How child support is calculated (and why your number isn't a guess)

Child support is one of the parts of divorce that intimidates people most. There’s a number, the number can be large, and it can feel like it was pulled out of the air by an unsympathetic judge. It wasn’t. Every state has a formula, a worksheet, and usually an online calculator. The number that comes out is determined by inputs you can predict, and once you know which inputs your state uses, the result stops being mysterious.

The two main models

American states use one of two basic models to calculate child support.

Income shares. Used by the majority of states. The formula adds both parents’ incomes together, looks up what a household with that combined income would be expected to spend on children of that age, then allocates each parent’s share of the total according to their share of the combined income. The non-custodial parent pays their share to the custodial parent.

Percentage of obligor income. Used by a minority of states. The formula starts with only the non-custodial parent’s income, then takes a percentage based on the number of children. The custodial parent’s income isn’t a direct input, though it can affect adjustments.

Both models produce a "guideline" number — what the formula spits out before any adjustments. The guideline number is what the court will use unless someone gives them a specific reason to deviate.

What counts as income

The biggest debate in many child-support cases isn’t the formula — it’s what number goes into the formula on each parent’s side.

Income generally includes:

• W-2 wages and salary
• 1099 self-employment income, net of legitimate business expenses
• Bonuses, commissions, tips
• Rental income (net)
• Interest, dividends, capital gains
• Pension or Social Security income
• Alimony from a prior marriage
• Sometimes: lottery winnings, royalties, distributions from a trust, recurring gifts

A few items courts watch closely: large one-time payments (bonuses, settlements), variable income that the obligor can manipulate, and self-employment where business expenses can be padded.

A new spouse’s income generally doesn’t count, with one exception: in some states it can be considered for "ability to pay" purposes if the obligor argues that the guideline number would create extreme hardship.

Parenting time changes the number

The amount of time each parent has the child directly affects the support calculation in most states.

The intuition is straightforward: the parent who has the kids less is paying for fewer direct expenses (groceries, electricity, etc.) at their home and contributing to the other parent’s expenses through support.

In broad strokes, in an income-shares state:

• A 50/50 schedule typically produces a small support transfer, or none, especially if incomes are close.
• A 70/30 schedule produces a meaningful obligation from the 30% parent to the 70% parent.
• A 90/10 schedule produces a substantial obligation.

The exact thresholds and formulas vary by state. Some states have a sharp cliff at certain percentages; others use a smooth curve.

Add-ons

Beyond the base guideline number, most states require parents to split certain additional expenses on top of it:

• Health insurance premiums for the child(ren). Often added to the guideline number and reallocated by income share.
• Childcare costs necessary for either parent to work. Usually split by income share.
• Uninsured medical expenses (copays, prescriptions, orthodontics, mental-health care). Usually split by income share or by some other agreed allocation.
• Extracurriculars of meaningful cost. Sometimes split by agreement; sometimes the court orders specific allocation.
• Educational expenses above what public school provides, where applicable.

These add-ons aren’t optional. The base guideline number alone doesn’t cover them.

Deviation: when the number isn’t right

The guideline number is what the court starts from, but it can be adjusted up or down based on the specific facts of your case.

Common reasons courts deviate:

• A child with significant special needs requiring extraordinary expenses
• A non-custodial parent with extraordinary travel costs for parenting time
• A custodial parent whose actual living expenses are unusually low or high
• A second household to support (other children, sometimes a current spouse)
• An obligor with substantial debt the marriage created
• Extraordinarily high combined parental income, where the formula extrapolates into very large numbers

Deviation requires the court to explain in writing why it’s deviating, so it’s not something a judge does casually. But if the formula produces a number that genuinely doesn’t fit your situation, asking for a deviation is the path.

Modification later

Child-support orders are not permanent. Either parent can ask the court to modify the order when circumstances change.

The standard for modification varies but commonly requires:

• A "substantial change in circumstances" since the last order
• Often a minimum amount of time since the last order (commonly one to three years), unless an emergency change happened
• A meaningful difference between the existing number and what a new calculation would produce (some states require at least a 10% or 15% gap)

Common modification triggers: a job loss or substantial raise, a custody arrangement that changes the parenting-time inputs, a major change in the child’s needs, or the obligor having additional children to support.

Using the actual calculator

Most state court websites have a child-support guideline calculator that uses your state’s formula. They’re usually free, fairly user-friendly, and accurate. Search "[your state] child support calculator" and use the official one rather than a third-party tool that may be running outdated rules.

Running the calculator with your real numbers, before any conversation about money, is the cheapest way to know what to expect. If the number feels off, you’ve at least set the conversation up well.