How Child Support Is Calculated by State
Most states use one of three models — income shares, percentage of income, or the Melson formula. Here is what each does and the variables that move the number.
Every U.S. state runs a child support program under federal law (Title IV-D of the Social Security Act), but each state writes its own guideline formula. The result is that the same two parents with the same incomes can owe very different amounts depending on where they live. Almost every state organizes its formula around one of three model families.
The three model families
| Model | How it works | Where it is used |
|---|---|---|
| Income shares | Combines both parents’ incomes as if the household were still intact, then splits the child’s share in proportion to each parent’s contribution. | Roughly 40 states |
| Percentage of income | Applies a flat percentage of the noncustodial parent’s income, scaled by the number of children (commonly ~20% for one, ~25% for two, ~30% for three). | A smaller group of states, often for sole-custody cases |
| Melson formula | Treats the child’s needs as primary and deducts each parent’s basic self-support needs before dividing remaining income. | Delaware (and studied by several others) |
The model determines the structure — not the final dollar figure. Two income-shares states can still land far apart because they define income differently and handle add-ons differently.
The variables that actually move the number
| Variable | Effect on the result |
|---|---|
| Gross vs net income | States differ on whether the formula starts from gross pay or net-after-tax pay; some allow deductions for taxes and prior support before applying the guideline. |
| Parenting time | Most states lower the base obligation when the noncustodial parent has substantial overnights, often at or above roughly 40% of overnights per year. |
| Add-on costs | Health insurance premiums, work-related childcare, and uninsured medical costs are usually split on top of the base amount, by income share or a fixed split. |
| High-income cap | Many states cap or allow deviation above a combined-income threshold (commonly in the $150,000–$250,000 range), since the guideline curves are built for typical incomes. |
| Number of children | Both the percentage model and the income-shares share scale up with the count of children covered by the order. |
Why the same salary differs across state lines
Suppose both parents earn similar amounts. In an income-shares state, support reflects the combined household and each parent’s share; in a percentage-of-income state, only the noncustodial parent’s pay drives the base; in Delaware, each parent’s own minimum living needs are carved out first. Layer on different income definitions, different parenting-time adjustments, and different rules for childcare and medical add-ons, and the same family can owe noticeably more in one state than in a neighboring one.