Child support in the United States is not calculated the same way in every state. Whether you're the paying parent or the recipient, the final number the court orders depends heavily on which of two fundamentally different models your state uses, how income is defined, and what adjustments apply to your specific situation. A $70,000 income can produce drastically different support obligations depending on whether you live in California or Texas.
Two Models: Income Shares vs Percentage of Income
The vast majority of US states use one of two calculation models:
Income Shares Model (used in approximately 40 states): Both parents' incomes are combined to estimate what the parents would have spent on the child if they were still living together. That total obligation is then divided proportionally based on each parent's share of combined income. A higher-earning non-custodial parent pays a larger portion.
Example: Parent A earns $6,000/month, Parent B earns $4,000/month. Combined income = $10,000. For one child, a typical guideline table might put the total support obligation at $1,400/month. Parent A's share = 60% = $840/month. Parent B's share = 40% = $560/month. Since Parent B is the custodial parent, Parent A pays $840/month to Parent B (Parent B's obligation is considered fulfilled by providing direct care).
Percentage of Income Model (used in approximately 10 states): The non-custodial parent pays a fixed percentage of their own income, regardless of the other parent's earnings. This model is simpler but doesn't account for the custodial parent's financial situation at all.
Flat percentage example (Wisconsin, one child = 17%): If the paying parent earns $5,000/month net, support = $5,000 × 17% = $850/month.
Which Model Your State Uses
| Model | States |
|---|---|
| Income Shares | Alabama, Arizona, California, Colorado, Connecticut, Florida, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Michigan, Minnesota, Missouri, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wyoming |
| Percentage of Income | Alaska, Arkansas, Illinois, Mississippi, Nevada, North Dakota, Texas, Wisconsin |
| Hybrid / Melson Formula | Delaware, Hawaii, Montana (some courts) |
Note: States revise their guidelines periodically. Always verify the current model with your state's child support enforcement agency or a family law attorney.
Income Calculation: Gross vs Net
How income is defined matters enormously because it sets the base for the entire calculation.
Gross income states calculate support before taxes and deductions. This approach is simpler but can result in obligations that consume a larger share of take-home pay.
Net income states (including Illinois and Texas) calculate support on income after taxes, Social Security, Medicare, mandatory retirement contributions, and sometimes union dues. Net income is closer to what the parent actually receives.
What counts as income for child support purposes is broader than most people expect:
- Wages, salaries, and tips
- Self-employment income (after legitimate business expenses)
- Bonuses and commissions
- Rental income
- Investment dividends and interest
- Social Security and disability benefits (including SSDI, but generally not SSI)
- Unemployment benefits
- Workers' compensation
- Lottery winnings and gambling proceeds
Typically excluded: public assistance (SNAP, Medicaid), child support received for other children, adoption assistance payments.
For self-employed individuals, courts scrutinize business expense deductions closely. Legitimate operating expenses reduce income; personal expenses run through the business do not.
Factors That Adjust the Amount
The guideline amount is a starting point, not the final number. Courts routinely adjust for:
Work-related childcare costs: In most states, childcare expenses necessary for the custodial parent to work are shared proportionally between parents — either added directly to the base obligation or treated as a credit.
Health insurance premiums: The cost of covering the child on either parent's employer-provided insurance is typically added to the child's total needs and shared proportionally.
Extraordinary medical expenses: Out-of-pocket medical costs above a threshold (often $250/year) are usually shared proportionally. Courts may specify a cost-sharing formula in the order.
Other children from prior or subsequent relationships: Most states allow a deduction (or adjustment) for legal child support obligations to other children, preventing the second family from being severely disadvantaged by the first.
Special needs: Educational costs for a child with disabilities, therapy, specialized schooling, or adaptive equipment can all increase the total obligation.
Parental assets vs income: Courts may impute income to a parent who is voluntarily unemployed or underemployed, basing calculations on what that parent could earn rather than what they actually earn.
Custody Time and Its Impact on Payments
In Income Shares states, the amount of parenting time each parent exercises directly affects the support amount through what is called a parenting time offset or parenting time adjustment.
The logic: when a non-custodial parent has the children, they're directly spending money on them (food, activities, household costs). The support calculation acknowledges this by reducing the obligation as parenting time increases.
Typical thresholds in many states:
| Non-Custodial Parenting Time | Adjustment |
|---|---|
| Less than 20% of nights | No offset; standard guideline applies |
| 20%–35% of nights | Graduated offset begins |
| 36%–50% of nights | Substantial offset; in some states, only the difference is owed |
| 50/50 custody | Some states require only the higher earner to pay; others reduce to near-zero |
In true 50/50 custody arrangements, the higher-earning parent typically pays support to equalize resources for the child across both households, but the amount is significantly less than in a primary custody arrangement.
Modification: When and How to Request Changes
A child support order doesn't change automatically when circumstances change. Either parent must petition the court for a modification, and the court will only grant one if a substantial change in circumstances has occurred since the last order.
What qualifies as a substantial change:
- A significant income change (most states define this as 15%–25% or more)
- Job loss or involuntary reduction in hours
- A new serious medical condition or disability
- A change in the child's needs (starting daycare, medical diagnosis)
- A significant change in parenting time arrangements
- The child reaching a milestone (turning 18, graduating, becoming emancipated)
The process typically involves:
- Filing a motion to modify with the court that issued the original order
- Serving the other parent with notice
- A hearing where both parties present updated financial information
- The court running the guideline calculation with current numbers
- A new order issued if the change is substantial enough
Retroactive modification is generally not available — the new order takes effect from the date the petition was filed, not from the date of the triggering event. This means delays in filing are costly. A parent who loses a job and waits six months to petition owes back support at the old rate for all six months, regardless of the new order.
Informal agreements to pay less than the court order are legally unenforceable and do not protect the paying parent from arrears accumulation. Any change to the obligation must go through the court.