How Illinois calculates child support
- Each parent’s gross monthly income is converted to net income with the state’s standardized conversion table, which has one column for the parent with the majority of parenting time and one for the other parent.
- The parents’ net incomes are added together, and the schedule of basic obligations gives the amount parents with that combined income spend on the number of children.
- Each parent is responsible for their percentage share of that amount; the parent without the majority of parenting time pays their share to the other parent.
- When each parent has the children at least 146 overnights a year, the basic obligation is multiplied by 1.5, each parent’s share is multiplied by the other parent’s percentage of overnights, and the two amounts are offset.
- A paying parent with gross income at or below 75% of the federal poverty guideline is presumed to owe a minimum of $40 a month per child, up to $120.
What the calculator asks
- How many children is the support for?
- How are the children’s overnights split?
- How many overnights a year are the children with you?
- Your gross income per month
- The other parent’s gross income per month
- Child support you pay under a court order for another child
- Child support the other parent pays under a court order for another child
- Does either parent support another child who is not in this case and has no support order?
- Does either parent pay or receive spousal maintenance?
- Does a child receive a Social Security dependent benefit because of a parent’s disability or retirement?
It never asks for names, Social Security numbers or account numbers, and your answers are not saved.
What the estimate leaves out
- Health insurance premiums for the children, which the worksheet shares between the parents separately
- Child care, and extraordinary school and extracurricular expenses, which the court may add
- Spousal maintenance received or paid
- Support for other children without a court order, and a child’s Social Security dependent benefit
- Individualized (non-standardized) taxes, which need the parties’ agreement or a court finding
- Split physical care and deviations the court may make (750 ILCS 5/505(a)(3.4))
When a situation is outside what the calculator covers, it says so and points you to the official worksheet instead of guessing.
Official sources
- 750 ILCS 5/505, child support guidelines
- HFS 2026 Gross to Net Income Conversion Table Using Standardized Tax Amounts (eff. March 20, 2026)
- HFS 2026 Illinois Schedule of Basic Obligations (eff. March 20, 2026)
- HFS 2026 Addendum to the Schedule of Basic Obligations and Standardized Net Income Table
- HFS Shared Physical Care Support Obligation Worksheet
- HFS Income Shares guidelines page
Illinois HFS income shares worksheets and tables →
Ready to estimate? The calculator shows each worksheet line so you can check it against the official form.
Open the Illinois calculator →An estimate from the published guideline formula, not a prediction of a court order. The official court worksheet and the court’s decision control. General information, not legal advice. LawCat is not a law firm.