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Disclaimer: Shared custody formulas, thresholds, and multipliers vary by state; this tool uses a simplified income shares cross-credit. Run your state's official worksheet. Not legal advice.
Forty-one states calculate child support with the income shares model: the parents' combined income sets a basic obligation, and each parent is responsible for their pro-rata slice. When one parent has primary custody, the other simply pays their slice. In shared physical custody, most states switch to a cross-credit (offset): each parent's obligation is weighted by the time the child spends in the other home, the two weighted figures are compared, and the parent with the larger one pays the difference.
The trigger and the math are state-specific. Illinois engages shared-care rules at 146 overnights (40%) and multiplies the basic obligation by 1.5 to fund duplicated fixed costs before cross-crediting. New Jersey moves to a shared-parenting worksheet at roughly 28% of overnights. Arizona and Indiana apply graduated parenting-time credits rather than a cliff. A few states give judges open discretion. The consequence: identical families can produce materially different support numbers across a state line, and the overnight count near a threshold is often the most heavily litigated fact in the case.
Two children; Parent A earns $90,000 (66.7% of combined income), Parent B earns $45,000 (33.3%). Suppose the combined basic obligation is $2,100/month, and time is split 60/40 in Parent B's favor being 40%. Cross-credit: Parent A owes 66.7% × $2,100 weighted by the child's 40% time with B = $560; Parent B owes 33.3% × $2,100 weighted by 60% = $420. Parent A pays the difference — about $140/month before add-ons, versus roughly $700 under a sole-custody calculation. In a 1.5-multiplier state like Illinois the duplicated-cost uplift raises both sides before the offset.
Talk to a family law attorney when your overnight count sits near your state's threshold, when the proposed schedule appears engineered around the support formula, when incomes need imputation before the worksheet is run, or when relocation or schedule changes will move the numbers. Small overnight swings routinely change support by several hundred dollars a month.
Explore more free tools on our Family Law Calculators hub, walk through our interactive Family Law Wizards to understand your situation step by step, or contact LexScale.ai to build calculators like this for your own law firm website.
Enter each parent's gross annual income, the number of children, and each parent's percentage of parenting time. The tool estimates each parent's full child support obligation using a simplified table approximation, then applies the set-off: the higher obligation minus the lower one, payable by the higher-income parent. If neither parent reaches the shared custody threshold, the tool tells you the ordinary sole-custody table amount applies instead. Because official tables and state worksheets are more granular than any approximation, treat the output as a negotiation starting point, not the final number.
The threshold creates a well-documented cliff effect: a few overnights per year can swing support by hundreds of dollars a month. Judges know this and scrutinize whether a proposed schedule is driven by the child's interests or by support arithmetic. A parenting plan that plainly serves the child — school proximity, work schedules, sibling arrangements — will survive that scrutiny; a plan reverse-engineered from the support tables usually will not.
Child Support Calculator · Parenting Time Percentage · Add-On Expenses Calculator · Imputed Income Calculator · Custody Time Calculator
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