Illinois Child Support Guidelines: How Support Is Calculated

In Illinois, child support is set using the "income shares" model: the court combines both parents' net monthly incomes, looks up a basic support figure on the state's Schedule of Basic Child Support Obligations for that combined income and the number of children involved, and then divides that figure between the parents based on each parent's percentage share of the combined income. This approach has applied since July 1, 2017, under Public Act 99-0764, replacing Illinois's older percentage-of-income model. (Source: 750 ILCS 5/505(a)(1.5).)

The basic formula: income shares

The income shares model is meant to approximate what the child would have received if the parents had stayed together, by pooling both parents' incomes rather than looking at only the paying parent's income. In broad strokes, the process is:

1. Each parent's net monthly income is calculated.
2. The two net incomes are added together to get a combined income figure.
3. The court finds the basic child support obligation for that combined income and the number of children on Illinois's official Schedule of Basic Child Support Obligations.
4. That basic obligation is allocated between the parents according to each parent's share of the combined net income.

(Source: 750 ILCS 5/505(a)(1.5).)

What counts as income

Illinois law defines gross income broadly as "the total of all income from all sources." Net income is then gross income minus a standardized or individualized tax amount and other allowed adjustments. Certain money is excluded from this calculation, including means-tested public assistance and support a parent already pays for other children. (Source: 750 ILCS 5/505(a)(3).)

Because "all sources" is a broad standard and the deduction rules are technical, the exact net-income number in a specific case is something the court (or the parties, with guidance) works out using the official worksheets — it is not something to estimate casually.

The official tables change over time

Time-sensitive: the Illinois Department of Healthcare and Family Services (HFS) publishes and regularly updates the Gross to Net Income Conversion Table and the Schedule of Basic Child Support Obligations that the formula above depends on. The most recent revision took effect March 20, 2026. If you are working from an older printout of these tables, the numbers may already be out of date — always confirm you are using the current version from HFS before relying on a calculation. (Source: Illinois Dept. of Healthcare and Family Services, Income Shares.)

Shared physical care can change the math

If parenting time is closely shared, Illinois applies a different calculation. Under the current rule, when each parent has 146 or more overnights per year with the child, the basic child support obligation is multiplied by 1.5 to produce a "shared care obligation." Each parent's proportional share of that shared care obligation is then offset against the other's, and the parent who owes more pays the difference. (Source: 750 ILCS 5/505(a)(3.8).)

Time-sensitive: this overnight threshold is scheduled to change. An amendment lowers the qualifying threshold from 146 overnights to 110 or more overnights per parent per year, effective January 1, 2027. Families who don't currently qualify for the shared-care calculation because they fall short of 146 overnights may qualify once the lower threshold takes effect. (Source: 750 ILCS 5/505(a)(3.8), as amended, eff. Jan. 1, 2027.)

A minimum floor for very low-income parents

Illinois also builds in a floor for parents with very limited income. If the parent who would pay support earns at or below 75% of the federal poverty guidelines, there is a rebuttable presumption that the minimum support obligation is $40 per month per child, up to a maximum of $120 per month total, regardless of what the standard guideline calculation would otherwise produce. Because this is a "rebuttable" presumption, either parent can put evidence before the court to show that a different amount is more appropriate. (Source: 750 ILCS 5/505(a)(3.3a).)

When a court can deviate from the guideline amount

The guideline number is not absolute. A court may set a different amount only if it finds that applying the guideline figure would be inequitable, unjust, or inappropriate under the circumstances. If a judge deviates, the law requires the judge to make written findings stating (1) the guideline amount that would otherwise apply and (2) the specific reasons for departing from it. (Source: 750 ILCS 5/505(a)(3.4).) This paper trail matters if either parent later wants to appeal or seek a modification.

Changing an existing order

Child support orders in Illinois are not necessarily permanent. An order can be modified upon a showing of a substantial change in circumstances since it was entered. In addition, in cases being handled through the state's IV-D child support enforcement program, a parent may seek a modification without having to prove a substantial change if at least 36 months have passed since the last order and a new guideline calculation would differ from the current order by at least 20%, but not less than $10 per month. (Source: 750 ILCS 5/510(a).)

How this connects to spousal maintenance

Child support and spousal maintenance (sometimes called alimony) are calculated under different sections of Illinois law and are not the same thing. In a divorce, legal separation, or related proceeding, a court may separately grant a maintenance award "in amounts and for periods of time as the court deems just," without regard to marital misconduct, based on the parties' income and property. (Source: 750 ILCS 5/504(a).) If both child support and maintenance are on the table in your case, expect the court to address them as related but distinct calculations.

Enforcement doesn't stop at the state line

Federal law backs up Illinois's child support system in several ways worth knowing about:

• Income withholding, license suspension, and liens: federal law (42 U.S.C. §§ 654, 659, 666) requires states, including Illinois, to run a IV-D child support enforcement agency and to use standardized enforcement tools such as automatic income withholding, paternity establishment procedures, license suspension, and liens. Federal wages and certain federal benefits can also be garnished for support obligations.
• Orders follow you across state lines: under 28 U.S.C. § 1738B, other states must enforce an Illinois child support order, and (with narrow exceptions) only one state at a time has authority to modify it — this works together with the state-adopted Uniform Interstate Family Support Act (UIFSA).
• Support survives bankruptcy: under 11 U.S.C. §§ 507 and 523, child support (a "domestic support obligation") cannot be discharged in bankruptcy and is paid ahead of most other unsecured debts. Property-settlement debts from a divorce decree are also generally non-dischargeable in a Chapter 7 case.
• Past-due support generally can't be erased retroactively once it has accrued (the federal Bradley Amendment, 42 U.S.C. § 666(a)(9)(C)), and unpaid support can be collected through federal tax-refund offset (42 U.S.C. § 664).

What you can do in Illinois

• Get current numbers before you estimate anything. Use the Illinois HFS Gross to Net Income Conversion Table and Schedule of Basic Child Support Obligations that are currently in effect — these tables are updated periodically, most recently as of March 20, 2026, so an old printout can give you the wrong number.
• Document both parents' income and overnights. Because the formula depends on combined net income and, for shared-care cases, on the actual overnight count, keep clear records (pay records, a parenting-time calendar) to support the calculation.
• Watch the overnight-threshold change. If your parenting schedule gives each parent somewhere between 110 and 145 overnights a year, note that the shared-care multiplier's qualifying threshold is set to drop to 110 overnights on January 1, 2027 — this could change your support calculation going forward.
• If income is very low, ask about the minimum-support presumption rather than assuming the standard guideline formula automatically applies.
• If you believe the guideline amount is unfair in your situation, understand that the court can only deviate with written findings — come prepared with evidence addressing why the guideline figure would be inequitable, unjust, or inappropriate.
• If you want to modify an existing order, gather evidence of what has substantially changed since the last order (income, custody, needs), and if your case is IV-D-enforced and the last order is 36+ months old, ask about the streamlined recalculation option.
• Confirm any specific dollar figure, deadline, or procedural rule with your local Illinois circuit court or the HFS Division of Child Support Services before you rely on it — guideline tables and thresholds are updated periodically.

This article is for general information only and is not legal advice; consult a licensed Illinois attorney about your specific situation.