What is the income shares model in Illinois?

The income shares model, adopted in Illinois in 2017, is based on the principle that children should receive the same proportion of parental income they would have received if the parents lived together. It combines both parents' net incomes, looks up a total support obligation from a state schedule based on economic data, then allocates that obligation between parents proportionally.

What income counts for child support calculations?

Illinois uses net income, which starts with gross income from all sources (wages, salaries, bonuses, commissions, investment income, unemployment, workers' comp, etc.) and subtracts specific allowable deductions: federal and state taxes, FICA/Medicare, health insurance premiums, union dues, mandatory retirement contributions, and prior court-ordered support obligations.

What is the shared physical care threshold?

If either parent has the children for 146 or more overnights per year (40% of the year), Illinois law requires using the Shared Physical Care formula. This formula multiplies the Basic Child Support Obligation by 1.5 to account for the duplicated costs of maintaining two separate households for the children.

How does the shared care formula work?

In shared physical care cases: (1) BCSO is multiplied by 1.5, (2) each parent's portion is calculated based on their income share, (3) these portions are cross-multiplied by the other parent's time percentage, and (4) the parent with the higher resulting obligation pays the difference to the other parent.

What are mandatory add-on expenses?

Illinois requires parents to share three types of add-on costs proportionally: (1) health insurance premiums paid for the children, (2) work-related childcare costs, and (3) court-approved extraordinary expenses like private school or special medical needs. These are added to the basic support amount.

Can child support deviate from the guideline?

Yes. Courts may deviate from the guideline if applying it would be unjust or inappropriate after considering statutory factors like the financial resources and needs of each party, the standard of living the child would have enjoyed, educational needs, and the physical and emotional condition of the child.

What is the low-income adjustment?

If a parent's gross income is at or below 75% of the Federal Poverty Level for a one-person family, the presumptive minimum child support obligation is $40 per month per child. This ensures support orders remain meaningful while recognizing severe income constraints.

How do I use the Illinois HFS calculator?

The Illinois Department of Healthcare and Family Services provides an official Child Support Estimator online. It implements the income shares formula and allows you to choose between standardized net income calculation (using state tables) or itemized deductions for more precise results.

Income Shares Model750 ILCS 5/505

Illinois Child Support Formula

Complete guide to understanding how Illinois calculates child support under the income shares model, including shared physical care rules and add-on expense allocation.

Calculate Child Support Family Law Guide

Quick Facts

Formula Type

Standard

Income Shares

Considers both parents' incomes

750 ILCS 5/505

Income Definition

Net Income

After statutory deductions

750 ILCS 5/505(a)(3)

Shared Care Threshold

Critical

146 Overnights

40% of year triggers shared formula

750 ILCS 5/505(a)(3.8)

Shared Care Multiplier

1.5x BCSO

Accounts for dual household costs

750 ILCS 5/505(a)(3.8)

Official Calculator

HFS Estimator

State-provided online tool

Illinois HFS

Effective Date

Historical

July 1, 2017

Income shares model adopted

Public Act 099-0764 (2016)

Overview of the Illinois Formula

Illinois adopted the income shares model for child support calculations on July 1, 2017, replacing the previous percentage-of-income approach. This model is based on economic research showing that children should receive the same proportion of parental income they would have received if their parents lived together.

The formula is codified in 750 ILCS 5/505 and uses a statewide schedule of Basic Child Support Obligations (BCSO) maintained by the Illinois Department of Healthcare and Family Services (HFS). The calculation considers both parents' net incomes, the number of children, parenting time arrangements, and mandatory add-on expenses.

Key Change: Illinois uses net income (after specific deductions) rather than gross income. This differs from states like California that use net disposable income or gross income models.

Step-by-Step Calculation Process

Step 1: Calculate Net Income

Determine each parent's net income using either standardized gross-to-net conversion tables or itemized deductions

Illinois allows two methods: (1) Use HFS Gross-to-Net Conversion Table for standard cases, or (2) Itemize specific deductions including federal/state taxes, FICA, health insurance, prior support orders, union dues, and mandatory retirement contributions.

Step 2: Determine Combined Net Income

Add both parents' net incomes together

Combined net income = Parent A net income + Parent B net income. This total is used to look up the Basic Child Support Obligation (BCSO) from the state schedule.

Step 3: Look Up BCSO

Find the Basic Child Support Obligation from the Illinois Schedule

Using the combined net income and number of children, reference the official Illinois Schedule of Basic Child Support Obligations published by HFS. This table reflects economic data on child-rearing costs.

Step 4: Apply Standard or Shared Care Formula

Use different calculation methods based on parenting time

If either parent has 146+ overnights per year (≥40%), use the Shared Physical Care formula with 1.5x multiplier. Otherwise, use the standard formula where the non-custodial parent pays their proportional share.

Step 5: Add Mandatory Add-Ons

Allocate health insurance, childcare, and extraordinary expenses

These expenses are divided proportionally based on each parent's share of combined income. The final payment is adjusted to account for who actually pays these costs.

Net Income Determination Methods

Illinois provides two methods for calculating net income, allowing flexibility based on case complexity:

Method 1: Standardized

Using HFS Gross-to-Net Conversion Table

This simplified method uses state-published lookup tables that account for standard tax withholdings and FICA/Medicare deductions.

Best for:

Standard W-2 employees
Typical tax situations
Quick estimates

Method 2: Individualized

Itemizing specific deductions

This method calculates net income by subtracting specific allowable deductions from gross income, providing precision for complex financial situations.

Allowable deductions:

Federal & state income taxes
FICA & Medicare
Health insurance premiums
Union dues
Mandatory retirement
Prior support orders

Shared Physical Care Formula

Critical Threshold: If either parent has the children for 146 or more overnights per year (≥40% of the year), the Shared Physical Care formula applies instead of the standard calculation.

The shared care formula recognizes that when children spend substantial time in both households, there are duplicated fixed costs (housing, utilities, furniture, supplies). To account for this:

Shared Care Calculation Steps

Multiply BCSO by 1.5: Shared Care Obligation = BCSO × 1.5
Calculate each parent's portion: Based on income share percentage
Calculate time percentages: Each parent's overnights ÷ 365
Cross-multiply: Parent A obligation × Parent B time percentage, and vice versa
Offset: Higher obligation pays the difference to the other parent

The 1.5x multiplier is a statutory proxy representing approximately 50% increased costs for maintaining two child-appropriate households compared to a single-household arrangement.

Mandatory Add-On Expenses

Illinois law requires parents to share three categories of additional expenses proportionally, based on their respective shares of combined net income:

Health Insurance

The marginal cost of adding the children to a parent's health, dental, and vision insurance plans. Only the incremental premium for the children counts.

Childcare Costs

Work-related childcare expenses necessary for a parent to maintain employment or attend education/training programs that will result in employment.

Extraordinary Expenses

Court-approved recurring expenses beyond basic needs, such as private school tuition or expenses related to a child's special medical or educational needs.

The final child support payment is adjusted to account for which parent actually pays these expenses. If Parent A pays an expense but is only responsible for 40%, Parent B must reimburse the remaining 60% through increased support payments.

Frequently Asked Questions

Related Resources

Illinois Child Support Calculator

Calculate your estimated support payment using our free online tool

Deviation Factors Guide

Learn when and how courts may deviate from guideline support

Child Support Enforcement

Options for enforcing payment of support orders

How to File for Child Support

Step-by-step guide to establishing a support order

References & Accuracy

Editorial Review

Last Reviewed: January 15, 2025

Next Review: July 15, 2025

Reviewed By: TheDivorceCalc Editorial Team

Primary Legal Sources

statute750 ILCS 5/505 – Child support calculation formula
officialIllinois HFS Income Shares – Official schedule and conversion tables
officialHFS Child Support Estimator – State-provided calculator tool

Internal Research: This page incorporates proprietary analysis of Illinois child support law, including technical specifications of the income shares model calculation logic. For additional context, see our Illinois Calculator and Family Law Guide.

Accuracy Statement: We update this content semi-annually to reflect changes in Illinois law and HFS administrative guidelines. Child support calculations are complex and fact-specific. This information is for educational purposes and does not constitute legal advice. For case-specific guidance, consult a licensed Illinois family law attorney.