Illinois Child Support Calculation Guide

Illinois uses the Income Shares model for child support, effective since July 1, 2017. The framework is codified in 750 ILCS 5/505, with annual updates to the schedule and conversion tables. Whether you're navigating an uncontested or contested divorce, understanding these calculations is essential.

The Income Shares Model

Illinois estimates what intact-household parents at the same combined income typically spend on their children, then allocates that cost between both parents:

• Step 1: Determine each parent's monthly net income
• Step 2: Add both nets to get combined monthly net income
• Step 3: Find the Basic Child Support Obligation (BCSO) from the schedule
• Step 4: Multiply BCSO by each parent's income share percentage
• Step 5: Apply parenting-time adjustments and add-ons

Calculating Net Income

Illinois uses either a standardized tax method (official Gross-to-Net Conversion Table) or individualized actual taxes if stipulated:

• Gross income: All sources except means-tested benefits (TANF, SSI, SNAP)
• Deductions: Federal/state income taxes, FICA/Medicare
• Multi-family adjustment: Support paid for other children may be deducted
• Business income: Net receipts minus ordinary expenses; add back excessive deductions

The 2025 Schedule (Effective March 5, 2025)

The Basic Child Support Obligation Schedule covers combined monthly net income from $875 to $26,724.99. For income above the schedule cap, courts have discretion but cannot order less than the highest scheduled amount.

Estimate Your Illinois Child Support

Use our calculator for a preliminary estimate. For official calculations, use the Illinois HFS Child Support Estimator.

146-Overnight Threshold (Shared Physical Care)

If each parent has 146 or more overnights per year, shared physical care rules apply:

• Multiply the BCSO by 1.5 to account for duplicated household expenses
• Allocate by income shares (each parent's portion)
• Multiply each portion by the percentage of time the child spends with the other parent
• Offset the two results; the higher amount pays the difference

Minimum Orders and Low-Income Protection

Illinois protects low-income obligors:

• At or below 75% FPG: Presumptive minimum of $40/month per child, capped at $120 total
• No income/means-tested only: Presumption of $0 order
• Medically disabled/incarcerated: $0 order presumption
• Schedule floor: $875 net/month aligns with 75% FPG

Add-On Expenses

Beyond basic support, certain expenses are prorated by income shares:

• Child care: Work, education, or job-search related costs, net of the child-care tax credit
• Health insurance: Child's premium allocated by shares; capped at 5% of providing parent's gross income
• Medical expenses: Unreimbursed medical, dental, and vision costs
• School/extracurriculars: May be ordered if reasonable

Parents with net income below 133% of FPG generally are not ordered to pay for private health coverage.

Modification Rules

Illinois allows modification when:

• Substantial change in circumstances
• For IV-D/HFS cases: 36+ months since last order AND 20% (minimum $10/month) inconsistency with current guidelines
• Automatic review: HFS reviews IV-D cases at least every 36 months

Retroactive modification is not permitted before the motion is filed.

Duration of Support

Support ordinarily ends at age 18, but may continue:

• Through high-school graduation (no later than age 19)
• For adult children with qualifying disabilities under 750 ILCS 5/513.5

Enforcement and Arrears

Illinois enforcement tools include:

• Income withholding: Standard method through State Disbursement Unit
• License suspension: Driver's, professional, and recreational licenses for 90+ day delinquency
• Tax intercepts: Federal and state refunds
• Passport denial: Until debt is paid
• Credit reporting: Delinquencies reported to credit bureaus
• Liens/levies: On property and bank accounts
• Contempt: Court sanctions for willful non-payment

Interest on arrears is 9% simple annually, but HFS only adjudicates interest upon specific criteria (typically at case closure upon timely request).

Key Takeaways

• Income-shares model: Both parents' incomes determine the child's needs
• 2025 schedule: $875 to $26,724.99 combined monthly net income
• 146-overnight threshold: Triggers shared-care formula (BCSO × 1.5)
• Minimum orders: $40/child, $120 cap for very low income
• Add-ons: Child care, health insurance (5% cap), medical expenses
• 36-month review: For IV-D cases with 20% guideline inconsistency
• 9% interest: Simple, on request only

For more information about Illinois divorce processes, see our Illinois divorce timeline and filing checklist.