To calculate child support based on income, you plug both parents’ incomes, the number of children, custody time, and specific costs like health insurance and childcare into your state’s worksheet. Most states combine both parents’ earnings to produce a base obligation, then split it in proportion to what each parent contributes. The arithmetic is mechanical. The work is in getting the inputs right.
Pull Your Financial Records First
Before you open a worksheet or calculator, assemble the raw numbers. Three to six consecutive months of pay stubs catch overtime, bonuses, and irregular hours that a single stub would hide. Add your federal income tax returns from the previous two years, with W-2s or 1099s, so annual totals back up what the monthly stubs show.
Every state publishes a standardized child support worksheet, usually through the court system’s website or the department of social services. The worksheet has specific lines for wages, self-employment income, investment returns, rental income, and other sources. You will also need documentation of what you pay for the child’s health insurance, any work-related childcare, and existing support obligations for children from other relationships. Getting these records in one folder before you start filling in boxes saves weeks of back-and-forth later.
What Counts as Income
Child support formulas cast a wide net. Wages, salary, and commissions are the obvious starting points, but the definition extends to overtime, bonuses, self-employment profits, rental income, dividends, interest, pensions, Social Security benefits, workers’ compensation, unemployment benefits, and disability payments. Non-cash perks like a company car or employer-provided housing count too, when they meaningfully reduce what you would otherwise spend on daily living.
The breadth of this definition catches people off guard. A parent with a modest salary but significant rental income or trust distributions will see all of it folded in. The goal is to capture actual economic capacity, not just what a single paycheck shows.
Imputed Income
If a parent quits a job, takes a pay cut, or stays unemployed without a legitimate reason, the court can assign an income figure based on what that parent could reasonably earn. This is called imputing income, and it exists to prevent someone from dodging support by deliberately earning less. Courts look at work history, education, professional skills, local job market conditions, and whether a real health issue limits the ability to work. A parent laid off during a downturn is treated very differently from one who walked away from a well-paying career weeks before the support hearing.
From Gross Income to Net Income
Once gross income is established, the worksheet subtracts specific deductions to produce net (or adjusted) income. The deductions allowed vary by state, but the most common ones include federal and state income taxes, Social Security and Medicare taxes (7.65% of earnings for employees), mandatory retirement contributions, union dues, and existing court-ordered support for children from a prior relationship. That last item matters because the law will not stack a new obligation on top of an old one without accounting for what you already have to pay.
Some states calculate the tax deductions using an assumed filing status rather than your actual return, which prevents manipulation through creative withholding elections. After the allowed deductions come out, the remaining figure is the net income that plugs into your state’s formula. A small error here compounds into thousands of dollars over the life of the order, so it is worth slowing down.
The Three State Calculation Models
Every state uses one of three models to turn income into a dollar amount. Which one applies depends on where the order is being established, and the differences are not trivial.
Income Shares Model
The overwhelming majority of states use this approach. The premise is that a child should receive the same share of parental income they would have gotten if the household had stayed together. Both parents’ net incomes are added to produce a combined figure, which is matched against a state lookup table that specifies a base obligation for that income level and that number of children. Each parent’s share of the total is proportional to their share of combined income. If you earn 65% of the combined household income, you cover 65% of the base obligation.
Percentage of Income Model
A handful of states use a simpler formula that applies a set percentage to the noncustodial parent’s income only. The custodial parent’s earnings are not part of the equation, on the theory that the custodial parent already contributes by covering day-to-day expenses directly. Percentages vary by state and rise with the number of children. The math is faster, but results can feel lopsided when the custodial parent earns significantly more.
Melson Formula
Only Delaware, Hawaii, and Montana use this model. It works in three steps. Each parent first keeps a self-support reserve covering their own basic needs. The child’s primary needs and childcare are funded next from what remains. If the higher-earning parent still has income left over, an additional amount is allocated so the child shares in that parent’s standard of living. Melson is the most complex of the three but the most responsive to families at both ends of the income range.
Adjustments Beyond the Base Amount
The number the formula produces is almost never the final number. Several categories of cost get layered on top, typically divided between the parents in proportion to their incomes.
Health Insurance
The cost of covering the child on a health insurance plan is one of the most common add-ons. If one parent carries the child on a family plan, the portion of the premium attributable to the child, not the parent’s own coverage, gets split between both parents. Isolating the child’s share from a family premium can be tricky. Some states have a specific formula for this; others accept the difference between individual and family rates as a reasonable approximation.
Childcare and Extraordinary Medical Costs
Work-related childcare for younger children is split proportionally when it is necessary for a parent to maintain employment or attend school to improve earning capacity. Extraordinary medical expenses not covered by insurance, including orthodontics, therapy, and treatment for chronic conditions, are also divided. Courts generally require receipts or invoices before adding these to the order, so keep organized records of out-of-pocket spending from the start.
Parenting Time Credits
Most states reduce the noncustodial parent’s obligation when that parent has the child for a significant number of overnights each year. The threshold and formula differ by state, but the logic is the same: a parent who has the child 40% of the time is already paying for food, utilities, and daily expenses during those stays, and the support figure should reflect that. In a shared-parenting arrangement, make sure the worksheet uses your actual overnight schedule rather than defaulting to standard visitation.
Low-Income Protections
Many states build in a self-support reserve that keeps a parent’s income above a minimum threshold, often pegged to the federal poverty guideline, before any support obligation kicks in. Driving a parent below subsistence helps no one, including the child who depends on that parent being housed and functional. If your income is near or below the poverty line, the worksheet may produce a reduced obligation, or a nominal minimum order in some states, rather than zero.
What the Finished Calculation Becomes
Once the worksheet is complete, it goes to the court or the state child support enforcement agency along with your supporting documents. A caseworker or court officer reviews the math, cross-checks figures against pay stubs and tax records, and flags discrepancies. The resulting order specifies the monthly amount, the payment schedule, and how funds move. Most support is collected through income withholding, where the paying parent’s employer deducts the amount from each paycheck and sends it to the state disbursement unit. In fiscal year 2024, roughly 71% of all child support collections came through income withholding.1Congress.gov. The Child Support Enforcement (CSE) Program
One tax note worth filing away: child support is not deductible for the parent who pays it, and the receiving parent does not report it as taxable income.2IRS. Publication 504 (2025), Divorced or Separated Individuals The rule is clean, with no exceptions or phase-outs, and it is not the same as the rules for alimony.
Updating the Number When Your Income Changes
A child support order is not permanent. Either parent can request a modification when a substantial change in circumstances makes the current amount unreasonable. Common triggers include a significant increase or decrease in either parent’s income, the loss or gain of health insurance coverage, a change in custody, or new legal obligations to additional children. Many states apply a threshold test: the modification must change the order by at least 20% or a minimum dollar amount before a court will act.
One rule catches many parents off guard. Under federal law, child support that has already come due cannot be reduced retroactively.3Office of the Law Revision Counsel. 42 USC 666 – Requirement of Statutorily Prescribed Procedures to Improve Effectiveness of Child Support Enforcement Every missed payment becomes a judgment by operation of law the moment it is due, and no court or bankruptcy proceeding can erase it after the fact. A modification only affects future payments, and generally only from the date the petition is filed. If your income drops, file for modification right away. Waiting and hoping a court will forgive the gap later does not work.
When the Obligation Ends
Child support does not last forever, but the end point varies by state. In most states, support terminates when the child turns 18. Some extend the obligation to 19 if the child is still in high school. A few require support through 21, or through college under certain circumstances. Support can also end earlier if the child marries, joins the military, or is legally emancipated. A child with a disability that prevents self-sufficiency may be entitled to support beyond the standard age, sometimes indefinitely.
The order itself does not dissolve automatically when the child ages out. In most states, the paying parent has to file a motion to formally terminate the obligation. Any arrears that built up before termination remain collectible regardless of the child’s age. Letting an order linger because you assume it ended on a birthday is how parents end up with continued withholding and enforcement actions long after the kids are grown.