Child support isn’t guesswork. Every state has a written formula — a guideline — and courts are required to use it. The number that comes out of that formula isn’t a judge’s opinion. It’s math.
What most people don’t realize is that the math is different depending on where you live. There are three distinct guideline models used across the United States, and they produce different results even when the parents earn exactly the same incomes. A parent paying support in Texas might owe significantly less — or more — than a parent in Illinois with identical finances.
How child support is calculated depends on which model your state uses, what counts as income under that model, how many children are involved, and what parenting time looks like. This article breaks down all three models, shows you how each one works in practice, and explains what the numbers actually mean.
Federal law sets the rules that every state must follow. Each state fills in the formula.
What Model Does Your State Use?
There are three child support guideline models used in the United States. Every state uses one of them.
Income Shares Model. The most common. Used by approximately 40 states. Both parents’ incomes are combined into a single figure. The state’s guideline table shows how much a household at that income level would typically spend on children. Each parent pays a proportional share of that amount based on their percentage of the combined income.
Percentage of Income Model. Used by approximately 10 states, including Texas, Wisconsin, and Alaska. Only the obligor’s income matters. Support is a fixed percentage of that income — the same rate regardless of what the other parent earns.
Melson Formula. Used only by Delaware, Hawaii, and Montana. A more complex variant of the Income Shares model. It first sets aside a self-support reserve for each parent, meets the child’s primary needs from what’s left, and then applies a Standard of Living Adjustment to any surplus. Named after the former Delaware Family Court Judge Elwood F. Melson Jr.
Federal law under 45 CFR §302.56 requires every state to maintain written child support guidelines and review them at least every four years. The amount the guidelines produce is the correct amount — rebuttably presumed correct. A judge who deviates from it has to put the reason in writing. This is the core reason how child support is calculated differs from state to state.
How the Income Shares Model Calculates Child Support
The Income Shares model is built on a single premise: a child should receive roughly the same proportion of parental income as if the parents were still together.
Here’s how it works in practice.
Both parents’ net incomes are added together to get a combined monthly income figure. The state’s child support schedule — a published table — shows the basic child support obligation for that combined income level and the number of children. That’s the total amount the child “costs” under state guidelines.
Each parent’s share of the combined income determines their share of the obligation. If Parent A earns 60% of the combined income, Parent A pays 60% of the guideline amount. The custodial parent’s share is presumed to be spent directly on the child. The non-custodial parent’s share is paid through the court system.
Take a situation where Parent A earns $5,000 per month and Parent B earns $3,000 per month. Combined income: $8,000. Under 750 ILCS 5/505, Illinois uses the Income Shares model — the guideline obligation at $8,000 combined for one child is approximately $1,440 per month. Parent A earns 62.5% of the combined income, so Parent A’s obligation is roughly $900 per month. Parent B’s $540 share is presumed to be spent on the child directly and doesn’t get paid out.
How income is defined under the Income Shares model varies by state. Illinois uses net income — gross income minus a standardized tax deduction. New York’s Child Support Standards Act uses adjusted gross income of both parents and applies percentages of 17% for one child, 25% for two, 29% for three, 31% for four, and 35% for five or more children to the combined parental income up to the statutory cap.
New York’s cap as of March 1, 2026 is $193,000 in combined parental income. Above that threshold, the court has discretion — it can apply the CSSA percentages or weigh 10 statutory factors instead.
How the Percentage of Income Model Works
The Percentage of Income model is simpler. One income. One percentage. One number.
Only the obligor’s income matters. The obligee’s income is irrelevant to the base calculation. The state applies a fixed percentage to the obligor’s income based on the number of children.
Texas Family Code §154.125 sets the percentages at 20% for one child, 25% for two, 30% for three, 35% for four, and 40% for five or more — applied to the obligor’s monthly net resources. Net resources in Texas means gross income minus federal income tax (calculated at the single-filer standard deduction rate), FICA, health insurance premiums for the child, and union dues.
Here’s how this plays out in practice. Say the obligor earns $6,000 per month gross. After taxes, FICA, and health insurance, net resources come to approximately $4,500. One child: 20% of $4,500 is $900 per month. Two children: 25% is $1,125 per month. The custodial parent’s income never enters the calculation.
Texas caps the guideline application at $11,700 per month in net resources, effective September 1, 2025 — updated from the prior cap of $9,200. For income above that cap, a court may order additional support based on the proven needs of the child under Texas Family Code §154.126.
Wisconsin uses the Percentage of Income model too — but with a significant difference. Where Texas calculates from net resources, Wisconsin calculates from gross income. Wisconsin Administrative Code DCF 150 sets the standard rate at 17% of the obligor’s gross income for one child, 25% for two, 29% for three, 31% for four, and 34% for five or more. And Wisconsin runs three income tiers — the percentages decrease as income increases, with reduced rates applying between $84,000 and $150,000 per year, and further-reduced rates above $150,000.
A person owing support in Texas and a person owing support in Wisconsin, with identical gross incomes and one child, will owe different amounts — and the gap can be significant.
What the Melson Formula Is and Which States Use It
Only three states use the Melson Formula: Delaware, Hawaii, and Montana. It’s more protective of the lower-earning parent than a straight Income Shares calculation, and more protective of the children’s standard of living than the Percentage of Income model.
The formula runs in three steps.
Step 1 — Self-Support Reserve. Before any child support is calculated, each parent keeps enough income to cover their own basic living needs. The reserve is based on wage surveys and cost-of-living data. A parent earning below the reserve owes no support until income exceeds that floor.
Step 2 — Primary Support Obligation. From the income remaining after both self-support reserves are set aside, the child’s primary needs are funded. This covers the baseline cost of food, clothing, shelter, and basic care. Both parents contribute proportionally.
Step 3 — Standard of Living Adjustment (SOLA). If either parent has income left over after steps 1 and 2, that surplus is shared proportionally with the child. The child’s standard of living rises along with the parents’ ability to provide.
Delaware’s Family Court administers the formula through official worksheets. The 2025 version — Form 509 — walks through all three steps using current income data and Delaware Department of Labor wage tables. Unlike the Income Shares model, the Melson Formula doesn’t produce a single lookup table result — it requires a step-by-step calculation for each family’s specific circumstances.
What Income Courts Include in the Calculation
Every model depends on income — but states define income differently, and those differences matter.
Broadly included in most states: wages, salary, overtime, bonuses, commissions, self-employment income (net of legitimate business expenses), rental income, investment income, pension and retirement distributions, SSDI, workers’ compensation, and unemployment benefits.
Military allowances: Wisconsin explicitly includes Basic Allowance for Housing (BAH) and Basic Allowance for Subsistence (BAS) under DCF 150.02(13)(a). Delaware’s Form 509i also includes military allowances in the calculation.
Excluded in most states: SSI, TANF, SNAP, foster care payments, and means-tested public assistance. Child support received for other children is also excluded in most states.
Imputed income. When a parent is voluntarily unemployed or underemployed — choosing not to work, or working below their capacity without good reason — courts can assign income based on what the parent is capable of earning. This is called imputed income. Courts look at prior work history, education, skills, and the local job market to determine earning capacity.
A parent who quits a $70,000-per-year job and claims zero income doesn’t automatically owe support based on zero income. Most courts will run the calculation based on what they could reasonably earn.
California’s formula is uniquely algebraic. California Family Code §4055 sets the statewide guideline as CS = K[HN − (H%)(TN)], where K represents the share of both parents’ combined income allocated to child support, HN is the high earner’s net monthly disposable income, H% is the high earner’s percentage of time with the child, and TN is the total combined net monthly disposable income. There is no lookup table. Attorneys and courts use licensed software — DissoMaster — to run the calculation. The formula was last amended by SB 343, effective January 1, 2024.
How Parenting Time Affects the Formula
Parenting time affects child support calculations — but not the same way in every state, and not always in the direction people expect.
Under the Income Shares model, states typically give parenting time credit when the non-custodial parent has the child above a threshold number of overnights. The theory is that when a parent has the child more, they’re directly spending more on the child, which should reduce the transfer payment.
Wisconsin has one of the clearest thresholds: DCF 150.035 triggers the shared placement formula at 92 overnights per year — 25% of the year. Below that threshold, standard percentages apply. Above it, the formula adjusts to reflect each parent’s proportional direct spending.
New York takes a different approach. The CSSA does not automatically reduce support based on additional parenting time. If parents have substantially equal time, the court may deviate from the guideline amount using 10 statutory factors — but there’s no automatic overnight-based credit built into the formula.
Texas falls somewhere in between. The standard percentage formula doesn’t automatically adjust for additional parenting time. A court can deviate under Texas Family Code §154.123 if the guideline amount would be unjust or inappropriate given the actual time-sharing arrangement — but the deviation requires a judicial finding, not an automatic calculation.
A lot of people assume that 50/50 parenting time means no child support. That’s not how it works in most states. Equal time doesn’t zero out the obligation — courts still run both incomes through the formula, and the parent who earns more typically owes something to the other. The size of the payment depends on the income gap, not just the custody split.
Mandatory Add-Ons: Healthcare, Childcare, and Special Expenses
The base child support obligation is just the starting point. Most states also require parents to share additional expenses on top of the guideline amount.
Health insurance. Courts almost universally order one or both parents to maintain health insurance for the child. In Texas, health insurance premiums paid by the obligor are deducted from gross income when calculating net resources — it reduces the income base. In other states, the cost of insurance is added to the base obligation and divided between parents proportionally.
Childcare costs. Work-related childcare expenses — daycare, after-school care, summer programs that allow parents to work — are divided proportionally between parents in Income Shares states. In New York, childcare costs are mandatory add-ons under FCA §413(1)(c)(5), split pro rata based on each parent’s share of combined income.
Unreimbursed medical expenses. Out-of-pocket medical costs not covered by insurance are typically divided between parents proportionally. New York treats unreimbursed medical expenses as mandatory add-ons in addition to the base CSSA amount.
Educational and extracurricular expenses. Some states allow courts to allocate private school tuition, tutoring costs, and extracurricular activity fees between parents. These are generally discretionary — the court can include them if the circumstances warrant.
The add-ons mean the number in the guideline table isn’t the final number. The actual monthly obligation can be significantly higher once insurance, childcare, and medical costs are divided.
How Courts Deviate from the Guidelines
The guideline amount is the presumptive correct amount. Courts can deviate from it — but only if the result would be unjust or inappropriate under the specific facts, and only if they put the reason in writing.
Deviation is the exception, not the rule. The presumption runs in favor of the guideline amount. A judge who wants to order less — or more — has to justify it with findings.
Common deviation factors include: the child’s specific financial needs or resources; each parent’s financial resources and debts; the child’s standard of living if the parents had stayed together; extraordinary medical, educational, or special needs; significant travel costs for visitation; and whether one parent is caring for other children without court-ordered support.
Here’s how this plays out differently by state. Under Illinois 750 ILCS 5/505(a)(2), courts have a defined list of factors to consider before deviating. In Texas, §154.123 provides a similar list — but deviation is still relatively rare for base support. In California, the algebraic formula automatically adjusts for parenting time and income differences, so formal deviation is less common than in lookup-table states.
A private agreement between parents to deviate from the guideline amount is not self-executing. Without court approval, it isn’t a court order — and without a court order, the obligee has no legal mechanism to enforce it if the obligor stops paying. Courts in most states can also reject private agreements that fall below the guideline amount if the result isn’t in the child’s best interest.
How Child Support Is Calculated – Key State Examples
| State | Example State Rule |
|---|---|
| Texas | Example State RulePercentage of Income model. Courts calculate support from the obligor’s net resources only. Standard guideline: 20% for one child, with a cap at $11,700 monthly net resources. |
| California | Example State RuleIncome Shares model using a statewide algebraic formula: CS = K[HN − (H%)(TN)], which factors both parents’ income and parenting time. |
| Wisconsin | Example State RulePercentage of Income model calculated from the obligor’s gross income. Standard guideline: 17% for one child, with tiered reductions at higher income levels. |
| Illinois | Example State RuleIncome Shares model combining both parents’ net incomes and dividing the obligation proportionally based on each parent’s share of total income. |
| New York | Example State RuleIncome Shares model under the Child Support Standards Act (CSSA). Percentages apply to combined parental income up to the statutory cap of $193,000. |
| Delaware | Example State RuleMelson Formula. Courts first protect each parent’s self-support reserve, then calculate primary child support needs and apply a standard-of-living adjustment. |
How Child Support Is Calculated Differently Across States
The same parents, same incomes, same number of children. Different state. Different number.
| State | Guideline Model | Key Rule |
|---|---|---|
| Texas | Guideline ModelPercentage of Income — obligor’s net resources only | Key Rule20% for 1 child; cap at $11,700/mo net resources (eff. Sept. 1, 2025) |
| California | Guideline ModelIncome Shares — both parents’ net disposable income; algebraic formula | Key RuleCS = K[HN − (H%)(TN)]; no lookup table; last amended SB 343, eff. Jan. 1, 2024 |
| Wisconsin | Guideline ModelPercentage of Income — obligor’s gross income (not net) | Key Rule17% for 1 child; three income tiers; shared placement formula at 92 overnights/year |
| Illinois | Guideline ModelIncome Shares — both parents’ net income combined | Key RuleStandardized tax deduction applied to gross; multi-family adjustment available |
| New York | Guideline ModelIncome Shares (CSSA) — combined parental income up to statutory cap | Key RuleCap: $193,000 combined (eff. Mar. 1, 2026); support runs to age 21 |
| Delaware | Guideline ModelMelson Formula — both parents’ net income | Key RuleSelf-support reserve → primary support → SOLA; one of only 3 Melson states |
What Happens When Income Is Very High or Very Low
Guidelines don’t apply the same way at the extremes of the income scale.
High income. Every state with a percentage-based or table-based system has a ceiling — a cap above which the standard formula stops applying automatically. In Texas, the guideline percentage applies only to the first $11,700 per month in net resources. Above that, a court can order additional support based on the proven needs of the child — but the obligor isn’t automatically paying 20% of $50,000 per month.
New York’s CSSA cap is $193,000 in combined parental income as of March 1, 2026. Above that cap, courts have discretion to apply the CSSA percentages or consider 10 statutory factors including each parent’s financial resources, the child’s accustomed standard of living, and the child’s educational needs.
California has no fixed cap — the algebraic formula continues to apply mathematically at all income levels, though high-income cases typically involve more complex calculations and more judicial attention to the specific circumstances.
Low income. States protect very-low-income obligors through self-support reserves and low-income schedules. Texas applies a reduced percentage table when the obligor’s net resources fall below $1,000 per month — the rate for one child drops to 15% instead of 20%. Wisconsin has a separate low-income table for obligors earning between 75% and 150% of the federal poverty guideline.
New York’s self-support reserve for 2026 is $21,546 per year — 135% of the federal poverty guideline. Courts are required to ensure that a support order doesn’t reduce the obligor’s income below that reserve. If the calculation produces a number that would do so, the court adjusts.
The Melson Formula’s self-support reserve works similarly — it’s the floor below which the obligor keeps income before any child support obligation is applied.
What Happens If You Don’t Pay Child Support
This is where federal law takes over — and it applies in every state without exception.
The standard payment path in virtually every IV-D case runs like this: the court issues an Income Withholding Order (IWO) to the employer. The employer withholds the support amount from each paycheck and sends it to the State Disbursement Unit (SDU), which forwards it to the obligee. The obligor doesn’t handle the payment. The employer does.
The Consumer Credit Protection Act (15 U.S.C. §1673) sets the federal ceiling on withholding: 50–60% of disposable earnings depending on whether the obligor supports a second family, rising to 55–65% if arrears exist. That ceiling is nationwide — not a state policy choice.
Miss a payment — and that payment is already a judgment against the obligor the moment the due date passes. No hearing required. Under the Bradley Amendment (42 U.S.C. §666(a)(9)(C)), courts are prohibited from retroactively reducing or forgiving child support that has already accrued. A judge can change future support. They cannot erase what’s already owed. Every missed payment stacks up as a legally enforceable debt — with interest in most states — that no court can wipe out retroactively.
Beyond the IWO, enforcement tools available in every state include federal and state tax refund intercept; passport denial at $2,500 in certified arrears (a federal threshold); driver’s, professional, and recreational license suspension; bank account levy; and credit bureau reporting.
Willful non-payment for a child in another state — where arrears exceed $10,000 or payments have gone unpaid for more than two years — is a federal felony under the Deadbeat Parents Punishment Act (18 U.S.C. §228). And child support is non-dischargeable in bankruptcy under 11 U.S.C. §523(a)(5). Filing doesn’t touch it.
Frequently Asked Questions
Does 50/50 custody mean I don’t have to pay child support?
Not necessarily. Equal parenting time reduces the income-transfer logic behind support, but it doesn’t automatically zero out the obligation. Courts still run both parents’ incomes through the guideline formula. If there’s a significant income gap between parents, the higher earner typically owes support even in an equal time-sharing arrangement. Wisconsin uses a shared placement formula above 92 overnights per year — but a reduced obligation is still an obligation.
How much child support will I actually pay?
That depends on your state’s model, your income, the other parent’s income (in Income Shares states), the number of children, parenting time, and mandatory add-ons like childcare and health insurance. The only reliable way to estimate the number is to run your state’s official guideline calculator or worksheet with actual income figures. Wisconsin offers a free official calculator. Most states publish their guideline worksheets online through the IV-D agency.
Can child support be more than the guideline amount?
Yes. Courts can deviate upward from the guideline if the result is justified by the child’s specific circumstances — extraordinary medical needs, private school expenses the child was accustomed to, or other documented costs. Above-cap income is the most common context where courts order more than the standard formula produces.
Does the custodial parent’s income affect child support?
It depends on the model. Under the Income Shares model — used by approximately 40 states — yes, both parents’ incomes go into the formula. Under the Percentage of Income model — used by Texas, Wisconsin, and others — the obligee’s income is not a factor in the base calculation. The obligor’s income is the only variable.
What income does the court actually use?
Courts start with gross income from all sources — wages, salary, self-employment, investment income, rental income, SSDI, unemployment benefits. Most states then apply deductions to get to a net figure. The specific deductions vary: Texas deducts taxes, FICA, and health insurance premiums for the child. Illinois uses a standardized tax deduction. California deducts taxes, mandatory retirement contributions, union dues, and health insurance. SSI, TANF, and SNAP are excluded in virtually every state.
Can child support be reduced if I have children from another relationship?
Some states allow a multi-family adjustment. Illinois allows a deduction for child support paid under a court order for children from another relationship. Texas has an adjustment table under Texas Family Code §154.129 that reduces percentages when the obligor has other children. Having additional children doesn’t automatically reduce support — you typically need to file for modification and show the court your actual obligations.
Is child support tax deductible?
No. Child support is not deductible by the obligor and is not taxable income to the obligee. That rule predates the Tax Cuts and Jobs Act of 2017 and was unchanged by it. Alimony paid under agreements finalized after December 31, 2018 is also no longer deductible — but child support has never been deductible under IRS rules.
What states use the Income Shares model?
Approximately 40 states use some version of the Income Shares model. Among the states covered in this article: California, Illinois, New York, Pennsylvania, and Florida. States using the Percentage of Income model include Texas, Wisconsin, Minnesota, Alaska, and Mississippi. Delaware, Hawaii, and Montana use the Melson Formula.
