How Is Child Support Calculated?

Updated October 13, 2026 · 2 min read · 3 sources

Short answer

Every state uses child support guidelines, usually a formula based on both parents' incomes, the number of children and parenting time. Most states use an income shares model, which estimates what the parents would spend together and splits it by income; others use a percentage of the paying parent's income. Health insurance, child care and other costs are added, and judges can deviate from the guideline in limited cases.

Child support is not left to a judge's general sense of fairness. Federal law requires every state to adopt guidelines that create a presumptively correct amount (42 U.S.C. 667).

The main models

Model How it works
Income shares (most states) Estimates what the parents would spend on the child if living together, based on combined income, then splits it in proportion to each parent's income
Percentage of income A set percentage of the paying parent's income, rising with the number of children
Melson formula (a few states) Covers each parent's basic needs first, then allocates support

Inputs that matter

  • Each parent's income: wages, self-employment, bonuses and often other sources. Courts can impute income to a parent who is voluntarily unemployed or underemployed.
  • Number of children.
  • Parenting time: many states adjust support when the paying parent has the children a significant share of overnights.
  • Health insurance premiums for the children.
  • Child care costs related to work or school.
  • Extraordinary expenses: medical, educational or special needs.
  • Other children a parent supports.

Federal rules require state guidelines to consider the parent's actual earnings and ability to pay, including for low-income parents (45 CFR 302.56).

Deviations

Judges can depart from the guideline amount if applying it would be unjust or inappropriate, but they must explain why in the order.

How to estimate yours

Most states publish an online child support calculator or worksheet. Results are estimates; the court order controls.

How long support lasts

Usually until the child turns 18 or finishes high school, sometimes longer for children with disabilities or by agreement for college, depending on the state.

Changing support

Either parent can ask to modify support after a significant change in circumstances, such as a major change in income or custody. Changes generally apply only from the date the request is filed, so file promptly. See child support arrears.

A family lawyer or your state child support agency can help calculate support and file for an order.

Sources

  1. Cornell Law School LII: Child support
  2. Cornell Law School LII: 42 U.S.C. 667, State guidelines for child support awards
  3. Cornell Law School LII: 45 CFR 302.56, Guidelines for setting child support orders

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This article is general legal information, not legal advice, and reading it does not create an attorney-client relationship. Laws differ by state and change over time; talk to a licensed attorney in your state about your situation.

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