Illinois · Guidelines explained

How Illinois calculates child support

Illinois uses the Income Shares model under 750 ILCS 5/505. Here's exactly how the math works.

The formula

The Income Shares model

Both parents' gross incomes are combined into a single "combined adjusted gross income." The state's published schedule table converts that figure into a base child support obligation. Each parent then pays their proportional share of that amount, weighted by their income. When custody is shared, parenting time credits proportionally reduce the obligation.

Quick reference

Key facts for Illinois

Income type: gross
Support ends: Age 18

Common questions

What the guidelines don't say

Not true: “50/50 custody means zero support”

Even with equal parenting time, the higher-earning parent typically still owes support if incomes are unequal. The formula accounts for time, but income differential drives the result.

Not true: “These numbers are the final word”

Courts can deviate from guideline amounts based on special circumstances — extraordinary expenses, disabled children, or other factors the formula doesn't capture. A calculator gives an estimate, not a court order.

Source

Official guidelines

This calculator implements the formula directly from Illinois's published guidelines. For the authoritative text: Illinois child support guidelines →

Last verified: 2024-01

Illinois transitioned to an income-shares model in 2017.

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