Illinois Alimony and Child Support Calculator: Formulas and Examples

Illinois runs two separate calculations for an Illinois alimony and child support calculator: maintenance (alimony) uses a percentage-of-income formula under 750 ILCS 5/504, and child support uses the Income Shares model under 750 ILCS 5/505, which combines both parents’ net incomes and compares the total to a statewide schedule published by the Department of Healthcare and Family Services. The HFS schedule and the Gross-to-Net Income Conversion Table were most recently updated effective March 20, 2026.¹

What You Need Before You Run the Numbers

Both formulas run on net income, so start by documenting every source of earnings for each party: wages, bonuses, commissions, rental income, investment returns, and self-employment profits. Gross income is then converted to net using the HFS Gross-to-Net Income Conversion Table, which is updated annually alongside the child support schedule.¹ Recent tax returns, pay stubs, and brokerage statements are the standard proof.

You also need documentation for recurring child-related costs: health insurance premiums for the child, work-related childcare, and any extraordinary medical bills. These are added on top of the basic child support obligation and divided between the parents, so leaving them out produces a figure that is too low.

All of this gets reported on a standardized Financial Affidavit, a sworn form every Illinois court must accept.² Illinois Legal Aid Online offers a free guided interview that fills in the form from your answers. Because the affidavit is sworn, inaccurate figures can lead to contempt proceedings.

One boundary worth knowing before you plug numbers in: if a parent is voluntarily unemployed or underemployed, the court does not simply accept the low reported figure. Under 750 ILCS 5/505(a)(3.2), a judge can impute income based on work history, occupational qualifications, local job opportunities, and whether the parent holds substantial assets that are not producing income.³ Your estimate should use the number a court is likely to use, not the one on a reduced paycheck.

How Illinois Calculates Maintenance

The guideline maintenance formula under 750 ILCS 5/504 applies when the couple’s combined gross annual income is below $500,000 and the paying spouse has no existing child support or maintenance obligation from a prior relationship.⁴ When those conditions are met, the math is straightforward:

Take 33⅓% of the paying spouse’s net annual income.
Subtract 25% of the receiving spouse’s net annual income.
The result is capped so that the receiving spouse’s own income plus maintenance cannot exceed 40% of the couple’s combined net income.

If combined gross income exceeds $500,000, or if the paying spouse already has a prior support obligation, the court sets maintenance based on statutory factors (earning capacity, standard of living during the marriage, each spouse’s needs and assets) instead of the formula. A judge can also depart from the guideline amount in any case where applying it would be inappropriate.⁴

How Long Maintenance Lasts

Duration depends on how long the marriage lasted. The statute assigns a multiplier to each range of years, and you multiply that factor by the length of the marriage to get the payment period.⁴ Common factors:

Under 5 years: 0.20
5 to under 6 years: 0.24
7 to under 8 years: 0.32
10 to under 11 years: 0.44
14 to under 15 years: 0.60
15 to under 16 years: 0.64
19 to under 20 years: 0.80
20 years or more: duration equal to the length of the marriage, or indefinite, at the court’s discretion

The factor rises by 0.04 for each additional year. One detail that catches people: the statute measures marriage length as of the date the divorce action was filed, not when the judgment is entered, and those dates can be months or years apart.

A Worked Maintenance Example

Say the paying spouse earns $90,000 net per year and the receiving spouse earns $40,000 net after a 12-year marriage. The formula gives: ($90,000 × 0.3333) − ($40,000 × 0.25) = $29,997 − $10,000 = $19,997 per year. Now check the cap. Adding $19,997 to the receiving spouse’s $40,000 produces $59,997, which is 46.2% of the combined $130,000, over the 40% ceiling. So the cap controls: 40% of $130,000 is $52,000, minus the receiving spouse’s own $40,000, leaving $12,000 per year in maintenance. Duration for a 12-year marriage uses the 0.52 factor: 12 × 0.52 is roughly 6.2 years.

How Illinois Calculates Child Support

Child support uses the Income Shares model, which estimates what the parents would have spent on the child if the household had stayed intact, then splits that cost in proportion to each parent’s earnings.³ Four steps:

Determine each parent’s monthly net income.
Add both figures together to get combined monthly net income.
Look up that combined amount on the HFS Schedule of Basic Child Support Obligations, adjusted for the number of children, to find the basic obligation.
Each parent’s share equals their percentage of combined income. A parent who earns 65% of the combined total owes 65% of the basic obligation.

The HFS schedule and the Gross-to-Net Income Conversion Table are revised annually, with the most recent version effective March 20, 2026.¹ Working from an outdated table will throw off the result.

HFS also publishes a free online Child Support Estimator that applies the current schedule to the net incomes you enter, which is a sensible way to cross-check your own math before filing.

Add-Ons for Health Insurance, Childcare, and Medical Costs

Health insurance premiums paid for the child are treated as an additional child support obligation and divided between the parents in the same income-based proportions.⁵ The court can also order parents to split deductibles, copayments, and uncovered medical expenses. Childcare costs tied to a parent’s employment or education are typically added the same way, though the statute gives the court broad discretion.

Shared Parenting Adjustment

When each parent has the child for at least 146 overnights per year, the calculation shifts to account for both parents maintaining a full household. The basic support obligation is multiplied by 1.5 to reflect the duplicated costs of two bedrooms, two sets of groceries, and two homes with utilities.⁶ The inflated amount is then divided based on each parent’s share of combined income and the percentage of overnights each parent exercises. In practice, this often lowers the transfer payment from the higher-earning parent because their direct spending during parenting time is already accounted for. If neither parent reaches 146 overnights, the standard calculation applies no matter how much daytime contact occurs.

When Child Support Ends

Any estimate should include the end date. Every Illinois child support order must specify termination, and at minimum support runs until the child turns 18. If the child is still in high school at 18, support continues until graduation or age 19, whichever comes first.³ Marriage, military service, or other legal emancipation before 18 can end the obligation earlier.

How Taxes Affect the Real Cost

Your estimate becomes the actual money that changes hands only because of a federal tax rule that often surprises people. For any divorce or separation agreement executed after December 31, 2018, alimony payments are neither deductible by the payer nor taxable to the recipient on federal returns.⁷ Agreements signed before 2019 may still follow the prior rules unless modified to adopt the new treatment.

Child support has always been tax-neutral. The paying parent cannot deduct it, and the receiving parent does not report it as income.⁸ Whatever figure comes out of the Income Shares calculation is the amount transferred, with no federal tax adjustment on either side.

1
Illinois Department of Healthcare and Family Services. Income Shares
2
Supreme Court of Illinois. Financial Affidavit
3
Illinois General Assembly. Illinois Code 750 ILCS 5/505 – Child Support, Contempt, Penalties
4
Illinois General Assembly. Illinois Code 750 ILCS 5/504 – Maintenance
5
Illinois General Assembly. Illinois Code 750 ILCS 5/505.2 – Health Insurance for Children
6
Illinois Department of Healthcare and Family Services. Income Shares FAQ
7
Internal Revenue Service. Divorce or Separation May Have an Effect on Taxes
8
Internal Revenue Service. Dependents 6