Child support is calculated using a formula that depends on both parents' income, custody time, and state law
Every state has its own formula for calculating child support, but they all start with the same basic inputs: how much each parent earns, how much time the child spends with each parent, and how many children need support. The court does not guess at these numbers — it applies a specific calculation method set by state law. The result is a monthly or annual amount one parent pays to the other.
The two most common calculation methods are called the income shares model and the percentage of income model. Most states use income shares. Under this method, the court adds both parents' incomes together, looks up what percentage of that combined income should go to the child based on the number of children, then divides that amount between the parents based on how much time each one spends with the child. The percentage of income model, used in a smaller number of states, straightforward takes a set percentage of the paying parent's income — often 17 percent for one child, 25 percent for two, and so on — regardless of the other parent's earnings.
The amount you see on a child support order is rarely the final number. Courts can adjust it up or down based on factors like health insurance costs, childcare expenses, overnight visitation schedules, and whether either parent has other children to support. Some states also adjust for the cost of living in that state or region.
Key Takeaways
- Every state uses either the income shares model (which combines both parents' incomes) or the percentage of income model (which takes a percentage of one parent's income only).
- The calculation starts with each parent's gross income, which usually includes wages, self-employment income, rental income, and sometimes benefits like unemployment or disability.
- The amount of time the child spends with each parent directly affects the calculation in most states, so custody arrangements matter to the final number.
- Courts can adjust the calculated amount for childcare costs, health insurance premiums, other children in either household, and extraordinary expenses like special education or medical needs.
- The formula itself is public and written into state law, so you can see exactly how your state calculates support before you go to court.
What counts as income for child support purposes
Income for child support is broader than just a paycheck. Courts include wages and salary, but also self-employment income, rental income, interest and dividends, bonuses, commissions, and income from a business or partnership. Some states also count unemployment benefits, workers' compensation, disability payments, and retirement income.
What does not count varies by state, but generally excludes means-tested benefits like food information or housing vouchers, and sometimes excludes child support or alimony a parent is already paying for other children. A few states exclude Social Security income if the parent is retired or disabled, though this rule differs widely.
If a parent is self-employed or has irregular income, the court usually looks at the average over the past two or three years rather than a single month. If someone recently lost a job or took a pay cut, the court may still use the previous income unless the change was involuntary and permanent — a voluntary career change to earn less does not automatically lower support.
How custody time affects the calculation
In income shares states, the parent who has the child less often typically pays support to the parent who has the child more often. The calculation adjusts based on the percentage of overnights or days each parent has. If both parents have roughly equal time, the support amount may be lower or even zero, depending on the income difference between them.
The exact threshold varies by state. Some states say that if a parent has the child at least 40 percent of the time, the calculation shifts to account for that. Others use different percentages. The point is that custody time is not just about who the child lives with — it directly changes the dollar amount of support because it affects how much each parent spends on the child's day-to-day costs.
If custody changes after an order is in place, either parent can ask the court to recalculate support. Courts typically require a significant change in circumstances — often defined as a 10 to 15 percent change in income or a substantial shift in custody time — before they will modify an existing order.
The difference between gross and net income
Most states calculate child support based on gross income, which is what you earn before taxes, Social Security, health insurance, or retirement contributions come out. A few states use net income instead. This matters because gross income is always higher than net income, so the support amount will be higher if the court uses gross.
Even in gross income states, the court may subtract certain things: court-ordered support for other children, union dues, or mandatory retirement contributions. Health insurance premiums for the child are usually added back in as a separate line item rather than subtracted from income, so the paying parent covers both the support amount and the insurance cost.
If you are self-employed, the court will look at your business income minus legitimate business expenses, but not minus personal expenses like a car payment or rent. The goal is to find what you actually have available to support a child, not to reduce your income based on your personal spending choices.
Adjustments courts make to the base calculation
After the formula produces an initial number, courts can adjust it. The most common adjustments are for childcare costs (the parent paying for daycare or after-school care can deduct this), health insurance premiums for the child, and extraordinary medical or educational expenses. Some states also adjust for the cost of living in different regions within the state.
A parent with other children in their household — either from a previous relationship or a new one — may be able to reduce their support obligation because they have additional financial responsibilities. The court does not eliminate the obligation, but it may lower it to account for the fact that the parent's income is stretched across more children.
If the calculated amount would be unusually high or low compared to what the state considers reasonable, the court can adjust it. Some states have a cap on the income used in the calculation — for example, they might only count the first $150,000 of a parent's income — so that support does not become disproportionately high for high-earning parents.
How to find your state's specific formula
Your state's child support formula is written into state law and is public information. You can find it by searching your state's statutes for "child support" or "support guidelines." Most state court websites also have a plain-language summary of how support is calculated in that state, and many have worksheets or calculators that let you plug in numbers and see what the formula produces.
If you are working with a family law attorney, they will have the formula and can walk you through the calculation with your specific numbers. If you are representing yourself, the court clerk's office can usually point you to the statute and any official worksheets the state provides. Some states require both parents to file a completed worksheet with the court showing how they arrived at the support amount.
Keep in mind that the formula is a starting point, not the final answer. The judge has discretion to adjust the result based on the factors listed in state law. If you disagree with a support order, you can ask the court to explain how it arrived at that number, and you can present evidence that an adjustment is warranted.
When and how support orders can be changed
A child support order is not permanent unless the court specifically says it is. Either parent can ask for a modification if circumstances change significantly. The most common reasons are a substantial change in income, a change in custody time, or a change in the child's needs (for example, the child now needs special education services or has a serious medical condition).
The threshold for "substantial" varies by state, but it is often a 10 to 15 percent change in income or a shift in custody of at least 30 days per year. If you have experienced a job loss, a significant raise, or a custody change, you may be able to ask the court to recalculate. The new calculation uses the same formula as the original order, just with updated numbers.
If you do not ask for a modification and circumstances change, you are still bound by the original order until a court changes it. Paying less than the order requires, even if your income dropped, can result in arrears (back support owed) and enforcement action. If your income has changed, the step is to file a motion to modify with the court, not to unilaterally pay less.
Frequently Asked Questions
Does child support end automatically when the child turns 18?
In most states, support ends when the child reaches the age of majority, which is usually 18. However, some states extend support through high school graduation or to age 19 or 20 if the child is still in school full-time. A few states allow support to continue beyond that if the child has a disability. The order itself will state the end date, so check what yours says.
What if one parent refuses to report their income honestly?
If you believe the other parent has hidden income or misrepresented their earnings, you can ask the court to investigate. The court can subpoena tax returns, bank records, and employer statements. If the court finds that a parent deliberately underreported income, it may order them to pay the difference plus penalties, and it may award attorney fees to the parent who had to prove the truth.
Can child support be modified if one parent loses their job?
Yes, but only if the job loss was involuntary and the parent is making a genuine effort to find new work. If a parent quit their job to avoid paying support, the court will not lower the obligation. If a parent was laid off or their position was eliminated, they can ask for a modification based on their new, lower income, though the court may use the previous income as a baseline for a period of time.
How does remarriage or a new child affect child support?
Remarriage itself does not change support — the new spouse's income is not counted. However, if the paying parent has a new biological or adopted child, that can be factored into a modification request because the parent now has additional financial responsibilities. The court will not eliminate support for the first child, but it may adjust it downward.
What if both parents have equal income and equal custody time?
If both parents earn the same amount and spend equal time with the child, the support obligation may be zero or very low, depending on the state's formula. Some states still order one parent to pay a small amount to cover shared expenses like health insurance or activities. The court will calculate it using the same formula, and the result may straightforward be that neither parent owes the other support.