How Illinois Child Support Is Calculated
Illinois switched to the Income Shares model in 2017 (750 ILCS 5/505). Both parents' net monthly incomes are combined and matched against the Illinois Minimum Child Support Obligation Schedule, then each parent's share is determined by their income proportion.
Calculate each parent's net monthly income
Gross income minus federal income tax, Illinois state income tax (4.95% flat rate), Social Security (6.2%), Medicare (1.45%), and mandatory health insurance premiums.
Look up combined net income in the IL schedule
The Illinois Minimum Support Obligation Schedule provides the base obligation based on both parents' combined net incomes and number of children.
Calculate the paying parent's income share
Divide the paying parent's net income by the combined net income. Multiply this percentage by the schedule obligation to get the NCP's base obligation.
Add healthcare and childcare costs
Health insurance premiums and work-related childcare are allocated proportionally between parents.
Illinois Minimum Support Obligation Schedule (Net Monthly, 2025)
| Combined Net Income | 1 Child | 2 Children | 3 Children | 4 Children |
|---|---|---|---|---|
| $1,500 | $354 | $530 | $604 | $639 |
| $2,500 | $544 | $815 | $930 | $984 |
| $4,000 | $818 | $1,223 | $1,395 | $1,477 |
| $5,000 | $997 | $1,490 | $1,700 | $1,800 |
| $6,000 | $1,174 | $1,753 | $2,002 | $2,119 |
| $8,000 | $1,519 | $2,272 | $2,595 | $2,748 |
| $10,000 | $1,857 | $2,779 | $3,175 | $3,362 |
| $12,000 | $2,190 | $3,275 | $3,742 | $3,963 |
| $15,000 | $2,687 | $4,016 | $4,587 | $4,857 |
Source: Illinois Minimum Child Support Obligation Schedule (750 ILCS 5/505). Values are the combined obligation — NCP pays their income share.
Illinois Child Support — Frequently Asked Questions
Illinois adopted the Income Shares model effective July 1, 2017, replacing the old flat-percentage model. Under the old model, the paying parent's net income alone determined the support amount (20% for 1 child, 28% for 2, etc.). The current Income Shares model uses both parents' net incomes and compares them to an Illinois schedule of minimum support obligations.
Illinois uses "net income" — gross income minus federal and state income taxes, Social Security (6.2%), Medicare (1.45%), and mandatory healthcare premiums. Illinois has a flat state income tax of 4.95% (as of 2025). Net income is sometimes also called "net monthly income" or "NMI" in Illinois courts.
Illinois courts may impute income to a parent who is voluntarily unemployed or underemployed at a level below their demonstrated earning potential. The imputed amount is based on education, work history, and available job opportunities, and is used in place of actual income for calculating support.
Illinois law provides an adjustment when the paying parent exercises at least 146 overnights (40%) per year. A "shared parenting" formula applies that can significantly reduce the support amount by accounting for direct expenses the paying parent incurs during their parenting time.
Yes. In addition to the basic obligation, Illinois courts can order add-ons for: childcare costs necessary for employment, health insurance premiums for the children, and extraordinary expenses such as special educational or medical needs.