A pays their share: $2,137 × 62.5% = about $1,336 a month.

Note what did not happen: nobody multiplied one salary by a fixed percentage. Parent B's income changed the answer, because it changed both the combined figure and the split.

Shared parenting and the 146-overnight rule

Illinois treats a case as shared physical care when each parent has the children for 146 or more overnights a year. When that threshold is met, 750 ILCS 5/505(a)(3.8) changes the arithmetic:

1. Multiply the basic obligation by 1.5, recognizing that running two households for the same children costs more.
2. Apportion that larger figure between the parents by income share.
3. Multiply each parent's amount by the other parent's percentage of parenting time.
4. Offset the two results. Whoever owes more pays the difference.

Two children, Parent A on $6,000 net and Parent B on $4,000, with time split 182 / 183 nights:

• Combined net $10,000 → basic obligation $2,499
• × 1.5 shared-care multiplier → $3,748
• Parent A's 60% share is $2,249; Parent B's 40% share is $1,499
• Cross-multiplied by the other parent's time: A owes $1,128, B owes $748
• Offset → Parent A pays about $380 a month

Almost equal time does not mean nobody pays. It means the income difference drives a much smaller number.

Changing on January 1, 2027. Senate Bill 3524 lowers the shared-parenting threshold from 146 overnights to 110, and adds a graduated adjustment of 10% down to 2% for parents with 110–145 overnights. The 1.5 multiplier is unchanged. This calculator applies the law as it stands today; if your case will be decided in 2027 or later, ask how the new threshold affects you.

What the guideline figure does not include

The schedule covers ordinary child-rearing costs. Several significant expenses sit on top of the basic obligation rather than inside it, which is why a court order often exceeds the calculator's number:

• Health insurance premiums for the children are added to the basic obligation.
• Childcare reasonably needed for work or education may be added.
• School and extracurricular expenses. A court may order either or both parents to contribute.
• Extraordinary medical expenses, and additional costs for a child with special medical, physical or developmental needs.

When a court departs from the guideline

The guideline result is a presumption, not a ceiling or a floor. A judge may order a different amount where applying the guideline would be inequitable, unjust or inappropriate, and must put the reasons in writing. Extraordinary medical expenditures needed to preserve a child's life or health, and the needs of a child with special medical, physical or developmental requirements, are named in the statute.

At the other end, Illinois sets a floor. For a parent whose actual or imputed gross income is at or below 75% of the federal poverty guidelines for a family of one person, there is a rebuttable presumption of a minimum obligation of $40 per child per month, capped at $120 a month in total and divided equally among that parent's children. Where a parent has no gross income, receives only means-tested assistance, or cannot work because of a medically proven disability, incarceration or institutionalization, a zero-dollar order is entered.

The published schedule also has a top. It runs to $26,024.99 of combined monthly net income; above that figure the court exercises its discretion rather than reading off a table.

Illinois child support: common questions

Who pays child support if we split parenting time 50/50?

Usually the higher earner still pays something. Once both parents pass 146 overnights the shared-care formula applies, the obligation is multiplied by 1.5 and the two parents' figures are offset against each other. Equal time narrows the gap but rarely closes it, because the calculation is still driven by the difference in income.

Is Illinois child support based on gross or net income?

Net. Specifically, gross income minus the standardized tax amount defined in 750 ILCS 5/505, or minus your individualized tax amount if the court uses your actual figures, and then minus any allowable adjustments.

Does the mother's income affect child support in Illinois?

Yes. Under the income shares model both parents' incomes go into the calculation regardless of which parent the children live with most. This is the single biggest change from the pre-2017 rules, where only the paying parent's income mattered.

How many overnights do I need for shared parenting to apply?

146 or more per year, for each parent, under the law as it stands. From January 1, 2027 that threshold drops to 110 overnights.

Does having children from another relationship reduce what I pay?

It can. The court deducts from your net income either the support you actually pay under an existing order, or 75% of the guideline amount for those children where no order exists, whichever is the smaller figure.

What is the maximum child support in Illinois?

There is no fixed cap. The published schedule stops at $26,024.99 of combined monthly net income; beyond that the court determines an appropriate amount using its discretion rather than the table.

How accurate is this calculator?

The arithmetic follows the statute exactly and uses the current HFS schedule, so for straightforward cases it is a genuinely good estimate. It cannot know about health insurance premiums, childcare, extraordinary expenses, disputed income, or the reasons a judge might deviate. Those are precisely the issues that move real numbers. Treat it as a starting point, not an answer.

Get the number that actually applies to your case

Attorney Eric Vernsten is a former Winnebago County prosecutor who has handled Illinois family law matters in Rockford for over a decade. Send your figures through the calculator above and he will review them personally and call you with a precise estimate, including the add-ons a guideline calculation leaves out. There is no obligation, and the call usually takes ten minutes.

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This page explains the Illinois child support guideline in general terms and is not legal advice. Reading it does not create an attorney-client relationship. Your obligation depends on the facts of your case and the discretion of the court.