Support in Illinois

How Illinois Calculates Child Support

Since a major reform in 2017, Illinois has used an income shares model for child support. This replaced the older percentage-of-income method and brought Illinois in line with the majority of states. The current framework is codified in 750 ILCS 5/505.

Under the income shares model, the court determines each parent’s net income, combines them, and then consults a guideline table that specifies the basic support obligation based on the combined net income and the number of children. Each parent’s share of the total obligation is proportional to their share of combined net income.

Net income is calculated by subtracting standardized taxes (federal, state, FICA, and Medicare) from gross income, along with certain other deductions. Illinois uses a standardized tax approach rather than actual tax withholdings to ensure consistency across cases.

For a broader overview of how income shares models work in different states, see our guide on how child support is calculated.

Key Components of the Calculation

The Illinois child support guidelines consider several factors beyond base income:

• Both parents’ gross income. This includes wages, salary, commissions, bonuses, severance pay, pensions, annuities, Social Security benefits, disability benefits, trust income, interest, dividends, rental income, business income, and any other source of recurring income.
• Imputed income. If a parent is voluntarily unemployed or underemployed, the court may impute income based on that parent’s employment potential, recent work history, and earning capacity.
• Child care expenses. Costs for child care necessary for either parent to work or attend school are added to the base support obligation and divided proportionally.
• Health insurance premiums. The cost of maintaining health insurance coverage for the child is added to the basic obligation.
• Extraordinary expenses. These may include special educational needs, extracurricular activities agreed upon by both parents, or recurring medical expenses not covered by insurance.

Shared Parenting Adjustment

Illinois provides a specific adjustment for shared parenting situations. When each parent exercises 146 or more overnights per year (40% or more of the time), the shared parenting formula applies. Under this formula, each parent’s child support obligation is calculated individually based on their income and the number of overnights, and the obligations are then offset against each other. The parent with the higher obligation pays the difference to the other parent.

This adjustment recognizes that parents who share significant time with their children incur substantial direct costs during their parenting time, and it prevents the duplication of expenses.

You can estimate your potential obligation using our child support calculator.

Modifying Child Support in Illinois

A parent may petition for modification of child support by demonstrating a substantial change in circumstances. Illinois also allows modification if the existing order was entered at least three years ago and applying the current guidelines to the parties’ present incomes would result in a change of at least 10% (either up or down) from the current order.

Modifications take effect only from the date the petition is filed — arrears that accrued before the petition date cannot be reduced retroactively.

Child support in Illinois ordinarily continues until the child turns 18, or until age 19 if the child is still attending high school. The court may also order support for a non-minor child attending college or trade school, covering a reasonable portion of tuition, room and board, and other educational expenses. This is a notable feature of Illinois law that does not exist in many other states.

Enforcement of Child Support Orders

Illinois enforces child support orders through the Division of Child Support Services, which has access to multiple enforcement tools:

• Income withholding notices (mandatory in most cases)
• Interception of federal and state tax refunds
• Liens on real property, vehicles, and financial accounts
• Suspension of driver’s licenses, professional licenses, and recreational licenses
• Reporting to credit bureaus
• Contempt of court, which can lead to fines or incarceration
• Passport denial for arrears exceeding $2,500

Illinois imposes 9% annual interest on unpaid child support, providing a strong incentive for timely payment.

Maintenance (Alimony) in Illinois

Illinois uses the term maintenance rather than alimony, and the current framework (750 ILCS 5/504) provides a formula-based approach for calculating both the amount and duration of maintenance.

Calculating the Amount

The guideline maintenance formula is:

33.33% of the payor’s net income minus 25% of the payee’s net income

However, the resulting amount is subject to a cap: when combined with the payee’s net income, the maintenance amount cannot exceed 40% of the parties’ combined net income. This cap prevents the payee from receiving a larger share of income post-divorce than during the marriage.

For example, if the payor earns $10,000 per month net and the payee earns $3,000 per month net:

• Formula result: ($10,000 x 0.3333) - ($3,000 x 0.25) = $3,333 - $750 = $2,583
• 40% cap check: $2,583 + $3,000 = $5,583, compared to 40% of $13,000 = $5,200
• Since $5,583 exceeds the cap, maintenance would be reduced to $2,200 ($5,200 - $3,000)

Duration of Maintenance

Illinois ties the duration of maintenance to the length of the marriage using a statutory multiplier:

• Marriage under 5 years: Duration = 20% of the marriage length
• 5-6 years: 40%
• 6-7 years: 44%
• 7-8 years: 48%
• 8-9 years: 52%
• 9-10 years: 56%
• 10-11 years: 60%
• 11-12 years: 64%
• 12-13 years: 68%
• 13-14 years: 72%
• 14-15 years: 76%
• 15-16 years: 80%
• 16-17 years: 84%
• 17-18 years: 88%
• 18-19 years: 92%
• 19-20 years: 96%
• 20 or more years: The court may order maintenance for a period equal to the marriage length or for an indefinite term

These multipliers are guidelines, and the court retains discretion to deviate based on relevant factors.

Factors Influencing Maintenance

Beyond the formula, the court considers factors listed in 750 ILCS 5/504, including:

• Each party’s income and property
• The needs of each party
• The present and future earning capacity of each party
• Any impairment of earning capacity due to devoting time to domestic duties or forgoing education or career opportunities for the benefit of the other spouse
• The time necessary for the payee to acquire education, training, or employment
• The standard of living established during the marriage
• The duration of the marriage
• The age and health of both parties
• Tax consequences of the property division
• Contributions and services by the payee to the payor’s education, training, career, or license
• Any valid prenuptial or postnuptial agreement

When to Seek Legal Guidance

Illinois family law provides relatively clear formulas for both child support and maintenance, but the interplay between the two — particularly in shared parenting arrangements or cases involving high income, self-employment, or business ownership — can create complexity. If you are facing a support dispute or seeking to understand your potential obligations, consider scheduling a free consultation to discuss your situation with a family law professional.

Statutes referenced: 750 ILCS 5/505 (child support), 750 ILCS 5/504 (maintenance).