Illinois Child Support Calculator

How Illinois Calculates Child Support — and Why the Formula Matters

Illinois determines child support obligations using the Percentage of Income model, one of three approaches used across the United States. The Income Shares model (used by 41 states) combines both parents' incomes to estimate what the family would have spent on the children if intact, then divides that obligation in proportion to each parent's share of the combined income. The Percentage of Income model (used in states like Alaska, Mississippi, Nevada, North Dakota, Texas, and Wisconsin) takes a fixed percentage of the non-custodial parent's income based on the number of children. The Melson Formula — adopted in Delaware, Hawaii, and Montana — layers a self-support reserve on top of the Income Shares approach so neither parent's basic needs are consumed before the child's obligation is met.

This calculator applies the Illinois-specific statutory tables and adjustments: parenting-time credit adjusts the obligation when the non-custodial parent has substantial overnight visitation, and healthcare insurance and work-related childcare are typically added and split proportionally. The result is an estimate of presumptive support — state courts can and do deviate upward or downward based on factors like high income, extraordinary medical expenses, or the non-custodial parent's other dependents. For a binding order, the official Illinois worksheet must be filed with the court. Pair this calculator with the per-state guideline page for current multipliers, deviation factors, and review/modification thresholds.

Source: Illinois Child Support Guidelines · Methodology: statutory tables and adjustments as published · Estimate only — a court order requires the official worksheet.