Family law

How Is Child Support Determined?

Quick answer

Child support is not a number a judge picks on a hunch. Every state uses a written guideline formula, and the result depends mostly on what each parent earns, how parenting time is split, and a few add-on costs for the children.

Researched by Dan Martin, Legal Researcher · Published by Onbello Legal · Last updated June 2026

Every state uses a guideline formula

Child support in the United States is set by state law, and each state has a guideline formula that courts are required to start from. The exact math differs from state to state, but the idea is the same everywhere: estimate what the children would have received if the parents lived together, then divide that responsibility between the parents based on their ability to pay.

Most states use one of two approaches. The income shares model (used in the majority of states, including California, Florida and Illinois) combines both parents' incomes and splits the support obligation in proportion to each parent's share. The percentage of income model (Texas is the best known example) applies a set percentage to the paying parent's net income based on the number of children. Either way, the guideline number is presumed correct, and a judge needs a stated reason to order something different.

The main inputs: income and parenting time

Income is the biggest driver. Courts look at gross income from nearly every source, including wages, self-employment profit, bonuses, commissions, rental income, and often unemployment or disability benefits. Some deductions are allowed, such as taxes, mandatory retirement contributions, union dues, and support already paid for other children. If a parent is voluntarily unemployed or underemployed, a court can impute income, meaning it calculates support as if that parent earned what they reasonably could.

Parenting time matters in many states but not all. In California, the percentage of time each parent has the children is a direct input to the formula, so a 50/50 schedule produces a very different number than every other weekend. Florida, Illinois and several others adjust support once the time share crosses a set threshold of overnights. Texas generally does not adjust the percentage for parenting time, although a court can consider it as a reason to deviate.

Add-ons that get layered on top

The base guideline amount usually covers everyday costs like housing, food and clothing. Certain expenses are added separately and split between the parents, most commonly work-related childcare, the children's share of health insurance premiums, and uninsured medical, dental and orthodontic costs. Many states also allow add-ons for special education needs, extracurricular activities, or travel costs when parents live far apart.

Because these items are handled outside the base number, two families with identical incomes and schedules can end up with different totals. It is worth listing every child-related expense you currently pay before any calculation, since items left off the list are easy to miss and hard to add later.

When a judge can depart from the formula

Guideline support is a presumption, not an absolute rule. A court can order more or less if applying the formula would be unjust or inappropriate in a specific case. Common reasons include a parent with extraordinarily high income, a child with significant special needs, parents who have agreed on a different amount that still meets the children's needs, or a situation where the paying parent's housing costs are unusually high because of the custody arrangement.

Support orders also are not permanent. Either parent can ask the court to modify support when there is a meaningful change in circumstances, such as a job loss, a large raise, a change in the custody schedule, or a child reaching the age where support ends. The modification request is filed in the same court that issued the original order, and in most states the new amount runs from the date the request was filed, not from the date circumstances changed. Filing promptly protects both sides.

Tools and resources

Where to check this

Forms, deadlines, and local rules change. Verify anything on this page against the official self-help resources for your state before you rely on it.

This page is general legal information, not legal advice, and does not create an attorney–client relationship. Laws and local rules vary and change. For advice about your situation, consult a licensed family-law attorney in your state.