How Child Support is Calculated in Illinois

Illinois income shares model (since July 2017)

Before July 1, 2017, Illinois used a percentage-of-income model for child support, where support was a flat percentage of the paying parent's net income: 20% for one child, 28% for two children, and so on. That model did not consider the receiving parent's income at all.

Since July 1, 2017, Illinois uses the income shares model under 750 ILCS 5/505. The income shares model combines both parents' net incomes to determine what the children's total basic support need would be if the family were living together, then allocates that obligation between the parents in proportion to their respective incomes.

Step 1: Calculate each parent's net income

Net income for child support purposes starts with gross income from all sources and subtracts allowable deductions. Gross income includes wages, salaries, bonuses, commissions, self-employment income, rental income, investment income, and any other regular source of income.

Allowable deductions from gross income include:

• Federal and state income taxes (based on filing status)
• FICA (Social Security and Medicare taxes)
• Mandatory union dues
• Prior child support obligations currently in effect for other children (not future or theoretical obligations)
• Health insurance premiums paid by the parent for the children who are the subject of the current support order
• Mandatory retirement contributions required by the employer

Step 2: Combine net incomes and find the basic support obligation

Once you have calculated each parent's net income, add them together to get the Combined Net Income. The Illinois Department of Healthcare and Family Services (BCSE) publishes an Income Shares Schedule — a table that shows the Basic Support Obligation for a given Combined Net Income and number of children.

For example, if Parent A has a net income of $4,000/month and Parent B has a net income of $2,000/month, the Combined Net Income is $6,000/month. You then look up $6,000 combined income with two children on the BCSE schedule to find the Basic Support Obligation — the total amount the court estimates those children need.

Step 3: Allocate the obligation between parents

Each parent is responsible for a share of the Basic Support Obligation proportional to their contribution to the Combined Net Income.

Using the example above:

• Parent A's share = $4,000 / $6,000 = 66.7%
• Parent B's share = $2,000 / $6,000 = 33.3%

If the Basic Support Obligation for two children at $6,000 combined income is, hypothetically, $1,200/month, then Parent A owes $800/month (66.7%) and Parent B owes $400/month (33.3%). The non-residential parent's share is the actual support payment that flows to the other parent.

Additional expenses

The Basic Support Obligation covers ordinary expenses. Certain additional expenses are added to the Basic Support Obligation and also split proportionally between the parents:

• Work-related childcare costs — daycare, after-school care, and summer camp necessary for a parent to work
• Health insurance premiums — the cost of the children's health insurance coverage (net of tax savings)
• Extraordinary medical expenses — medical costs not covered by insurance that exceed a de minimis threshold

Modification of child support

An existing child support order can be modified under 750 ILCS 5/510 in two circumstances:

• Substantial change in circumstances — a significant change in either parent's income, the child's needs, or the amount of parenting time since the last order was entered.
• Three-year review — if three years have passed since the last support order, you can request a modification if the new calculated amount would differ from the current amount by at least 20% or $10 per month, whichever is greater. No showing of substantial change is required for a three-year review.

Either parent can request a BCSE administrative review every three years. The BCSE will recalculate support based on current income information and, if the recalculated amount differs sufficiently, submit the modification to the Circuit Court for approval.