Illinois Child Support and Custody: How Overnights Drive the Calculation

The Income Shares Model

Since July 1, 2017, Illinois has used the Income Shares Model to calculate child support under 750 ILCS 5/505. The core principle: the child should receive the same share of parental income they would have received if the household had stayed intact.

Here's the calculation in five steps:

1. Determine each parent's gross monthly income from all sources (wages, self-employment, investments, rental income). Means-tested benefits like TANF and SNAP are excluded.

2. Convert to adjusted net income by subtracting standardized tax deductions, court-ordered child support paid for other children, and maintenance paid to a current or former spouse.

3. Combine both parents' net incomes to get the total family income.

4. Calculate each parent's percentage share — their net income divided by the combined total.

5. Look up the basic support obligation on the HFS (Department of Healthcare and Family Services) Schedule of Basic Child Support Obligations. This table maps the combined income and number of children to a dollar amount based on economic data about the cost of raising children.

The basic obligation is then split proportionally. In a standard arrangement, the parent with the majority of overnights spends their share directly on the child. The other parent pays their share to the majority-time parent.

The 146-Overnight Shared-Care Rule

The math changes significantly when the non-majority parent has 146 or more overnights per year — approximately 40% of the year. At that threshold, the arrangement is classified as "shared physical care" and a modified formula applies:

Step 1: Multiply the basic support obligation by 1.5. This increase accounts for duplicate household costs — separate bedrooms, clothing, toiletries at both homes.

Step 2: Split the increased obligation proportionally by each parent's income share.

Step 3: Multiply each parent's share by the percentage of parenting time exercised by the other parent.

Step 4: Offset the two amounts. The parent with the larger obligation pays the difference to the other.

The practical effect: equal parenting time doesn't eliminate child support if there's a significant income gap. A parent earning twice as much as the other will still owe support under the shared-care formula. But the amount is typically lower than it would be under the standard calculation.

Worked Example

Parent A earns $6,000/month net. Parent B earns $4,000/month net. Combined: $10,000. Two children. Assume the HFS table shows a basic obligation of $1,800.

Standard calculation (Parent B has majority time): Parent A's share is 60% × $1,800 = $1,080/month to Parent B.

Shared-care calculation (182 overnights each): $1,800 × 1.5 = $2,700 total. Parent A owes 60% × $2,700 = $1,620. Parent B owes 40% × $2,700 = $1,080. Parent A's adjusted: $1,620 × 50% (Parent B's time share) = $810. Parent B's adjusted: $1,080 × 50% (Parent A's time share) = $540. Offset: $810 - $540 = $270/month from Parent A to Parent B.

The difference between $1,080 and $270 per month illustrates why the overnight count is one of the highest-stakes numbers in an Illinois custody case.

What Changes in 2027 Under SB3524

Senate Bill 3524 takes effect January 1, 2027, and it restructures the shared-care framework:

Lower threshold: Shared-care calculations apply when each parent has at least 110 overnights (down from the current 146-overnight rule). This means parents with an every-other-weekend-plus-midweek arrangement may now qualify.

Overnight equivalents: For non-traditional schedules, the court can credit "overnight equivalents" when a parent exercises significant daytime care blocks (after-school through evening, full weekend days) without the child sleeping over.

Transition adjustment table: For arrangements between 110 and 145 overnights, a statutory adjustment table applies. This eliminates the current "cliff" at 146 nights, where crossing from 145 to 146 overnights dramatically changes the support amount.

Cap rule: The shared-care support obligation can never exceed what the standard calculation would produce — a protection for lower-income families.

Minimum obligation: For an obligor whose gross income is at or below 100% of the Federal Poverty Guidelines for a single-person household, a rebuttable presumption of $40/month per child applies (maximum $120/month total).

Annual income exchange: All support orders must include a mandatory annual income verification exchange between parents, reducing the need for modification petitions.

Add-On Expenses

Standard child support covers basic housing, food, and clothing. The court can order proportional contributions to additional expenses under 750 ILCS 5/505 and 505.2:

• Health insurance premiums for the child
• Unreimbursed medical, dental, and vision costs
• Childcare costs needed for a parent to work or attend school
• Educational expenses including private school, tutoring, and school supplies
• Extracurricular fees — sports, music, camps

The court may allocate these expenses proportionally based on each parent's income share, on top of the base support obligation.

Using the HFS Estimator

The Illinois Department of Healthcare and Family Services maintains an online child support estimator. Because SB3524 changes the shared-care rules effective January 1, 2027, check the estimator and applicable HFS materials for the version that applies to your case before relying on a result.

The Illinois Child Custody & Parenting Plan Guide includes an overnight-counter worksheet and a child support estimator walkthrough that shows how different schedule proposals affect your financial obligations — so you can evaluate the financial impact of each arrangement before you negotiate.

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