Child support is calculated using a formula that depends on both parents' income, custody time, and state law

Child support amounts are not set by a single national rule. Each state has its own formula, and the amount you pay or receive depends on how much each parent earns, how much time the child spends with each parent, and what state you live in. Most states use one of three main approaches: the income shares model, the percentage of income model, or the combined parental income model. The court or child support agency in your state will apply whichever method your state uses to the specific numbers from your case.

The calculation starts with gross income — that is, income before taxes. Gross income typically includes wages, salary, bonuses, self-employment income, rental income, and sometimes unemployment or disability benefits. Some income sources are excluded, such as means-tested benefits like TANF or SSI. Once the court determines each parent's gross income, it applies the state's formula to reach a base amount. From there, adjustments are made for custody time, health insurance costs, childcare expenses, and other factors that vary by state.

Key Takeaways

  • Every state uses a different formula to calculate child support, so the amount depends on which state the order is issued in.
  • Gross income — not take-home pay — is the starting point, and it includes wages, self-employment earnings, bonuses, and some benefits.
  • The amount changes if custody time is split roughly equally between parents, and most states adjust for childcare and health insurance costs.
  • If either parent's income changes significantly, you can ask the court to modify the order, though the change usually has to be substantial and lasting.

The Three Main Calculation Models States Use

The income shares model is used by most states. It assumes both parents should contribute to the child's support in proportion to their income. The court calculates what it costs to raise the child at the combined income level of both parents, then divides that cost between them based on each parent's share of the total income. For example, if one parent earns 60 percent of the combined income, that parent pays 60 percent of the calculated support amount.

The percentage of income model is used by a smaller number of states. Under this approach, child support is simply a percentage of the paying parent's gross income — typically 17 to 25 percent for one child, with higher percentages for more children. This model is simpler to calculate but does not account for the other parent's income in the same way the income shares model does.

The combined parental income model is less common. It combines elements of the other two: the court adds both parents' incomes together, calculates support based on that combined figure, then assigns a portion to each parent. This approach is used in a handful of states and produces results similar to the income shares model in most cases.

What Income Counts and What Does Not

Gross income for child support purposes is broader than what appears on a tax return. It includes W-2 wages, salary, hourly pay, bonuses, commissions, self-employment income, rental income from property, income from investments, and unemployment or workers' compensation benefits. Some states also count disability benefits, military allowances, or pension income.

Income that is typically excluded includes means-tested public benefits such as TANF, SSI, or SNAP; child support or alimony received from another relationship; and tax refunds. Self-employment income is counted, but the paying parent can deduct legitimate business expenses before the amount is used in the calculation. If someone is self-employed, the court will often look at tax returns from the past two years to establish a pattern of income.

If a parent is unemployed or underemployed, the court may impute income — that is, assign an income level based on what the parent could earn. This prevents someone from deliberately working less to reduce their support obligation. The court typically imputes income based on the parent's education, work history, and local job market, or sometimes at minimum wage if no other information is available.

How Custody Time Affects the Calculation

Most states adjust the base child support amount if the child spends a significant amount of time with both parents. If custody is split roughly equally — often defined as each parent having the child at least 40 to 50 percent of the time, depending on the state — the support amount is usually reduced. The logic is that the parent with less custody time is already spending money on the child during their parenting time, so they should not pay the full amount calculated for a parent with minimal custody.

The adjustment is not automatic or uniform. Some states reduce the support amount by a fixed percentage when custody is equal; others use a more complex calculation that accounts for the exact number of overnight stays. A few states do not adjust for equal custody at all. If one parent has the child far more than half the time, the other parent typically pays the full calculated amount with no reduction.

Custody time is measured by overnights or days per year, not by who makes decisions about schooling or medical care. If you have a custody order, that order defines the parenting time used in the calculation. If there is no formal order but the parents have an informal arrangement, the court will look at the actual pattern of time spent.

Adjustments for Childcare, Health Insurance, and Other Costs

After the base support amount is calculated, most states allow adjustments for certain expenses. The most common adjustments are for childcare costs needed so one parent can work, and for health insurance premiums paid by either parent for the child. Some states also adjust for the cost of uninsured medical expenses, or for a parent's support of other children from different relationships.

Childcare adjustment works like this: if the custodial parent pays for daycare or after-school care so they can work, that cost is often added to the base support amount, and the paying parent's share of that cost is added to their monthly obligation. Health insurance premiums for the child are typically split between the parents in proportion to their income, or the paying parent is ordered to carry the insurance and the cost is factored into the support amount.

Some states have a cap on the income used in the calculation — for example, support might be calculated only on the first $150,000 of combined parental income, with income above that amount not counted. A few states allow adjustments for a parent's substantial debt, or for the cost of supporting a disabled adult child. These adjustments vary widely by state.

Modifying Child Support When Income Changes

A child support order is not permanent if circumstances change. Either parent can ask the court to modify the amount if there has been a substantial and lasting change in income. What counts as "substantial" varies by state — some require a change of 10 percent or more, others use different thresholds. The change must be expected to last for more than a short period; a temporary job loss or a one-time bonus usually does not trigger a modification.

To request a modification, you file a motion with the court that issued the original order, or with the child support enforcement agency in your state. You will need to provide recent income documentation — pay stubs, tax returns, or a letter from your employer — to show the change. The other parent has the right to respond and present their own income information. The court will then recalculate the support amount based on the new income figures.

If your income has decreased, you should request a modification as soon as the change appears to be lasting. If you do not, you remain legally obligated to pay the original amount, and arrears can accumulate. If your income has increased, the other parent can request a modification to increase the amount you pay. Some states allow modifications to be requested every two or three years even without a change in income, as a way to account for cost-of-living increases.

How State Differences Affect Your Calculation

Because each state has its own formula and rules, the same two parents with the same income and custody arrangement could owe very different amounts depending on which state issues the order. A state that uses the income shares model and adjusts heavily for equal custody will produce a different result than a state using the percentage of income model with no equal custody adjustment.

If you are ordered to pay child support in one state but move to another, the original order usually remains in effect unless it is modified. However, if the other parent requests a modification in the new state, the new state's formula will be applied. This can result in a significant change to the amount owed. If you are considering a move, or if the other parent has moved, it is worth understanding how the new state's rules would affect the calculation.

You can find your state's specific formula and rules through your state's child support enforcement agency or the state court system's website. Many states publish guidelines that show the calculation method and any income caps or adjustment factors. If you need to understand how your specific numbers would be calculated, a family law attorney in your state can walk you through the process.

Frequently Asked Questions

Does the paying parent's new spouse's income count toward child support?

No. The income of a stepparent or new spouse is not counted in the child support calculation. Only the income of the two biological or adoptive parents is used. However, if the paying parent's household expenses decrease because they now live with someone else, that might be considered in some states when deciding whether to modify the order.

What happens if the paying parent is in jail or prison?

Incarceration does not automatically stop a child support obligation, but it can be grounds for a modification. The court may reduce or suspend the obligation if the parent has no income while incarcerated. However, arrears continue to accumulate unless the court specifically suspends them. Once the parent is released, the obligation resumes, and they may owe the back amount that accumulated during incarceration.

Can child support be calculated if the other parent refuses to disclose their income?

Yes. If a parent refuses to provide income information, the court can impute income based on their education, work history, and earning potential, or assign income at minimum wage or a higher rate. The court can also hold the parent in contempt for refusing to comply with the disclosure order. You do not need the other parent's cooperation to proceed with a calculation.

Does child support end automatically when the child turns 18?

In most states, child support ends when the child reaches the age of majority, usually 18. However, some states extend the obligation if the child is still in high school, or if the child has a disability. The order itself will specify the end date. Support does not stop automatically — either parent can file paperwork to terminate it once the child reaches the age specified in the order.

If I have shared custody 50-50, do I still pay child support?

Possibly. Even with equal custody, if one parent earns significantly more than the other, that parent typically pays some support to equalize the child's standard of living in both homes. The amount is usually less than it would be with unequal custody, but it is not zero unless both parents have identical income and the state's rules allow for no payment in that situation.