How Child Support Is Calculated: The Three Models Explained

Federal law requires every state to keep one set of child support guidelines and to treat the guideline number as presumptively correct in court (45 CFR 302.56). The law does not say what the formula has to be. So each state built its own, and they sort into three basic families.

Of the 51 jurisdictions (50 states and DC), 39 use income shares, 7 use a percentage of income, and 3 use the Melson formula; California follows its own statewide formula and the District of Columbia blends two approaches.

The model your state picked shapes everything downstream: whose income matters, what counts as income, and how parenting time moves the number. That is why a family earning the same money can owe very different amounts on either side of a state line.

Income shares is the majority model. The idea: a child should get the same share of parental income they would have gotten in one household. The math runs in three steps. Add both parents’ incomes together. Look up a basic support amount for that combined income in a state table built from studies of what intact families spend. Then split that amount between the parents in proportion to their incomes.

Pennsylvania is a clean example. Its schedule runs from $0 to $30,000 in combined monthly net income. Two parents netting $4,000 combined with two children find a $1,377 basic obligation, and a parent earning 60 percent of the combined income carries 60 percent of it. Georgia runs the same logic on gross income, with a table that now reaches $40,000 a month under its 2024 rewrite.

The paying parent is usually the one with fewer overnights, and the receiving parent is presumed to spend their own share directly on the child. Add-ons like health insurance premiums and work-related child care typically ride on top, split the same pro rata way.

Percentage-of-income states apply a set percentage to the paying parent’s income. Texas is the classic version: 20 percent of monthly net resources for one child, 25 percent for two, stepping up to 40 percent for five, applied only to the noncustodial parent. The other parent’s income does not enter the guideline formula at all.

New York runs a twist on this model. Its Child Support Standards Act takes flat percentages of the parents’ combined income, 17 percent for one child up to 35 percent for five or more, then allocates the result pro rata. Both incomes matter, but there is no expenditure table, just the statutory percentages and an income cap that resets every two years.

This model is simple to compute and predict. Its trade-off is bluntness: a flat percentage does not bend for the real cost curve of raising children at different income levels, so these states lean harder on caps, low-income schedules, and deviation findings.

The Melson formula, named for a Delaware judge, is the rarest model. It works in layers. First, each parent keeps a self-support allowance so they can stay solvent and employed. Next, the children’s primary needs are funded from what remains. Only after those needs are met does a percentage of leftover income get added so children share in a parent’s higher standard of living.

Delaware, Hawaii, and Montana use Melson. It is the most protective of low-income parents by design, and also the most complex to run by hand, which is part of why it never spread.

Two jurisdictions fit none of the three boxes cleanly. California applies its own statewide algebraic formula to both parents’ net disposable incomes and the high earner’s share of parenting time. The District of Columbia blends a percentage model with a reduction based on the custodial parent’s income.

A model label alone does not predict your number. Two income shares states can produce different results because one measures gross income and one measures net, or because their tables were rebuilt in different years. Federal rules require each state to review its guidelines at least every four years, so tables shift on their own schedules.

The reliable path is your own state’s current formula, run with current-year figures. Each of our state calculators encodes that state’s statute or rule, names its effective date, and links the official source, so you can check the math rather than trust it.