Child Support Calculator

Introduction: how the income shares model splits child costs

Most child support systems in the United States start from a single idea: the money spent raising a child should be shared by both parents roughly in proportion to what each of them earns. That idea is called the income shares model, and it is now the most widely adopted approach among state guidelines. It works in three steps. First, the parents' incomes are added together to describe the household economy the child would have had. Second, a guideline schedule or percentage converts that combined income into a basic child support obligation, an estimate of what a family at that income level typically spends on children. Third, that obligation is divided between the parents pro rata, meaning each parent is assigned the same fraction of the obligation as that parent's fraction of the combined income.

Parenting time enters afterwards. A parent who houses and feeds the children for part of the year is already meeting part of their assigned share directly, in kind rather than in cash. Guidelines therefore credit each parent for the time the children are actually in their care and settle only the difference in cash. This calculator implements that arrangement in its simplest and most transparent form, so you can see exactly how a change in income or in the parenting calendar moves the estimated transfer. It is a teaching and planning tool. It does not produce a court-ready worksheet, and nothing on this page is legal advice.

How to use this child support calculator

Enter each parent's gross monthly income, the number of children covered by the arrangement, and the percentage of overnight parenting time assigned to Parent A. The labels “Parent A” and “Parent B” are neutral placeholders; assign them in whichever order you prefer, because the arithmetic is symmetric and reversing the two parents simply reverses the direction of the reported transfer. The custody percentage is expressed as a share of overnights, which is how most guidelines count parenting time. Entering 60 means the children sleep at Parent A's home about 219 nights a year and at Parent B's home about 146 nights.

The result panel shows every intermediate quantity rather than only the final number: combined income, the guideline percentage applied, the basic obligation, each parent's pro rata share in dollars, the parenting-time split translated into overnights, both cross obligations, and the net monthly transfer with its direction. Change one input at a time and watch which line moves. That habit is the fastest way to understand why two families with similar incomes can end up with very different orders. Use the Reset button to clear the form and start a fresh scenario, and use the shareable link in your address bar to return to a scenario later.

Formula for the income-shares child support estimate

Write I_A and I_B for the two monthly incomes and I for their sum. The guideline percentage G depends on the number of children. The basic obligation B and the two pro rata shares S_A and S_B are:

Let t_A and t_B be the fractions of parenting time, so t_A + t_B = 1. Each parent owes their own pro rata share for the portion of the year the children are with the other parent. Those two cross obligations, and the net transfer P received by Parent A, are:

Substituting the shares and simplifying gives a compact identity that makes the whole model easy to reason about. Because S_A + S_B = B, the net transfer is the basic obligation multiplied by the gap between Parent A's share of parenting time and Parent A's share of combined income:

A positive P means Parent B is the paying parent; a negative P reverses the direction and the calculator reports the absolute value together with the correct payer. The estimate reaches exactly zero only when a parent's share of parenting time equals that parent's share of combined income. That is why an even 50/50 schedule does not automatically produce a zero order: with equal time the net transfer settles at half the difference between the two pro rata shares, which is nonzero whenever the incomes differ.

Guideline percentages used by this calculator

The percentages below convert combined monthly income into the basic obligation. They match the statutory child support percentages in the New York Child Support Standards Act, which are among the most widely quoted numeric guideline rates and are applied to combined parental income before the pro rata split. Other states use dollar schedules built from economic estimates of child-rearing costs rather than flat percentages, so treat this table as one concrete, sourced example rather than a national rule:

Notice that the rate rises with each additional child but the increments shrink, reflecting the economies of scale in a household. Real schedules add features this table omits: a self-support reserve that protects a low-earning parent's basic living costs, a cap on the combined income to which the percentages apply, and judicial discretion above that cap. New York, for example, applies its percentages to combined parental income only up to a cap that is updated every two years, with the court exercising discretion on income above it.

Worked example: a two-child shared parenting scenario

Parent A earns $3,000 per month, Parent B earns $2,000 per month, there are two children, and Parent A has 70% of the overnights. Combined income is $5,000, so the two-child rate of 25% gives a basic obligation of $1,250. Parent A's income share is 3,000 / 5,000 = 60%, worth $750; Parent B's share is 40%, worth $500. Parent A owes their $750 share for the 30% of the year the children are with Parent B, which is $225. Parent B owes their $500 share for the 70% of the year the children are with Parent A, which is $350. The net transfer is $350 − $225 = $125 per month from Parent B to Parent A.

The compact identity gives the same answer in one step: $1,250 × (0.70 − 0.60) = $125. Now change only the parenting time to an even split. Parent A's cross obligation becomes $750 × 0.50 = $375 and Parent B's becomes $500 × 0.50 = $250, so the net transfer is $250 − $375 = −$125, which means Parent A now pays Parent B $125 per month. The direction flipped because Parent A's 60% income share is larger than their 50% share of parenting time. This reversal is the single most useful thing the model teaches, and it is the reason the calculator determines the payer from the arithmetic rather than from whoever has more custody.

Adjustments this child support estimate leaves out

Real orders routinely include facts this calculator never asks for. The most common additions are health insurance premiums attributable to the children, work-related childcare, and extraordinary or uninsured medical expenses; these are typically added to the basic obligation and then divided pro rata in the same proportions. Other frequent adjustments include credits for other children a parent supports, travel costs for long-distance parenting time, private school or special-needs expenses, spousal maintenance paid or received, mandatory retirement and union dues, and self-employment tax treatment.

Jurisdictions also differ sharply in how they handle parenting time itself. Some apply a straight pro rata offset like the one modeled here. Some apply no time credit at all until a threshold number of overnights is crossed, then apply a multiplier to reflect duplicated fixed costs across two households. Some use a published table indexed by overnights. Because of those thresholds and multipliers, a real order can change abruptly at a particular number of overnights, whereas this calculator moves smoothly and continuously. Read that smoothness as a simplification, not as a prediction.

Limitations, assumptions and legal disclaimer

This calculator is an educational estimate and is not legal advice. It does not create, modify or predict any legal obligation, and its output is not a court order. Actual orders follow the guidelines of the state or jurisdiction where the case is heard, subject to a rebuttable presumption that the guideline amount is correct and to written findings when a court deviates from it. Its assumptions are deliberately simple: gross monthly income is taken as entered with no deductions, the guideline percentage is a flat rate applied to unlimited combined income with no cap and no self-support reserve, parenting time is measured purely as a share of overnights, and the time credit is a straight pro rata offset with no threshold and no duplicated-cost multiplier.

Two further limitations are worth stating plainly. First, the model is symmetric between the parents and carries no assumption about which parent is the primary caregiver; the roles “payer” and “recipient” fall out of the arithmetic alone. Second, the estimate is only as good as the income figures entered, and disputes about income — irregular earnings, unreported income, or a request to impute income to an underemployed parent — are usually the largest source of difference between an estimate and an order. If support is genuinely at issue, gather pay stubs, tax returns and childcare and insurance records, then work through the official worksheet for the relevant jurisdiction or consult a qualified family law professional.

Common questions about child support estimates

Sources for the child support formula on this page

The structure of the calculation and the guideline percentages used above are drawn from the following primary sources. Confirm the rules that apply to a specific case with the guidelines published by the relevant state agency or court.

• 45 CFR § 302.56, Guidelines for setting child support orders — requires every state to adopt one set of numeric guidelines, to base them on the parents' earnings and ability to pay, to review them at least once every four years, and to treat the guideline amount as a rebuttable presumption.
• New York Domestic Relations Law § 240(1-b), Child Support Standards Act — source of the 17%, 25%, 29%, 31% and 35% child support percentages, and of the rule that the combined parental income is multiplied by the percentage and then “prorated in the same proportion as each parent's income is to the combined parental income.” The parallel provision for Family Court is Family Court Act § 413(1)(b)(3).
• U.S. Office of Child Support Services, Administration for Children and Families — federal program office for child support, and a directory of the state and tribal agencies that publish the official worksheets and guideline schedules.