How the Income-Shares Model Calculates Child Support in Most US States

If you ask a family-law attorney in most U.S. states how child support is calculated, the answer starts with the same phrase: "It's an income-shares state." Roughly 38 of the 50 states use some version of the income-shares model — a methodology that treats both parents' incomes as a single pot, looks up a basic child-rearing cost from a statewide schedule, and divides that cost between the parents in proportion to their incomes, then adjusts for parenting time, child care, health insurance, and extraordinary medical. This guide walks through how the model actually works, runs a detailed calculation example with two parents earning $60,000 and $45,000 with two kids, compares parenting-time adjustment formulas in Colorado, Illinois, and New Jersey, and explains the self-support reserve, high-income adjustment, add-ons, and modification triggers.

Where the income-shares model came from

Income-shares was developed in the mid-1980s by Robert Williams, an economist at Policy Studies Inc., under contract with the U.S. Department of Health and Human Services. The federal Child Support Enforcement Amendments of 1984 (Public Law 98-378) required every state to adopt numeric child support guidelines, and the Family Support Act of 1988 (Public Law 100-485) made those guidelines presumptively binding — meaning judges could only deviate with written findings. States had to pick a methodology, and most adopted the income-shares approach Williams had published as a model. The federal regulatory framework is now codified at 45 CFR Part 302.

The conceptual goal was to give a child of divorced or separated parents roughly the same financial support they would have received if the family had stayed intact. The model assumes that in an intact household, child-rearing costs scale with combined household income; so in a separated household, both parents should contribute to those costs in proportion to their respective incomes. The economics underlying the schedules are based on USDA's Expenditures on Children by Families report, which estimated per-child spending by household income bracket. The USDA discontinued that annual report in 2017, and most states have since re-based their schedules on consumer expenditure survey data, but the income-shares architecture remains. The 2016 federal child support enforcement rule updated the framework to require evidence-based schedules and a self-support reserve.

The other two models states use

Two alternative methodologies survive. Percentage-of-income — used in Wisconsin, Texas, Alaska, Georgia, Mississippi, North Dakota, South Dakota, and Tennessee (with variations) — calculates support as a flat percentage of the non-custodial parent's income, ignoring the custodial parent's income entirely. Wisconsin uses a percentage based on the number of children (17% for one, 25% for two, 29% for three, 31% for four, 34% for five or more) on gross income, with a shared-placement adjustment when parenting time crosses a threshold. The Wisconsin methodology is documented on the Wisconsin DCF child support page.

The Melson Formula, used in Delaware, Hawaii, and Montana, is a more elaborate three-step calculation: each parent's primary support needs are subtracted from their income, and the remaining "available income" is allocated to the child. The Melson formula explicitly protects a low-income payor's basic needs before calculating support, which income-shares handles only through a low-income adjustment.

Income-shares remains the most common model, used in states including Illinois, New York, Pennsylvania, Colorado, New Jersey, Virginia, Arizona, Florida, Michigan, Ohio, Minnesota, Maryland, North Carolina, and the District of Columbia. Each state publishes its own schedule and its own worksheets.

Step 1: Determine each parent's net income

The first step in an income-shares calculation is determining each parent's monthly net income. "Net" here has a specific meaning that varies by state, but typically it means gross income minus a defined list of deductions: federal and state income taxes, FICA, mandatory retirement contributions, union dues, prior child support orders actually paid, and a small maintenance allowance for other dependents. Some states (Illinois since its 2017 reform, New York, New Jersey) work from gross income and apply a standardized tax-adjustment table; others (Colorado, Virginia, Pennsylvania) use net income computed on a worksheet.

"Income" is defined broadly. The Uniform Interstate Family Support Act and most state guidelines include wages, salaries, commissions, self-employment income, rental income, interest and dividends, disability and workers' compensation benefits, unemployment insurance, social security retirement and disability benefits (but typically not SSI), and — in most states — imputed income when a parent is voluntarily unemployed or underemployed. Imputation requires evidence: the parent's earning capacity based on recent work history, education, and local labor market data. A parent who quits a $80,000/year job to take a $30,000/year position without cause can be imputed at the higher figure.

A worked example: $60k + $45k, two children

Let's run a concrete calculation under the Illinois gross-income income-shares schedule (effective 2024), the most common state framework for examples because the schedule and worksheets are publicly available. The parents: Parent A earns $60,000/year gross ($5,000/month); Parent B earns $45,000/year gross ($3,750/month). Two children. Parent B is the primary residential parent with roughly 80% of parenting time; Parent A has the children roughly 20% of overnights.

Step 1 — Combined net income. Illinois applies a standardized tax adjustment to gross income. For a single parent with two children claimed as dependents at $60,000 gross, the standardized net is approximately $4,325/month. For Parent B at $45,000, approximately $3,250/month. Combined net income: $7,575/month.

Step 2 — Find the basic obligation. Look up the Illinois schedule for combined net income of $7,575/month and two children. The schedule produces a basic obligation of approximately $1,615/month. This figure represents the state's estimate of total monthly child-rearing costs for two children at that combined income level.

Step 3 — Allocate proportionally. Parent A's income share is $4,325 / $7,575 = 57.1%. Parent B's share is 42.9%. Parent A's share of the basic obligation: $1,615 × 0.571 = $922/month. Parent B's share: $1,615 × 0.429 = $693/month. Because Parent B is the primary residential parent, Parent A pays Parent B.

Step 4 — Parenting-time adjustment. Under the Illinois shared-physical-care formula (750 ILCS 5/505(a)(3.5)), when Parent A has more than 146 overnights per year (40% of the year), a parenting-time multiplier applies. At 20% of overnights (about 73 nights), Parent A is below the threshold, and no adjustment is made — Parent A pays the full $922/month. If Parent A had 40% of overnights, the multiplier would reduce the obligation. We'll work through the parenting-time adjustments in detail in the next section.

Step 5 — Add-ons. Add the following on top, allocated proportionally:

• Childcare: $600/month (after-school care for both children) — Parent A's share $343, Parent B's share $257.
• Health insurance: $250/month to add the children to Parent A's employer plan — Parent A pays the full premium; Parent B owes Parent A $107/month as Parent B's share (42.9% × $250).
• Extraordinary medical: $0 below the $250/child/year threshold.

Final order: Parent A pays Parent B $922 (basic) + $343 (childcare) − $107 (health-insurance reimbursement) = $1,158/month. Total child-rearing spend: $1,615 (basic) + $600 (childcare) + $250 (health insurance) = $2,465/month, split 57.1/42.9 between the parents.

Parenting-time adjustment formulas: CO, IL, NJ compared

Parenting-time adjustments are where state schedules diverge most sharply. The three main approaches:

Colorado (CRS § 14-10-115, "Worksheet B" for >93 overnights). Colorado uses a gross-income schedule with two worksheets: Worksheet A for the parent with fewer than 93 overnights (no adjustment) and Worksheet B for shared parenting time of 93 or more overnights per year. Worksheet B multiplies each parent's basic obligation by the percentage of overnights the child spends with the other parent, then offsets the two results. Example: if Parent A has 175 overnights (48% of the year) and Parent B has 190 (52%), Parent A's obligation = (basic obligation × 0.571) × 0.52 = the share Parent A owes for time the child is with Parent B. Parent B's obligation = (basic obligation × 0.429) × 0.48. Offset: Parent A pays Parent B the difference. At our worked-example numbers, the parenting-time adjustment would reduce Parent A's payment from $922 to roughly $410/month — a meaningful reduction reflecting the duplicated costs of two households.

Illinois (750 ILCS 5/505(a)(3.5), shared physical care). Illinois applies a graduated multiplier when each parent has at least 146 overnights (40% of the year). The formula multiplies each parent's basic obligation by a "parenting time multiplier" that reflects the increased fixed costs when a child has two households. The multiplier is computed as: 1.5 × (overnights / 365). At 50/50 parenting time, the multiplier is 0.75 (1.5 × 0.5). Each parent's basic obligation is multiplied by the other parent's multiplier, then offset. Below 146 overnights, no adjustment.

New Jersey (Court Rule 5:6A, shared parenting). New Jersey uses a cross-threshold approach: the parenting-time credit kicks in only when the non-custodial parent has more than 2 overnights per week (roughly 28% of the year). The credit is calculated by multiplying the basic child support obligation by the non-custodial parent's overnight percentage, then applying a 50% reduction (on the theory that the non-custodial parent saves only half of variable costs during their parenting time — fixed costs like housing remain). At 28% overnights, the credit is 28% × 50% × $1,615 = roughly $226/month reduction. At 50% overnights, the credit is 50% × 50% × $1,615 = $404/month reduction.

The table below summarizes:

Step 2 revisited: schedule ceilings and floors

Once both parents' net incomes are determined, they're combined and matched against the state's schedule. The schedule is a table that takes combined monthly net income (in increments — typically $100 or $50 bands, up to a ceiling around $20,000–$30,000/month) and number of children (typically one through six), and returns a basic child support obligation. That obligation represents the table's estimate of the total monthly cost of supporting the child(ren) at that combined income level.

For example, on the Illinois schedule effective July 1, 2024, a combined net income of $8,000 per month with two children produces a basic obligation of approximately $1,684. On the Colorado schedule, the same income and number of children yields a different figure because the schedule is built on different economic assumptions. The Illinois Courts child support page publishes the current schedule and worksheets. These schedules are revised every few years when states update their underlying economic data.

At the high end, schedules typically cap at a maximum combined income ($30,000/month in Illinois, $20,000 in Virginia). For incomes above the cap, courts have discretion to extrapolate or apply a discretionary add-on, but the schedule itself does not provide figures. At the low end, every state has either a self-support reserve (a floor below which the payor's income cannot be reduced) or a low-income adjustment that lowers the obligation.

Low-income adjustment and the self-support reserve

The 2016 federal child support enforcement rule (45 CFR § 302.56) required every state to implement a self-support reserve — a floor designed to ensure that a child support order does not push the payor below a basic subsistence level. The reserve is typically set at 100% to 115% of the federal poverty guideline for a single individual — roughly $1,255/month in 2024 (the 2024 federal poverty guideline for a single person is $15,060/year, or $1,255/month). If the guideline calculation would reduce the payor's net income below the reserve, the obligation is reduced to the reserve level.

Worked example: a payor earning $1,800/month net. The guideline calculation produces a $700/month obligation. $1,800 − $700 = $1,100 net remaining — below the $1,255 reserve. The obligation is reduced to $545/month, leaving the payor at the reserve level. Most states publish a low-income obligor table that produces these reduced obligations automatically.

Some states (Illinois, New York) have a separate low-income schedule that produces lower obligations for payors under a threshold income (typically 75% to 100% of the federal poverty guideline). At very low incomes, the obligation can be as low as $50/month — symbolic rather than substantive, but preserving the legal obligation so that arrearages accrue if the payor's income later increases.

High-income adjustment

At the other end of the spectrum, schedules cap at a maximum combined income. Illinois caps at $30,000/month combined net income; Virginia at $20,000/month; New York at $9,000/month combined parental income under the 2024 advisory guidelines. For incomes above the cap, courts apply discretion — typically by extrapolating the schedule's marginal rate or by computing an add-on based on the child's actual standard-of-living expenses (private school, summer camp, travel sports).

Worked example: in Illinois, a couple with combined net income of $40,000/month and two children. The schedule caps at $30,000/month, which produces a basic obligation of roughly $4,200/month. The court must then decide whether to extrapolate (using the schedule's marginal rate at the cap) or apply a discretionary add-on based on the child's actual expenses. Many high-income parents negotiate an agreed order at or near the cap figure, with documented add-ons for private school and enrichment activities — typically resulting in a total obligation of $5,000–$8,000/month for combined parental income of $40,000.

High-income deviations are common in disputed cases and frequently appealed. Federal regulation 45 CFR § 302.56(c) requires written findings of fact for any deviation. A judge cannot simply say "the guideline is too high"; they must identify the specific factor, quantify it, and explain the math.

Add-ons: childcare, health insurance, extraordinary medical

The basic obligation covers ordinary expenses: food, clothing, shelter, transportation, and routine recreation. Three categories of expense are typically added on top, allocated in proportion to each parent's income share:

• Childcare — necessary to allow a parent to work or attend education/training. Most states require the actual cost, documented by the provider, and allocate it proportionally. Some states cap childcare at a "reasonable" amount or require the parent to seek subsidized care if available.
• Health insurance — the cost of adding the child to a parent's employer-sponsored or marketplace health plan, less any employer contribution. If neither parent has access, the cash medical support obligation is set in lieu of insurance.
• Extraordinary medical and extracurricular — unreimbursed medical expenses above a threshold (often $250 per child per year), and sometimes extraordinary extracurricular or educational expenses. Ordinary medical costs below the threshold are presumed included in the basic obligation.

Travel costs for long-distance parenting time are sometimes treated as an add-on, sometimes as a deviation factor, depending on the state. The order will typically specify how add-ons are paid — directly to the provider, reimbursed within a set number of days after receipt of an invoice, or routed through the state disbursement unit.

Worked add-on example: a parent with 60% income share pays $1,200/month for the children's employer-sponsored health insurance and $800/month for after-school care. Add-ons total $2,000/month. The other parent (40% income share) owes 40% × $2,000 = $800/month in add-ons, usually paid as reimbursement after receipt of an invoice.

When courts deviate from the guideline amount

The guideline figure is presumptively correct, but every state permits deviations. Common grounds include: special needs of the child (e.g., therapeutic services, private school for a documented disability), extraordinary visitation travel costs, a parent's other dependents from a new household, significant income disparity producing an unjust result, and agreed-upon deviations that the court finds reasonable. Federal regulation 45 CFR § 302.56(c) requires states to permit rebuttal of the guideline.

Deviation requires written findings of fact. A judge cannot simply say "I think the guideline is too high"; they must identify the specific factor, quantify it, and explain the math. Appeals of guideline deviations are common in high-income cases where the schedule's upper cap makes the guideline figure an unreliable proxy for the child's actual standard of living.

Modification triggers: 10% income change, job loss, and review

Guidelines change, and so do circumstances. Most states permit modification when there has been a "substantial change in circumstances" — historically defined as a 15% or 20% change in either parent's income or in the child's needs. The 2016 federal child support enforcement rule (45 CFR § 302.56) pushed states to allow modification based on a 15% change (or 10% in low-income cases) without requiring other findings. The federal Office of Child Support Enforcement maintains a state-by-state directory of modification procedures.

Common modification triggers:

• 10% or 15% income change — most states allow modification when either parent's income changes by this threshold. Some states (Illinois under 750 ILCS 5/510) require a "substantial change" — generally interpreted as 10% or more in the absence of a statutory figure.
• Job loss or involuntary reduction in hours — a payor who loses their job through no fault of their own (layoff, plant closure) is typically entitled to a downward modification, often with arrears accruing at the old rate unless the modification is filed promptly. Voluntary unemployment without cause typically results in imputation of income at the prior rate.
• Change in parenting time — if the parenting schedule shifts from 20% to 40% overnights, the parenting-time adjustment kicks in and the obligation drops.
• Change in childcare or health insurance costs — a parent whose employer raises the health insurance premium from $250 to $500/month can file for modification.
• Emancipation of an older child — when the oldest child turns 18 (or 19 and graduates high school, depending on the state), the obligation for that child terminates and the schedule is recomputed for the remaining children. This is not automatic — the payor must file for modification.

Some states, including Illinois, allow modification based solely on a substantial change in the schedule itself — so when the state updates its economic tables every few years, parties can petition to bring their order in line with the new schedule. Reviews can also occur every three years as a matter of right under federal law.

Best practice: file the modification petition immediately when circumstances change. Modifications are generally prospective only — arrears continue to accrue at the old rate until the new order is entered. A payor who loses their job in January and waits until July to file will owe six months of arrears at the old rate even if the modification is granted.

Takeaways

The income-shares model is a structured but flexible framework. The basic obligation comes from a statewide schedule, gets divided by income share, then gets adjusted for parenting time and topped with childcare, health, and extraordinary medical add-ons. Two cases that look identical on the surface can produce meaningfully different orders depending on whether the state uses gross or net income, how the parenting-time adjustment is structured (Colorado's offset, Illinois's multiplier, New Jersey's cross-threshold credit), and whether add-ons are large relative to the basic obligation. Watch for the self-support reserve at low incomes and the schedule cap at high incomes — both require specific handling. Modification is available on a 10% income change or job loss, but the new amount is prospective only, so file promptly. Run the worksheet for your own state — or use an estimator that follows the same methodology — rather than relying on rules of thumb.