Child support in the United States is calculated using a state-specific formula, not judicial discretion. Every state is required by federal law under 45 CFR Part 302.56 to maintain numeric guidelines that create a rebuttable presumption — meaning courts must follow the formula unless they find in writing that the result would be unjust or inappropriate.
Three calculation models exist across all 50 states. The model your state uses determines which income figures go into the formula, how parenting time affects the result, and how add-on expenses like childcare and health insurance are handled.
Model 1: Income Shares (41 States)
The Income Shares model is used by the majority of states. The core principle is that children should benefit from the same proportion of parental income they would have received if the family remained intact. To achieve this, the formula pools both parents' incomes and divides the resulting obligation proportionally.
How It Works Step by Step
- Calculate combined gross monthly income. Add both parents' gross monthly income (before taxes). Include wages, salary, overtime, bonuses, self-employment income, and rental income.
- Look up basic child support obligation. Each state publishes a schedule table that shows the basic monthly obligation for a given combined income and number of children. For example, at $7,000 combined monthly income for one child, a typical income shares table shows approximately $1,100 in basic support.
- Calculate each parent's income share. Divide each parent's income by the combined income. Parent A earning $4,000 of the $7,000 combined holds a 57% income share.
- Allocate the obligation. Each parent is responsible for their percentage of the basic obligation. The non-custodial parent pays their share directly to the custodial parent.
- Add childcare and health insurance. Work-related childcare and the cost of the child's health insurance are added to the basic obligation and split between parents by their income shares.
- Apply parenting time adjustment. Most income shares states reduce the non-custodial parent's obligation when they have significant overnight time — typically above 92 overnights per year.
Worked example (Income Shares): Parent A earns $5,000/mo, Parent B earns $3,000/mo. Combined = $8,000/mo. Basic obligation for 1 child at this income level ≈ $1,200/mo. Parent A's share = 62.5%, so Parent A's base obligation = $750/mo. Add $200/mo childcare split 62.5% = $125. Total: $875/mo if Parent A is the non-custodial parent.
Model 2: Percentage of Income (6 States)
The Percentage of Income model uses only the non-custodial parent's income. It applies a fixed percentage based on the number of children. This model is computationally simpler and is used in Texas, Wisconsin, Nevada, Illinois, Mississippi, and North Dakota.
The standard percentages for net income (after taxes):
| Children | % of Net Income | Example: $4,000 net/mo |
|---|---|---|
| 1 child | 20% | $800/mo |
| 2 children | 27% | $1,080/mo |
| 3 children | 33% | $1,320/mo |
| 4 children | 36% | $1,440/mo |
| 5+ children | 40% | $1,600/mo |
Texas and North Dakota apply percentages to net income. Illinois, Mississippi, Nevada, and Wisconsin use gross income. Always verify which your state uses before estimating.
Model 3: Melson Formula (Delaware, Hawaii, Montana)
The Melson Formula is the most complex model. It first ensures each parent retains enough income for their own basic needs (the self-support reserve), then determines the primary support need for the children, then allocates any remaining income toward a standard of living adjustment for the children.
Because of this complexity, the Melson Formula is difficult to estimate without the official state worksheet. Use the calculator on our homepage for a rough estimate, but confirm with your state's official form.
Add-On Expenses in All Models
All three models handle add-on expenses similarly. The following costs are typically added to the base obligation and split between parents based on income share or by order:
- Work-related childcare and daycare costs
- Health insurance premiums for the child
- Uninsured medical expenses (typically split 50/50 or by income share above a threshold)
- Extraordinary educational expenses
- Extracurricular activity costs (varies by state — not all states include these)
How Parenting Time Affects Child Support
More overnights with the non-custodial parent = lower support obligation. The threshold that triggers a reduction varies by state. Common thresholds:
- Under 92 overnights/year: standard obligation, no reduction
- 92–145 overnights/year: modest reduction in most states
- 146–182 overnights/year: significant reduction
- 183+ overnights (50/50): reduced obligation or cross-calculation — higher earner pays the difference
Estimate Your Child Support
Enter both parents' income, number of children, and parenting time to get an estimate using your state's model.
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