How Child Support Is Calculated in the United States
Child support is a court-ordered payment from one parent to the other to cover a child's living expenses, healthcare, education, and day-to-day needs after a separation or divorce. Because the U.S. has no single federal formula, each state sets its own guidelines — but virtually every state follows one of three models: the Income Shares Model, the Percentage of Income Model, and the Melson Formula.
The Three State Models
Income Shares Model (~40 states): This is the most common approach. It assumes that a child should receive the same proportion of parental income as they would if the parents lived together. The court adds both parents' gross incomes, finds a basic support obligation from a state-published schedule (expressed as a percentage of combined income), then divides that obligation between the parents in proportion to their income share. The paying parent's share is their support obligation before add-ons.
Percentage of Income Model (Texas, Wisconsin, and a few others): The obligation is a fixed percentage of only the paying parent's income — gross in Wisconsin, net in Texas. The percentages are published in state law and do not change based on the receiving parent's income. Texas uses 20% for one child, 25% for two, 30% for three, 35% for four, and 40% for five or more, capped at a net income of $9,200 per month.
Melson Formula (Delaware, Hawaii, Montana): A more complex hybrid that first reserves a self-support allowance for each parent, then computes a primary support need for the child, and finally adds a percentage of surplus income. The Melson formula generally produces results similar to the Income Shares Model for middle-income families.
What Income Is Included
Courts cast a wide net when calculating income. Most states define gross income to include wages, salaries, tips, bonuses, commissions, overtime, self-employment income (net of ordinary business expenses), rental income, interest and dividends, pension and retirement distributions, disability payments, workers' compensation, and Social Security benefits. Courts may also impute income — assign a hypothetical earning capacity — to a parent who is voluntarily unemployed or underemployed to avoid paying support. The imputed amount is typically based on recent earnings history or the local minimum wage, whichever is higher.
What Income Is Excluded
Public assistance (TANF, SNAP, housing assistance), Supplemental Security Income (SSI), and most means-tested benefits are excluded from gross income. A new spouse's or new partner's income is generally not counted, though it may affect a modification request in some states. Child support received for a child from another relationship is typically excluded as well.
How Custody Affects Child Support
The number of overnight visits with the paying parent directly affects the support amount in most states. The logic is that a parent who has the child more often is already spending money directly on the child's day-to-day needs. Under a primary custody arrangement (roughly 80/20 or more), the standard guideline amount applies without adjustment. Under 50/50 shared custody, many states apply an offset method: each parent's obligation is calculated independently, and the difference flows from the higher earner to the lower earner — often reducing the paying parent's obligation by 20–30% compared to a primary-custody calculation. Even at exactly 50/50, support is rarely zero unless incomes are nearly equal.
Under a 60/40 or 70/30 split, states apply a proportional adjustment. Some states have explicit tables for various custody percentages; others apply a formula that scales linearly with overnights above the standard threshold.
Add-On Expenses
Most states allow add-on expenses on top of the basic support obligation. The most common are:
- Work-related childcare costs: Daycare, after-school care, or summer camp that allows the custodial parent to work. These are generally split between parents in proportion to their income shares, and the paying parent's share is added to the base support obligation.
- Unreimbursed medical and health insurance premiums: If one parent carries the children on their employer health plan, the premium cost is often shared. Unreimbursed out-of-pocket medical costs above a threshold (commonly $250 per year per child) are also split by income ratio.
- Extracurricular activities: Some states include sports, lessons, or school-related activities as add-ons, especially if both parents agree or if the court finds them reasonable.
Health Insurance Obligation
Federal law (under the Child Support Enforcement program) requires that child support orders address health insurance coverage when available at a reasonable cost — typically defined as no more than 5% of gross income. If the paying parent has access to employer-sponsored family coverage, the court may order them to enroll the child and may adjust the support payment accordingly. If neither parent has affordable coverage, the order may require one parent to seek individual coverage or the state to enroll the child in CHIP.
How to Modify Child Support
A child support order is not permanent. Either parent can file a motion to modify when there has been a substantial change in circumstances. Most states define "substantial" as a change that would alter the guideline amount by 15–20% or more. Common triggers include:
- Job loss or significant reduction in income
- A major raise, promotion, or new source of income
- Birth or adoption of a new child with a different partner (recognized in most states)
- A change in custody arrangement
- Significant change in the child's needs (new medical condition, change in school costs)
- The child reaching the age of emancipation for a sibling, reducing the number of children covered
Modifications are not retroactive to the date of the changed circumstances — they apply from the date the motion was filed. This makes prompt filing critical when circumstances change.
Back Child Support and Enforcement
Unpaid child support accumulates as arrears (back child support) and accrues interest in most states. Enforcement tools available to state child support agencies include:
- Wage garnishment: Up to 50% of disposable income when supporting another family; up to 60% if not. Add 5% for arrears more than 12 weeks old.
- Tax refund interception: Both federal and state tax refunds can be seized.
- License suspension: Driver's license, professional licenses, and hunting/fishing licenses can be suspended for non-payment.
- Passport denial: The U.S. State Department will not issue or renew a passport when arrears exceed $2,500.
- Credit bureau reporting: Unpaid support is reported to the major credit bureaus.
- Contempt of court: A paying parent can be held in contempt and face fines or jail time for willful non-payment.
Child Support and Taxes
Under the Tax Cuts and Jobs Act of 2017 (effective for agreements on or after January 1, 2019), child support is not deductible by the paying parent and not taxable income to the receiving parent. This is different from alimony, which has its own tax treatment for pre-2019 agreements. The dependency exemption for the child and the Child Tax Credit go to the parent who has the child for more than 183 nights per year by default, but can be transferred to the non-custodial parent by written agreement (IRS Form 8332).
Self-Employed Parents and Child Support
For self-employed parents, courts look at net business income after legitimate business expenses but before personal deductions. Courts are skeptical of claimed deductions that primarily benefit the owner personally, such as large vehicle expenses, travel, or meals. If a self-employed parent's reported income appears inconsistent with their lifestyle, the court may order a forensic accountant to review business records. Courts can also impute income to a business owner who pays themselves an artificially low salary from a profitable company.
How Long Child Support Lasts
In most states, child support continues until the child turns 18 or graduates from high school, whichever is later (up to age 19 in some states). Some states extend support to age 21 for college attendance, particularly if agreed to in the divorce settlement. For a child with a permanent disability who cannot support themselves, support may continue indefinitely. Child support terminates automatically in most states when the child marries, joins the military, or is legally emancipated.
What to Do If Your Ex Won't Pay
If the paying parent misses payments, contact your state's child support enforcement agency (part of the IV-D program established by Title IV-D of the Social Security Act). IV-D services are free to custodial parents and include locating the non-custodial parent, establishing paternity, enforcing the order through wage garnishment, and intercepting tax refunds. You can also file a motion to enforce with the court that issued the original order. For interstate cases (paying parent lives in a different state), the Uniform Interstate Family Support Act (UIFSA) governs enforcement.
College Expenses and Child Support
Basic child support typically ends at 18 or high school graduation, but some states — including New York, New Jersey, Massachusetts, and Connecticut — allow courts to order contribution to post-secondary education costs. Even in states that do not mandate it, parents can agree in a settlement to share college costs. If college contribution is important to you, negotiate it explicitly in your separation agreement rather than assuming a court will order it later.
International Child Support
If the paying parent lives in another country, enforcement depends on whether that country participates in the Hague Convention on the International Recovery of Child Support (in force for the U.S. since 2017). For countries not covered by the Hague Convention, enforcement is more complex and may require retaining an attorney in the other country. For interstate (state-to-state) enforcement within the U.S., UIFSA provides a streamlined process.
Reference Tables
Table 1: Income Shares Percentages by Number of Children
The percentages below represent simplified national-average Income Shares guidelines. Individual state schedules vary.
| Number of Children | Income Shares % (of combined) | TX Guideline % (of payer net) | Example ($8,000 combined) |
|---|---|---|---|
| 1 child | 17% | 20% | $1,360/mo |
| 2 children | 25% | 25% | $2,000/mo |
| 3 children | 29% | 30% | $2,320/mo |
| 4 children | 31% | 35% | $2,480/mo |
| 5+ children | 34% | 40% | $2,720/mo |
Table 2: Custody Arrangement and Support Adjustment
| Custody Arrangement | Paying parent overnights/year | Typical support adjustment | Notes |
|---|---|---|---|
| Primary custody (80/20) | 73 nights | No adjustment (baseline) | Standard guideline applies |
| 70/30 split | 110 nights | ~10% reduction | Varies by state formula |
| 60/40 split | 146 nights | ~15% reduction | Moderate shared custody |
| 50/50 shared | 183 nights | ~20–30% reduction | Offset method; income gap still drives support |
Table 3: State Model Types
| Model | Which States | Key Feature | Income Used |
|---|---|---|---|
| Income Shares | ~40 states (AL, AZ, CO, FL, GA, IL, MD, MA, MI, MN, MO, NJ, NY, NC, OH, PA, VA, WA, and others) | Both parents' incomes combined; split by income ratio | Gross income |
| Percentage of Income | Texas, Wisconsin (and a few others) | Fixed % of paying parent's income only | Net (TX) or Gross (WI) |
| Melson Formula | Delaware, Hawaii, Montana | Self-support reserve first, then proportional need | Gross income |