How is Child Support Calculated in Illinois?

Illinois calculates child support using both parents' incomes. The state follows the Income Shares Model under 750 750 ILCS 5/505 — both parents' gross incomes are converted to net income using a standardized table, those net incomes are combined, and each parent pays their proportional share of the resulting obligation. Illinois updates its support schedules every year, making it one of the more current systems in the country. The shared parenting adjustment kicks in when each parent has at least 146 overnights per year.

The standardized Gross-to-Net Conversion Table

Most states calculate child support by subtracting a parent's actual taxes from their gross income to arrive at net income. Illinois takes a different approach — and it's worth understanding why.

Illinois uses a standardized Gross-to-Net Income Conversion Table published by the Illinois Department of Healthcare and Family Services (HFS). Instead of calculating each parent's actual tax liability — which varies based on filing status, additional deductions, and other individual factors — the table applies standardized tax amounts based on gross income level alone.

The practical effect is that Illinois child support calculations are more predictable than in states where actual tax returns drive the net income figure. A parent who itemizes aggressively or runs significant deductions through a business doesn't automatically get a lower net income for child support purposes. The table is the table.

What counts as income in Illinois

Illinois defines income broadly. The starting point for gross income includes wages, salary, tips, bonuses, commissions, overtime, self-employment income, Social Security benefits, Veterans Administration benefits, pension and retirement distributions, interest, dividends, rental income, and gambling winnings. Courts take a wide view of what constitutes a parent's financial resources.

If a parent is voluntarily unemployed or working below their capacity, Illinois courts may impute income — assigning earnings based on what that parent could reasonably earn given their education, experience, and the local job market. Choosing not to work, or choosing lower-paying work, generally doesn't reduce a child support obligation.

How the Income Shares formula works

The non-custodial parent's share is then paid to the custodial parent. Health insurance costs and work-related childcare expenses are added on top of the basic obligation and shared proportionally between the parents.

Worked Example — Two incomes, one child

The 146-overnight shared parenting threshold

Illinois has a specific, defined threshold for shared parenting — and it's higher than most people expect. The shared parenting formula only applies when each parent has at least 146 overnights per year. That's roughly 40% of the year for each parent.

When both parents meet this threshold, Illinois multiplies the basic child support obligation by 1.5 before calculating each parent's share. The 1.5 multiplier reflects the reality that two households are both actively incurring the child's everyday expenses — food, clothing, transportation, activities — rather than costs being concentrated in one home.

If only one parent has 146 or more overnights — or if the total adds up to 146 for one parent but the other falls short — the standard formula applies without the multiplier. The threshold matters: 145 overnights and 146 overnights can produce different outcomes under Illinois law.

For parents with arrangements close to the 146-night threshold, this is worth discussing specifically with a family law attorney, because the difference in support amounts can be meaningful.

Low-income protections

Illinois has clear protections for parents with very limited incomes. If the paying parent's gross income is at or below $978 per month — 75% of the federal poverty level — a minimum order applies rather than the standard formula.

Situation	Minimum order
Gross income at or below $978/month (75% of federal poverty level)	$40 per child per month (max $120/month total)
Receiving means-tested public assistance only (TANF, SNAP, SSI, Medicaid)	$0 order may be entered
Medically unable to work	$0 order may be entered

The minimum order still creates a legal obligation on record — which matters for modification purposes if the paying parent's financial situation improves later. Even a $40/month order preserves the legal framework for future review.

Add-ons: childcare and health insurance

The basic support obligation from the schedule is only part of the picture. Two categories of expenses are added on top and shared between the parents in proportion to their net income.

Work-related childcare costs — daycare, after-school programs, or similar care that allows a parent to work or attend school — are totaled and split proportionally. Health insurance for the children works similarly: if one parent is providing coverage, the children's share of the premium is factored into the overall calculation. Uninsured medical, dental, and mental health expenses are also typically shared proportionally when they arise.

When a judge can order a different amount

Illinois courts treat the guideline amount as the presumptive starting point. A judge may deviate from it — upward or downward — based on circumstances the court finds relevant. There is no strict cap on how far the deviation can go, provided the court's reasoning is documented.

Deviation factors courts may consider include the child's specific needs and circumstances, extraordinary medical or educational expenses, a parent's financial situation beyond what gross income reflects, non-monetary contributions to the child's care, and any other factors the court finds material to the child's best interest. Illinois courts have meaningful discretion here — the guidelines are a framework, not a ceiling.

If you're navigating divorce finances in Illinois more broadly, our Illinois divorce finances overview covers property division, spousal maintenance, and retirement accounts. We also have a dedicated guide on how spousal maintenance is calculated in Illinois, including the statutory formula that ties duration directly to marriage length.

Common questions about Illinois child support

Does Illinois child support end at 18? Generally yes — Illinois child support ends when the child turns 18 or graduates from high school, whichever is later. If the child is still in high school at 18, support typically continues through graduation. Courts may order support beyond 18 for a child with a disability that requires ongoing care, depending on the circumstances.

Can child support be modified in Illinois? Yes. Either parent may seek a modification if there's been a substantial change in circumstances — typically a significant change in either parent's income, a change in the parenting schedule, or a change in the child's needs. Illinois also allows a review every three years simply based on the passage of time and updated income, without needing to show a change in circumstances.

What if a parent is self-employed? Self-employment income is included in gross income. Courts look at business revenue minus ordinary and necessary business expenses to arrive at a net profit figure. Expenses that appear inflated or personal in nature may be scrutinized and added back in. Illinois courts have the ability to examine tax returns, business records, and financial statements when self-employment income is disputed.

How does Illinois handle parents who live in different states? When parents live in different states, Illinois courts apply the Uniform Interstate Family Support Act (UIFSA) to determine which state's law governs the support order. Generally the state that issued the original order retains jurisdiction, provided at least one parent or the child still lives there.