Child support isn't a number someone pulls out of thin air. Every state uses a formula — and while the formulas differ, they all share the same goal: figure out what it would cost to raise this child, then divide that cost between both parents based on their incomes and how much time each parent spends with the child.
The number that comes out isn't always what people expect. It can feel too high, too low, or just confusing. This article walks you through how the math actually works — what goes in, what comes out, and what can change the final number either direction.
The Three Models States Use
Every state calculates child support using one of three frameworks. Knowing which one your state uses tells you a lot about how your number will be determined.
The Income Shares Model is the most common by far — roughly 40 states use it. The idea behind it is straightforward: a child should receive the same share of parental income they would have received if the family had stayed together. Both parents' incomes are combined, a base support amount is looked up on a state schedule, and that amount is then split proportionally between the parents based on each one's contribution to the combined total. The parent who has less parenting time pays their share to the parent who has more.
The Percentage of Income Model is simpler and used in about six states, including Texas. Under this model, only the paying parent's income matters. The court applies a fixed percentage of that parent's net income to determine support — roughly 20% for one child, 25% for two, 30% for three, and so on, though exact percentages vary by state. Texas updated its income cap in September 2025 to $11,700 in net monthly resources, up from $9,200, which meaningfully increased guideline maximums for higher earners.
The Melson Formula is used in three states — Delaware, Hawaii, and Montana. It's a more complex version of the Income Shares Model that first ensures each parent can meet their own basic needs before calculating child support, then applies a higher percentage to remaining income. The idea is to prevent child support from pushing a lower-earning parent into poverty.
What Counts as Income
This is where people are often surprised. States take a broad view of what counts as income for child support purposes — much broader than just your W-2 wages.
Most states include all of the following: wages and salary, overtime pay, bonuses and commissions, self-employment income (net of legitimate business expenses), rental income, investment income and dividends, pension and retirement distributions, Social Security benefits, disability payments, workers' compensation, and unemployment benefits.
What typically doesn't count: means-tested public assistance like SNAP or Medicaid, Supplemental Security Income (SSI), and in most states, the new spouse's income if you remarry — though a new spouse's income can sometimes affect the analysis indirectly.
How the Base Amount is Determined — Income Shares Step by Step
Since roughly 40 states use the Income Shares Model, here's how it works in practice.
Step 1: Determine each parent's gross monthly income. Add up all income sources for both parents. Most states work from gross income (before taxes); some, including California under its 2025 SB 343 update, now work from net income.
Step 2: Add the incomes together. This is the combined monthly income that will be used to look up the base support obligation.
Step 3: Look up the base obligation on the state schedule. Every Income Shares state publishes a table that shows how much a two-parent household at a given combined income level would spend on a child. The table shows different amounts for one child, two children, three children, and so on. This number is the total base child support obligation.
Step 4: Calculate each parent's income share. Divide each parent's income by the combined total. If Parent A earns $4,000 and Parent B earns $6,000, the combined income is $10,000. Parent A's share is 40%, Parent B's share is 60%.
Step 5: Assign each parent their proportional share of the base obligation. If the base obligation from the state table is $1,200 per month, Parent A owes 40% ($480) and Parent B owes 60% ($720).
Step 6: Adjust for parenting time. The parent with more parenting time is assumed to be spending their share directly on the child day-to-day. So typically, the parent with less parenting time pays their share to the other parent. If Parent A has less parenting time, they pay $480 per month. Then add-ons get calculated on top of this base.
A Worked Example
Marcus earns $5,500 per month gross. His ex-spouse, Diane, earns $4,500 per month gross. They have one child. Marcus has the child 30% of nights per year; Diane has the child 70%. They live in an Income Shares state where the base obligation for one child at $10,000 combined monthly income is $1,150 per month.
Base calculation:
Add-ons (split 55/45 by income share):
Result:
Note: Diane's $518 base share is considered paid directly through her day-to-day care of the child during her 70% of parenting time. Marcus's payments cover his share of all shared costs.
How Custody and Parenting Time Affect the Number
Parenting time matters a lot in Income Shares states — and it works differently from what many people assume.
More parenting time for the paying parent generally means a lower child support obligation, because that parent is directly covering more of the child's day-to-day costs during their time. Most states use a parenting time adjustment that kicks in once the non-custodial parent reaches a threshold — often around 20–30% of overnights per year (roughly 73–110 nights). Some states have a specific formula for shared custody situations; others leave it to judicial discretion.
The common misconception is that 50/50 custody means zero child support. That's rarely true. If one parent earns significantly more than the other, the higher earner typically still pays child support even in a true equal-time arrangement — just a reduced amount that reflects the income gap without the time gap. Equal time reduces the obligation; it doesn't eliminate it when incomes are unequal.
Add-On Expenses: What Gets Tacked On Top
The base support amount is designed to cover the basics: food, shelter, clothing. It doesn't cover everything. Most states require both parents to share certain additional expenses on top of base support, split proportionally by income share.
Work-related childcare costs are the most common add-on. If you need to pay for daycare, after-school care, or a nanny so you can go to work, those costs are shared. Only reasonable, work-necessary childcare counts — a luxury nanny arrangement won't be fully covered.
Health insurance premiums for the child are split between both parents proportionally. Whichever parent carries the child on their plan gets a credit — the other parent's proportional share of the premium is added to (or deducted from) the base support payment.
Unreimbursed medical expenses — co-pays, deductibles, prescriptions, dental, vision — are typically split proportionally as they occur. Courts usually set a threshold (often $250 per year per child) below which the custodial parent absorbs costs, with amounts above the threshold shared.
Educational expenses may be added on in some states — private school tuition, tutoring, special educational needs — particularly when both parents agreed to that standard of education during the marriage.
Extracurricular activities are handled inconsistently. Some states require proportional sharing of reasonable activities; others leave it to the parents to negotiate. If it's not explicitly in your court order, don't assume your co-parent is required to share the cost.
When Judges Deviate from the Formula
State guidelines produce a presumptive amount — the number the formula spits out. Judges are required to use this as the starting point, but they can deviate from it when circumstances justify doing so. The judge has to explain in writing why the formula amount would be unjust or inappropriate in the specific case.
Common reasons for an upward deviation include a child's special medical or educational needs, unusually high costs for the child's activities that both parents previously agreed to support, or a parent's extraordinarily high income where the formula amount far exceeds what the child actually needs.
Common reasons for a downward deviation include significant travel costs a parent incurs to exercise parenting time, other children in the household for whom the parent is already paying support, or a paying parent's income that is so low that the formula amount would leave them unable to meet their own basic needs.
Which States Use Which Model
| Model | How It Works | States (examples) |
|---|---|---|
| Income Shares | Combines both parents' incomes; splits the base obligation proportionally | California, Florida, New York, Illinois, Pennsylvania, Ohio, Georgia, and ~35 others |
| Percentage of Income | Applies a fixed percentage to the paying parent's income only | Texas, Wisconsin, Alaska, Nevada, North Dakota, and a few others |
| Melson Formula | Income Shares variant that protects each parent's basic needs first | Delaware, Hawaii, Montana |
How Child Support Can Be Modified
The number you start with isn't necessarily the number you pay forever. Child support can be modified when there is a substantial change in circumstances — typically defined as a significant shift in either parent's income, a change in the custody arrangement, the child developing new needs, or the paying parent losing their job.
Most states also require periodic reviews of support orders — often every three years — to make sure the amount still reflects current incomes and circumstances. In some states, the child support agency initiates these reviews automatically. In others, a request is required to initiate one.
One thing to know clearly: if your income drops and you can't afford the current payment, file for a modification immediately. Do not simply stop paying or pay less than the order requires. Unpaid child support accrues as a legal debt, earns interest in most states, and cannot be discharged in bankruptcy. Enforcement tools include wage garnishment, tax refund interception, license suspension, passport denial, and in serious cases, contempt of court.