How to lower child support in Illinois

The Illinois test, in plain words

Threshold: Inconsistency of at least 20%, but no less than $10 per month, between existing order and §505 guideline result — without proving substantial change. Unavailable if inconsistency exists because existing order was a deviation and deviation circumstances have not changed (750 ILCS 5/510(a)(1)-(2)(A))

Illinois uses income shares: both parents' net incomes combine, the 2026 HFS Schedule of Basic Obligations gives the basic amount, and each parent carries their income share. When each parent has at least 146 overnights a year, the shared-care formula applies: basic obligation × 1.5, then a cross-credit for time. The HFS practice rule (a 20% change, on a 3-year review cycle) is a separate authority from the statutory test in 750 ILCS 5/510(a)(2)(A) and is cited separately.

Agency route: HFS Child Support Services (CSS) — Modification review. Court route: Circuit court — Own petition/motion under §510

One trap to know: 20%-inconsistency route unavailable if inconsistency exists because existing order was a deviation and deviation circumstances have not changed (750 ILCS 5/510(a)(2)(A))

Exactness: Computed from the 2026 HFS schedule and gross-to-net table (effective 2026-03-20). If the individualized net-income method is used instead, the figure becomes an ESTIMATE and both nets must be shown.

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Your stateTexasCaliforniaFloridaNew YorkIllinoisOhioAnother state

What you pay now ($ / month)

Children in this case123456

Months since the order was made or last changed

Texas

Your monthly net resources ($)Net resources as Texas defines them (Tex. Fam. Code §154.062). The $11,700 monthly cap is applied for you.

Children you support in other households01234567Uses the §154.129 multiple-family adjusted percentages.

California — ESTIMATE only, permanently

Your monthly net disposable income ($)

Other parent's monthly net disposable income ($)

Your timeshare (%)

California's formula (Fam. Code §4055) turns on actual tax liability and approximate timeshare, so no California figure on this site is ever labeled exact.

Florida

Combined monthly net income, both parents ($)

Your monthly net income ($)

Your overnights per yearAt 73 or more overnights, the §61.30(11)(b) adjustment is mandatory and is applied.

New York

Combined parental income ($ / year)

Your gross income ($ / year)

Mandatory add-ons ($ / year: health insurance, unreimbursed medical, childcare)

Change in either parent's gross income since the order (%)

The basic obligation below the $193,000 combined cap is computed. The payable amount is always labeled ESTIMATE — add-ons and above-cap discretion sit on top.

Illinois

Your monthly gross income ($)Converted with the 2026 HFS gross-to-net table (effective 2026-03-20).

Other parent's monthly gross income ($)

Your overnights per yearAt 146 overnights each, the §505(a)(3.8) shared-care formula applies and is used.

Ohio

Combined gross income, both parents ($ / year)Run through the §3119.021 bracket formula.

Your gross income ($ / year)

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Track B — the evidence checklist (never scored)

• Substantial change in circumstances (foreseeability is not a defense unless the future event was expressly specified in the order/agreement)
• Termination letter / unemployment filing / final pay stub
• Before/after pay stubs and employer letter
• SSA determination / physician statement
• Overnight log — 12-month calendar with the 146-overnight line marked
• Health-insurance premium statements; evidence for child's health-care needs route §510(a)(2)(B)

Sources

• IL modification — 750 ILCS 5/510(a)(1)-(2)(A) — ilga.gov, pulled 2026-10-27
• IL guideline — 750 ILCS 5/505, incl. §505(a)(3.8) shared care
• IL agency review — HFS official page, hfs.illinois.gov

Rules checked October 2026.