How Child Support Is Calculated
Three formula families decide support across the fifty states. Here is how each works and which states use which.
Most states use income shares (combine incomes, split by share, credit parenting time); some use percentage of income (flat cut of the noncustodial parent's pay); a few use the Melson formula (basic needs first, then a standard-of-living add-on). All adjust for insurance, childcare, and overnights.
The income-shares model
Used by most states. Add both parents' incomes, find the basic support obligation for that combined income and child count in the state table, split it by each parent's income percentage, then adjust for parenting time and add-ons like insurance and childcare.
The philosophy: children should get the same share of total parental income they would have enjoyed in an intact household. The tables are built from economic studies of what families actually spend on children at each income level.
The percentage-of-income model
Used by a minority of states, including Texas and New York in modified forms. Take a flat percentage of the noncustodial parent's income: 20 percent for one child, 25 percent for two, and so on, sometimes with caps at high incomes.
It is simpler and more predictable, but critics say it ignores the custodial parent's income entirely, which can produce odd results when the custodial parent earns far more.
The Melson formula
Used by Delaware, Hawaii, and Montana. It works in layers: first cover each parent's basic self-support needs, then the children's primary needs, then split any remaining income so the children share in the parents' higher standard of living.
Melson is the most elaborate model and the most protective of low-income parents, because the self-support reserve comes off the top before any support is assessed.
Adjustments that move the number
Health insurance premiums for the children are typically added to the obligation and split by income share. Work-related childcare costs are added the same way. Extraordinary medical expenses are usually split in proportion to income.
Deductions vary: some states credit support paid for other children, alimony paid, or the cost of the payer's own health coverage. These line items are where real cases are won and lost.
Why estimates differ from orders
Online calculators, including this one, simplify tables and skip state-specific deductions. Two calculators can disagree by hundreds of dollars on the same facts and both be reasonable simplifications.
The number that matters is the one from your state's official worksheet, completed with verified income documents. Use estimates to set expectations, then let the official process set the order.
Skip the arithmetic
Run a simplified estimate with the free child support calculator.
Calculation models
Which states use income shares?
Most states use the income-shares model, including large states like California, Florida, Illinois, and Pennsylvania. Texas, New York, and a few others use percentage-of-income variants, while Delaware, Hawaii, and Montana use the Melson formula.
Do both parents' incomes matter?
It depends on the model. Income-shares and Melson states combine both incomes and split the obligation proportionally, so both matter. Percentage-of-income states base support primarily on the noncustodial parent's income.
Are online child support calculators accurate?
They give reasonable ballparks for expectation-setting. But they simplify state tables, skip deductions like other-child support, and cannot verify income. Treat the output as a starting point and complete your state's official worksheet for the binding number.