Not every state calculates child support the same way. Roughly 40 states use "income shares," about a dozen use "percentage of income," and three (Delaware, Hawaii, Montana) use a variant of the "Melson formula." The model your state uses changes which numbers actually matter.
Income shares (the majority model)
Income shares combines both parents' income, looks up (or calculates) what a household at that combined income level would typically spend raising the number of children involved, then splits that amount between the parents in proportion to their income share. Both parents' income matters directly.
Percentage of income
States like Texas and Wisconsin instead apply a flat percentage to just the paying parent's own income — the other parent's income generally isn't part of the base calculation at all (though it can matter for parenting-time credits). Fewer inputs, but the outcome can differ sharply from an income-shares state with the same facts.
The Melson formula
Delaware, Hawaii, and Montana use a more layered approach: each parent first keeps a self-support allowance, then a primary support need for the children is prorated by remaining income, then a standard-of-living adjustment shares any income above that. It's the most complex of the three models but aims to protect a parent's own basic needs before calculating support.
FAQ
Which model does my state use?
Check your state's calculator page on this site — the formula model and the exact statute it's based on are listed in the "How This Calculator Works" section of every state page.
Does the model change how much I'll pay?
It can — the same income and custody facts can produce meaningfully different results depending on which model your state uses, since each one weighs the two parents' incomes differently.