Calculation is client-side, income figures are never transmitted or stored.
How Illinois Child Support Is Calculated
- Determine each parent's net income (gross income minus the standardized tax amount from the official Gross-to-Net Income Conversion Table).
- Combine both parents' net incomes to get combined net income.
- Look up the basic child support obligation from the official Schedule of Basic Obligations for that combined income and number of children.
- Each parent's share equals their income divided by the combined income, applied to the basic obligation.
- If each parent has at least 146 overnights/year, a Shared Physical Care worksheet applies instead of straight proration (threshold drops to 110 overnights starting 2027-01-01).
- Above $27,375/month combined net income, courts determine support case-by-case.
Illinois Child Support Worksheet Walkthrough
Example: combined net income $3,000/month, 2 children, Parent A earns $1,800/mo (60% share), Parent B earns $1,200/mo (40% share) and has primary custody.
- Basic child support obligation for $3,000 combined, 2 children: $1,070/month.
- Parent A's share: 60% × $1,070 = $642/month guideline child support paid by Parent A.
How This Calculator Works — Formula & Constants
Source: Illinois Department of Healthcare and Family Services (HFS) · Calcul déterministe — no AI, no arbitrary estimate.
Constants used
| Constant | Value | Source |
|---|---|---|
| Overnights threshold | 146 nights/yr | 750 ILCS 5/505 |
Formula
base_obligation = schedule_lookup(combined_income, children)
share_B = parentB_income / combined_income
total_obligation = base_obligation + childcare_cost + health_insurance_cost
obligation_B = total_obligation × share_B
obligation_B = obligation_B × (1 - custody_credit(overnights))
Deterministic calculation based on Illinois's official guideline schedule table. Verify against Illinois's official calculator for a court-ready figure.
FAQ
How is child support calculated in Illinois?
Determine each parent's net income (gross income minus the standardized tax amount from the official Gross-to-Net Income Conversion Table). Combine both parents' net incomes to get combined net income. Look up the basic child support obligation from the official Schedule of Basic Obligations for that combined income and number of children. Each parent's share equals their income divided by the combined income, applied to the basic obligation. If each parent has at least 146 overnights/year, a Shared Physical Care worksheet applies instead of straight proration (threshold drops to 110 overnights starting 2027-01-01). Above $27,375/month combined net income, courts determine support case-by-case.
Does Illinois reduce child support for shared parenting time?
Only if both parents clear a specific overnight threshold — it isn't a sliding scale. Illinois requires each parent to have at least 146 overnights per year (dropping to 110 starting 2027-01-01) before a Shared Physical Care worksheet replaces the standard proration calculation. Below that threshold for either parent, the standard formula applies regardless of how time is actually split.
Is Illinois child support based on gross or net income?
Net income in Illinois is not your literal take-home pay — it's gross income minus a standardized tax amount pulled from the state's official Gross-to-Net Income Conversion Table, based on income level and filing status assumptions, rather than each parent's actual individual tax situation.
What happens if combined income is above $27,375/month?
Above $27,375/month in combined net income, Illinois's official schedule doesn't provide a table value, and courts determine support case-by-case rather than automatically extending the formula.
Methodology & Source
Formula model: income shares. Effective 2026-03-20, guideline version 2026-IL-v1, last reviewed 2026-07-11.
Official source: Illinois Department of Healthcare and Family Services (HFS) (750 ILCS 5/505).
Courts may deviate from the guideline amount for documented reasons — this calculator shows the presumptive guideline amount only, not a court order. Above $27,375/month combined net income, courts determine support case-by-case.
Guideline figures transcribed from the primary source above and cross-checked against Illinois's official calculator/worksheet for multiple test scenarios — see our verification methodology.