How child support is calculated in Illinois (2025 guide)

Last updated 2026-07-09

What model does Illinois use to calculate child support?

Illinois uses the Income Shares Model. The idea is simple: child support should roughly match what both parents would have spent on the child if the family had stayed together. So the court doesn't just look at the paying parent's paycheck. It adds both parents' incomes together and treats the total as one household figure.

The state switched to this model on July 1, 2017, under Public Act 99-0764, which rewrote 750 ILCS 5/505. ^[1] Before that date Illinois used a flat percentage of the noncustodial parent's net income (20% for one child, 28% for two, and so on). That old percentage model is gone for orders entered after July 1, 2017, though it still surfaces in some limited modifications of pre-2017 orders.

More than 40 states now use Income Shares. ^[10] New Jersey is one of them. If you've searched "how is child support calculated in NJ," you'd find the same core model, but different schedules and a different definition of net income. Illinois is worth learning on its own terms, because the Illinois schedule, the parenting time credit, and the way "net income" gets defined are all specific to state law.

Here's the short version. The formula has two inputs, both parents' net incomes, and one lookup table, the Schedule of Basic Obligations. Everything after that is an adjustment on top of the base.

How does Illinois define 'net income' for child support?

Net income for child support is a defined legal term, and it is not the same as your take-home pay. The statute at 750 ILCS 5/505(a)(1) defines it as gross income minus certain allowable deductions. ^[1]

Allowable deductions include federal and state income tax (at the actual or estimated rate), Social Security and Medicare taxes, retirement contributions your employer requires, union dues, and health insurance premiums for the parent and the child. Prior child support or maintenance orders the parent is already paying also come out.

Here's what does not come out: voluntary retirement contributions above the mandatory amount, a new spouse's income (courts can weigh it in a hardship analysis, but it doesn't lower your net income directly), and ordinary lifestyle expenses.

Gross income is defined broadly. It covers wages, salary, tips, bonuses, commissions, self-employment income, rental income, interest, dividends, Social Security benefits, pension income, workers' compensation, and unemployment benefits. ^[1] Illinois courts also impute income when a parent is voluntarily unemployed or underemployed without a good reason.

Self-employed parents get different math. Gross income is business revenue minus ordinary and necessary business expenses, but courts read those deductions hard and often add back depreciation or personal costs run through the business.

Subtract the allowable deductions from gross, and you have the net income that feeds the formula.

What is the Illinois Schedule of Basic Obligations?

The Schedule of Basic Obligations is a table from the Illinois Department of Healthcare and Family Services. It shows the estimated combined parental spending on a child at a given combined net income level, broken out by number of children. ^[2] It's the heart of the whole calculation.

The table starts at a combined net income of $10,000 a year and runs up through high income levels. Here's a simplified excerpt to show the shape:

Combined Net Monthly Income	1 child	2 children	3 children	4 children
$2,000	$404	$582	$677	$764
$3,000	$574	$826	$961	$1,085
$4,000	$711	$1,024	$1,191	$1,344
$5,000	$818	$1,178	$1,370	$1,547
$7,500	$1,095	$1,577	$1,834	$2,071
$10,000	$1,357	$1,954	$2,272	$2,566

Source: Illinois IDHFS Schedule of Basic Obligations (values here illustrate the table's shape; always pull the official current table for a real calculation). ^[2]

For incomes above the top of the table, the court uses its discretion, guided by the statutory formula extended proportionally. The schedule gets updated periodically, and the current version lives on the IDHFS website.

One point trips people up constantly. The schedule figure is not what either parent pays. It's the starting point. The next step splits it.

How is the base child support obligation split between parents?

Once you find the combined basic obligation on the schedule, you figure each parent's percentage share of the combined net income. Apply that percentage to the schedule amount, and you get each parent's share of the base obligation.

Example. Parent A nets $4,000 a month. Parent B nets $2,000. Combined: $6,000. Parent A's share is 67%. Parent B's share is 33%.

For two children at $6,000 combined monthly net income, the schedule obligation is about $1,300 (interpolated). Parent A's share is $871. Parent B's share is $429.

In a traditional sole-custody arrangement, the non-residential parent pays their share to the residential parent, because the residential parent is presumed to spend their own share directly on the child. So Parent A pays $871 a month.

This is exactly why a child support calculator earns its keep: interpolating the table and running the percentages correctly takes a few careful steps, and one slip throws off the whole order. Illinois courts use a worksheet that mirrors this math, and you can download the official version from the IDHFS site. ^[2]

What is the parenting time credit and how does it affect the amount?

If the paying parent has 146 or more overnights a year with the child, Illinois requires a parenting time adjustment that lowers the base obligation. That 146-night threshold is written into 750 ILCS 5/505(a)(3.8). ^[1]

The adjustment works like this:

1. Take the basic obligation. 2. Multiply by 1.5, to account for two households now duplicating some fixed costs. 3. Multiply that figure by each parent's share of overnights. 4. Subtract the paying parent's resulting share from the basic obligation. That's the adjusted amount.

In plain English: the more overnights the paying parent has, the bigger the credit, because they're already spending on the child during those nights. At 50/50 parenting time, the formula usually produces a small net payment from the higher earner to the lower earner. It rarely hits zero unless the parents earn nearly the same.

Under 146 overnights, no parenting time credit applies. The paying parent owes their full proportional share of the base obligation. There's no partial credit below the line.

Courts can deviate from the formula in either direction. Common reasons: a child's extraordinary medical needs, education costs, documented financial hardship, or a parental agreement the court approves. Any deviation requires written findings of fact explaining why the formula amount is wrong for this family. ^[1]

What additional costs get added on top of the base obligation?

The base obligation from the schedule covers routine costs: food, clothing, housing, transportation, and similar everyday expenses. It does not automatically fold in several real costs that parents fight about.

Health insurance premiums get handled separately. The court orders one or both parents to carry coverage for the child when it's available at a reasonable cost through work. That premium is split by the same income-share percentages used in the base calculation. ^[1]

Unreimbursed medical expenses, meaning out-of-pocket costs after insurance pays, usually get split by income share too. If Parent A has 67% of combined income, they pay 67% of the uninsured medical bills.

Childcare needed so a parent can work or attend school gets added to the base and split proportionally. This runs high in Chicago, where full-time infant care commonly costs $2,000 to $2,400 a month. ^[3]

Extracurriculars and private school tuition are discretionary add-ons. Courts can order them when the family's prior lifestyle included such expenses, but nothing here is automatic.

College is a separate statute. Under 750 ILCS 5/513, courts can order post-secondary support up to the cost of four years at the University of Illinois at Urbana-Champaign. ^[4] That obligation ends when the child turns 23 or finishes a bachelor's degree, whichever comes first.

What is a real-world example of the Illinois child support calculation?

A concrete example is the fastest way to see how the pieces fit.

Parent A earns $6,500 gross a month. After taxes, Social Security, Medicare, and their own health insurance, net income is $4,800.

Parent B earns $3,000 gross a month. Net income after deductions is $2,300.

Combined net monthly income: $7,100. Two children.

From the Schedule of Basic Obligations at $7,100 combined, the base obligation for two children is about $1,530 (interpolated from the official table).

Parent A's income share: $4,800 / $7,100 = 67.6%. Parent B's income share: $2,300 / $7,100 = 32.4%.

Parent A's share of the base: $1,530 x 67.6% = about $1,034 a month. Parent B's share: about $496 a month.

Say Parent B is the residential parent. Parent A pays $1,034 a month as the base, before add-ons.

Now add health insurance. Parent A covers the children through work at $280 a month. Parent A's 67.6% share of that premium is $189; Parent B's 32.4% share is $91. Because Parent A already pays the full premium, Parent B's $91 share gets credited against Parent A's obligation. Net payment: $1,034 minus the $91 credit, roughly $943 a month, plus each parent's proportional share of uninsured medical and childcare paid as it comes up.

If Parent A also has 160 overnights a year (over the 146 threshold), the parenting time credit cuts the number further. That takes running the adjustment formula from the section above.

Can parents agree to a different amount, and will the court accept it?

Yes. Parents in an uncontested divorce or parentage case can agree to a different support amount. Illinois courts will generally approve an agreed number that varies from the formula, but the agreement has to include written findings that the deviation serves the child's best interest and has to say why the formula amount is wrong. ^[1]

In practice, agreements within a reasonable range of the formula amount go through cleanly. Agreements far below the formula draw much harder scrutiny, because a parent can't waive support that belongs to the child.

Parents sometimes agree to more than the formula produces, often when they're also splitting property unevenly. Courts wave upward deviations through with little fuss.

If you're doing your own paperwork, documenting an agreed amount with the required deviation findings (when it varies from formula) is one of the details that quietly sinks filings. DivorceClear's $149 document packet includes an Illinois parenting plan and a child support worksheet that walks you through the formula, so your filed numbers actually match.

One thing that never flies: agreeing to zero support with no findings. Courts reject that in nearly every case, unless one parent is getting a compensating financial benefit that serves the child.

How does Illinois child support work when parents have similar or equal parenting time?

Equal or near-equal parenting time (roughly 50/50) does not automatically zero out child support. Illinois has no blanket equal-parenting rule that cancels the obligation. The formula still runs, and the parenting time credit still needs the 146-overnight threshold.

At a true 182.5/182.5 overnight split, the parenting time adjustment shrinks the net number, but the higher earner usually still owes the lower earner something. The reasoning: the child's standard of living should be similar in both homes, and a big income gap between parents means some transfer is needed to keep the child's experience roughly even.

Courts handle real 50/50 cases by running the formula and ordering whatever net payment the math spits out. When that number is tiny (say, under $100 a month), some parents agree to waive it and offset it elsewhere. That still needs court approval with the deviation findings covered above.

The messier real-world problem in equal-parenting cases is childcare: who pays which provider, and how the income-share split of those bills gets enforced month to month.

How does the court handle self-employed parents or parents who hide income?

Self-employment makes net income harder to pin down, because the parent controls what runs through the business. Illinois courts pull business bank statements, tax returns (usually two to three years), 1099s, and sometimes business financials. Courts can, and do, add back personal expenses run through the business, above-market salaries paid to family members, and aggressive depreciation that inflates deductions.

When a parent looks like they're deliberately working below their earning capacity, courts impute income based on what the parent could earn given their education, work history, and the local job market. There's no fixed formula for imputation; the judge makes a factual finding. If a parent with a law degree claims they can only find $15-an-hour work, expect the court to impute something closer to prevailing attorney wages in the area.

Cash-heavy businesses (restaurants, contractors, retail) draw the most scrutiny. In higher-asset cases, a forensic accountant sometimes gets hired to reconstruct true income. In a DIY divorce, if your spouse is self-employed and you suspect unreported income, that's one place where a session with a divorce attorney before you sign off on an agreed amount is money well spent.

When and how can Illinois child support be modified?

Illinois allows modification when there's a substantial change in circumstances. Under 750 ILCS 5/510, a "substantial change" includes a change of at least 20% in either parent's net income, or a change in the child's financial needs. ^[5] Courts also weigh a change in parenting time when it crosses the 146-overnight line.

Modification is not automatic. One parent has to file a petition with the court that entered the original order. The court then recalculates using the current formula and current incomes.

There's a rebuttable presumption that an order is inadequate if applying the current formula to current incomes would move it by more than 20%. That presumption gives the party seeking modification a fairly low bar to clear.

Modifications take effect from the date the petition is filed, not the date the court rules. So if your income drops, filing fast protects you.

The Illinois Department of Healthcare and Family Services runs support enforcement through the State Disbursement Unit and can start administrative reviews every three years for cases in the IV-D program (cases where IDHFS is the Title IV-D agency). ^[2] For private orders with no IDHFS involvement, one parent has to file for modification on their own.

What is Illinois child support enforcement, and what happens if someone doesn't pay?

Illinois has heavy enforcement tools for unpaid support. IDHFS runs the Title IV-D child support program, which handles income withholding, license suspension, passport denial, and tax refund interception. ^[2]

Income withholding is the main tool. Illinois requires income withholding in nearly every support case, which means the paying parent's employer deducts the amount from each paycheck and sends it to the State Disbursement Unit, which forwards it to the receiving parent. ^[1]

If payments get missed outside of wage withholding, the receiving parent or IDHFS can pursue driver's license suspension, professional license suspension, interception of state and federal tax refunds, liens on real estate and personal property, credit bureau reporting, and in bad cases, criminal contempt.

Statewide, Illinois child support arrears run into the billions, a figure IDHFS reports annually. ^[2] Arrears accrue interest at 9% a year under 735 ILCS 5/2-1303. ^[9] Past-due support cannot be discharged in bankruptcy. It does not disappear.

One warning if you're the paying parent. Lose your job or take a big income hit, and do more than stop paying and hope the arrears get forgiven. File a modification petition immediately. Courts cannot retroactively reduce arrears that piled up before the petition was filed.

How does child support interact with taxes and divorce paperwork?

Child support is not tax-deductible for the payer and not taxable income for the recipient. That's federal law, and it applies the same everywhere. ^[6] Alimony now works the same way: under federal law, spousal support in agreements made after December 31, 2018 is also non-deductible for the payer and non-taxable for the recipient. For that distinction, see our overview of alimony.

The dependency claim and Child Tax Credit are separate questions. Federal tax law lets parents agree, or courts order, which parent claims the child. The default under IRS tiebreaker rules (Publication 501) is the parent with more overnights. Parents can override that with IRS Form 8332, signed by the custodial parent. ^[6]

For your paperwork, the support amount has to appear in the Marital Settlement Agreement (or Judgment of Dissolution), the Parenting Plan, and a separate Income Withholding Order. A missing document here is a common reason courts bounce uncontested divorce filings. If you're building your own divorce papers, make all three documents agree, and make the support number match your worksheet.

For the full Illinois filing process, the Illinois Courts self-help resources at illinoiscourts.gov walk through required forms by county. ^[7]

Frequently asked questions

What percentage of income is child support in Illinois?

Illinois no longer uses a flat percentage. It moved to the Income Shares Model on July 1, 2017. The share of the paying parent's income that goes to support depends on both parents' net incomes and how the combined obligation gets split. As a rough benchmark, for one child a paying parent with $4,000 net monthly income might pay somewhere between 15% and 22% of net, depending on the other parent's earnings.

Is there a minimum child support amount in Illinois?

The Schedule of Basic Obligations has a floor at its lowest income level. For a very low-income paying parent, courts can set a nominal amount, sometimes $40 to $50 a month, to keep the order on the books rather than at zero. Courts avoid zero because support can be modified upward when income improves, and a nominal order keeps that door open.

Does remarriage affect child support in Illinois?

A parent's remarriage doesn't directly change the calculation. A new spouse's income is generally not counted in the parent's net income for support. Courts can consider a new spouse's financial contributions when weighing a hardship deviation, but that's the exception. The child's need drives the number, not the parent's new household setup.

At what age does child support end in Illinois?

Support ends when the child turns 18 or graduates from high school, whichever is later, but no later than age 19. So a 17-year-old still in high school is supported through graduation even if that falls after their 18th birthday. Separate college support under 750 ILCS 5/513 can run to age 23 if a court orders it.

Can child support be waived in an Illinois divorce agreement?

No. Parents can't permanently waive support, because it belongs to the child, not either parent. Courts reject agreements that try to erase support entirely without compelling justification and written best-interest findings. An agreement setting support below the formula can be approved with proper deviation findings, but zero is almost never accepted.

How does Illinois child support handle multiple children from different relationships?

When a parent already pays support under a prior order, those payments come out of gross income before net income is calculated for the new case. That keeps a parent from being ordered to pay more total support than their income can carry. The deduction is limited to orders currently in effect and actually being paid.

How is child support calculated in Illinois when one parent is unemployed?

If unemployment is involuntary and temporary, courts sometimes set a nominal order and revisit it. If the court finds the unemployment is voluntary, or the parent is underemployed to dodge support, income gets imputed at what the parent can earn based on education, skills, and local wages. Courts look at recent work history and may use Department of Labor wage data for the occupation.

Does Illinois child support cover private school or extracurriculars?

Not automatically. The Schedule of Basic Obligations covers ordinary expenses. Private school tuition and extracurriculars can be ordered as add-ons under 750 ILCS 5/505 when they reflect the family's pre-divorce lifestyle and the court finds them appropriate for the child. Parents can also agree to split those costs proportionally, and courts will enforce that agreement.

What is the difference between the Illinois child support formula and New Jersey's?

Both states use the Income Shares Model, but the details differ. New Jersey uses its own Schedule of Basic Child Support Needs, a different definition of net income (it nets out fewer items), and different parenting time thresholds. Same concept, different tables and rules, so identical incomes produce different dollar amounts in each state.

How do I file to modify child support in Illinois?

File a Petition to Modify Child Support in the circuit court that entered the original order. You have to show a substantial change in circumstances: at least a 20% change in either parent's net income is the clearest qualifier under 750 ILCS 5/510. Attach updated income documentation. File fast, because modifications only reach back to the petition date, not earlier.

Where can I find the official Illinois child support worksheet and schedule?

The Illinois Department of Healthcare and Family Services posts the current Schedule of Basic Obligations and the official calculation worksheet on its child support page. The Illinois Courts self-help section at illinoiscourts.gov also links these resources and lists circuit court self-help centers by county. Always use the current schedule; the table gets updated periodically.

How long does it take to get a child support order in an uncontested Illinois divorce?

In an uncontested divorce where both parents agree on the amount and file complete paperwork, courts usually enter the support order as part of the final divorce judgment. The realistic minimum is roughly 60 to 90 days, driven mostly by court scheduling and complete filings. Courts with heavy caseloads, like Cook County, often run longer.

Sources

1. Illinois General Assembly, 750 ILCS 5/505 (Illinois Marriage and Dissolution of Marriage Act, child support provisions): Illinois Income Shares Model, net income definition, parenting time credit threshold of 146 overnights, income withholding requirement, modification standard, and deviation findings requirements
2. Illinois Department of Healthcare and Family Services, Child Support Services: Schedule of Basic Obligations, official calculation worksheets, Title IV-D enforcement program, State Disbursement Unit, and statewide arrears data
3. U.S. Department of Labor, Bureau of Labor Statistics, Occupational Employment and Wage Statistics: Childcare cost benchmarks for urban areas including the Chicago metropolitan region
4. Illinois General Assembly, 750 ILCS 5/513 (post-secondary educational support): Illinois courts can order college support up to University of Illinois at Urbana-Champaign cost, ending at age 23 or degree completion
5. Illinois General Assembly, 750 ILCS 5/510 (modification of child support orders): Substantial change in circumstances standard, 20% income change rebuttable presumption for modification
6. IRS Publication 501, Dependents, Standard Deduction, and Filing Information: Child support is not deductible by the payer and not taxable income to the recipient; dependency tiebreaker rules and Form 8332
7. Illinois Courts, Self-Help Resources: Official Illinois court self-help pages listing required divorce forms, county self-help centers, and filing guidance
8. Illinois General Assembly, Public Act 99-0764 (amending 750 ILCS 5/505 to adopt Income Shares Model effective July 1, 2017): Illinois switch from percentage-of-income to Income Shares Model on July 1, 2017
9. Illinois General Assembly, 735 ILCS 5/2-1303 (interest on judgments): Child support arrears accrue interest at 9% per year in Illinois
10. U.S. Department of Health and Human Services, Office of Child Support Services: Income Shares Model is used in more than 40 states; general description of how states implement the model