Illinois combines both parents' monthly net income, reads the obligation off the HFS Income Shares Schedule, and splits it by what each parent earns. The rule worth understanding before you negotiate overnights is shared physical care: once each parent reaches 146 overnights, the calculation changes shape entirely, and not always in the direction people expect.
Four questions. The estimate updates as you type, and you can open the working to see which figure drives it.
Net income, not gross: take gross pay and subtract tax, mandatory retirement, union dues, health-insurance premiums and any court-ordered support already being paid.
Count the nights the children sleep at your home. This matters financially: 90 overnights is the threshold for an automatic 10% reduction.
Enter both incomes to see an estimate.
Estimate onlyBasic support before health insurance, child care and medical costs are added. Courts can depart from the guideline figure. Based on 750 ILCS 5/505 and the HFS Income Shares Schedule, last checked 2026-08-25.
Income shares: combined monthly net income is read against the HFS schedule, then split by each parent's share of that income — with a separate formula once both parents pass 146 overnights.
Priya and Marcus have one child. Priya's monthly net income is $4,000 and Marcus's is $2,000. The child is with Priya 20% of overnights.
About $807 a month. Note what happens if they move to a near-even split instead: the $1,211 becomes $1,816 before it is divided, and after the offset Priya would still be paying — the extra time does not simply cancel the obligation.
This is the number to know. Once each parent reaches 146 overnights, Illinois multiplies the basic obligation by 1.5 — recognising that two households cost more than one — then allocates and offsets. At 145 overnights each, none of it applies. It is a cliff, and it does not always reduce the order.
750 ILCS 5/505(a)(3.8)Because the obligation is multiplied by 1.5 before it is split, a higher earner with close to equal time can end up owing more than under a straightforward primary-care calculation, not less. Worth modelling before agreeing a schedule on the assumption that more time means less support.
750 ILCS 5/505(a)(3.8)The schedule is built on combined monthly net income. You can use your actual net figures or the state's standardized tax amounts, but entering gross pay will overstate the result substantially.
750 ILCS 5/505(a)(3)A payer earning at or below 75% of the federal poverty guideline for one person pays $40 per child per month, capped at $120 total. For a parent with no income, only means-tested assistance, or an inability to work through disability or incarceration, a zero-dollar order is presumed instead.
750 ILCS 5/505(a)(3.3a) and (3.3b)The figures used here are the 2026 HFS schedule, effective 20 March 2026. A calculator using an earlier schedule will be low.
HFS Income Shares Schedule (2026)Kidtime records every overnight, swap and handover as it happens, so the parenting-time figure behind your support order is evidence rather than an estimate. Free to start.
Open a case through Illinois Healthcare and Family Services, or ask for support inside an existing divorce or parentage case. HFS can establish parentage, set the order and enforce it at no cost.
Either parent can seek a modification on a substantial change in circumstances, or every three years on request through HFS. A change generally takes effect from the date the request is filed rather than when the circumstances changed.
146, and both parents need it — that is 40% of the year each. At 145 overnights the shared care formula does not apply at all. It is worth counting precisely, because the calculation changes shape rather than adjusting gradually.
Not necessarily. The obligation is multiplied by 1.5 before it is split, so a higher earner with near-equal time can owe more than under a primary-care calculation. Model it rather than assuming.
Net. The Illinois schedule is built on combined monthly net income. Subtract tax and mandatory deductions, or use the state's standardized tax amounts.
$40 per child per month, capped at $120 total, for a payer at or below 75% of the federal poverty guideline. A parent with no income or who cannot work may instead have a zero-dollar order entered.
This calculator gives an estimate of basic guideline support for general information. It is not legal advice and it is not the figure a court will order — judges can and do depart from the guideline, and the calculation leaves out health insurance, child care and medical costs. For the controlling text see 750 ILCS 5/505, compare against the state's own calculator, and consult a licensed Illinois family-law attorney about your situation. Last reviewed 2026-08-25.