How Illinois Calculates Child Support
Illinois uses the Income Shares model to determine child support obligations. This page explains the guidelines, formula, and key factors.
Model Type
Income Shares (schedule-based; changed from flat percentage model in 2017)
Illinois adopted the income shares model effective July 1, 2017, replacing the prior flat-percentage model. Combined net income of both parents is used to look up base support obligation from a schedule. Each parent pays a proportional share. Parenting-time adjustment applies when each parent has the child at least 146 overnights.
Income Basis & Definitions
combined net income (both parents)
Selects base support amount from the income shares schedule.
Source: 750 ILCS 5/505(a)(1) income shares scheduleNet income after standardized tax deductions per 750 ILCS 5/505(a)(3).
Source: 750 ILCS 5/505(a)(3) net income definitionNet income of the other parent.
Source: 750 ILCS 5/505(a)(3) net income definitionParenting Time Handling
Shared parenting adjustment applies when each parent has at least 146 overnights (40% of year). Multiplier of 1.5 applied to basic obligation and allocated by income share.
Source: 750 ILCS 5/505(a)(3.3) shared parenting adjustmentDeductions & Adjustments
Existing child support obligations for other children are deducted from net income.
Source: 750 ILCS 5/505(a)(3.7) multi-family adjustment