Most US states calculate child support one of two ways. The Income Shares model combines both parents’ incomes, looks up what an intact family at that income level spends on a child, and splits that cost proportionally. The Percentage of Income model applies a set percentage to the paying parent’s income alone and ignores what the other parent earns. Custody time and specific expenses then adjust the result.
The difference between the two is not academic. Run the same two incomes through each formula and you get different numbers, and more importantly, they respond to different facts. Below is that exact comparison, worked in dollars.
What are the two main child support models?
The Income Shares model dominates. <cite index="2-3,2-5,2-6">The National Conference of State Legislatures counts 41 states plus Guam and the Virgin Islands using income shares, while six states — Alaska, Mississippi, Nevada, North Dakota, Texas, and Wisconsin — use the percentage of income model, split between a flat percentage version (Alaska, Mississippi, Nevada, Wisconsin) and a varying percentage version (North Dakota, Texas)</cite>.
Income shares rests on a specific premise: the child should receive roughly the same share of parental resources they would have received if the household had stayed together. Each state publishes a schedule — a long table of combined parental income on one axis and number of children on the other — that converts combined income into a single "basic child support obligation." That obligation is then divided between the parents in proportion to income.
Percentage of income skips the schedule entirely. There is no combined income, no economic table, just a statutory percentage applied to the obligor’s income. <cite index="16-4,16-5,16-6">Texas starts with the paying parent’s net resources, applies a fixed percentage based on the number of children, and does not consider the other parent’s income in the basic calculation; the guideline amount is presumed correct and a judge who departs from it has to explain why.</cite>
A third approach, the Melson formula, is an income shares variant that carves out a self-support reserve for each parent before anything is allocated to the child, then lets the child share in any remaining income. <cite index="5-1,5-4">It works like income shares but adds a step: it first sets aside enough income for each parent’s basic self-support needs, then calculates support, then allows the child to share in surplus income — used in Delaware, Hawaii, and Montana.</cite>
One detail trips up almost everyone: the income figure the worksheet asks for is not the same everywhere. <cite index="24-7">Washington’s statutory economic table is indexed to combined monthly net income.</cite> <cite index="29-3,29-5">Georgia’s basic child support obligation table, published in O.C.G.A. section 19-6-15(o), runs on combined monthly adjusted gross income from $800 to $40,000.</cite> Plugging gross income into a net-income table inflates the answer badly.
Income shares model: a worked dollar example
Take two parents with one child. Parent A brings in $5,000 a month, Parent B brings in $3,000.
Step 1 — combine. $5,000 + $3,000 = $8,000 combined monthly income.
Step 2 — look up the basic obligation. This is the step with no shortcut, because every state publishes its own schedule. <cite index="29-6">Georgia’s table sets the basic child support obligation at $8,000 combined monthly income and one child at $1,095.</cite> <cite index="24-9">Washington’s economic table lists $1,270 for one child at $8,000 combined monthly net income.</cite> <cite index="25-9">Missouri’s Form 14 schedule lists $1,008 at $8,000 combined.</cite> So the same combined income produces a basic obligation somewhere around $1,000 to $1,300 depending on which state you are standing in. Use $1,095 for this example.
Step 3 — find each parent’s share. Parent A earns $5,000 of the $8,000, which is 62.5 percent. Parent B earns 37.5 percent. <cite index="29-7,29-8">Each parent pays their proportionate share based on income percentage — a parent earning 65 percent of combined income pays 65 percent of the obligation.</cite>
Step 4 — apply the share to the obligation. $1,095 x 0.625 = $684.38. That is Parent A’s base order, assuming Parent B has the child the large majority of the time.
Step 5 — add the add-ons. The base figure is not the final figure. Health insurance premiums for the child and work-related childcare are typically added to the obligation and split in the same income proportion. <cite index="4-9,4-10,4-11">Illinois treats the child’s health insurance premium as a shared child support expense allocated proportionally — a $100 monthly premium with a 60/40 income split means $60 falls on the higher earner and $40 on the other.</cite>
Say Parent B pays $400 a month for daycare and $150 a month to add the child to a health plan. That is $550 in add-ons. Parent A’s 62.5 percent share is $343.75, which gets tacked onto the base order:
$684.38 + $343.75 = $1,028.13 per month
The base order nearly doubled. This is why parents who look up their state’s schedule and stop there are routinely surprised at the hearing.
Percentage of income model: the same example recalculated
Now move the same family to Texas. Parent B’s $3,000 income disappears from the math entirely.
<cite index="16-10">Texas Family Code section 154.125(b) sets 20 percent of the obligor’s net resources for one child, 25 percent for two, 30 percent for three, 35 percent for four, and 40 percent for five.</cite> Net resources is not gross pay and not take-home pay either. <cite index="14-3">The calculation starts with gross income from nearly every source, subtracts statutory deductions — Social Security taxes, federal income tax, union dues, and the child’s health and dental insurance — to reach net resources, then applies the 20 percent guideline.</cite>
For $5,000 a month in gross wages, net resources typically land somewhere in the high $3,000s after Social Security, Medicare, and federal income tax. Published worked examples bracket it: <cite index="19-8">guideline support for one child comes to about $631 a month at $4,000 monthly gross and about $1,204 at $8,000 monthly gross.</cite> Interpolating, a $5,000 gross earner pays roughly $780 to $800 a month for one child.
Compare the two results:
| Income shares (Georgia-style) | Percentage of income (Texas) | |
|---|---|---|
| Parent B’s $3,000 income | Reduces Parent A’s share to 62.5% | Ignored |
| Base order | $684 | about $790 |
| With $550 in add-ons | $1,028 | Insurance already deducted from net resources; childcare handled separately |
| If Parent B earned $11,000 instead | Parent A’s share falls to 31.25% | Still 20% of Parent A’s net resources |
That last row is the real difference between the models. Under income shares, the other parent’s earnings directly move your number — a raise on their side lowers your share, and in a high-enough swing the direction of payment can reverse. Under percentage of income, your obligation is a function of your own paycheck and nothing else.
Texas also caps the input. <cite index="16-8">Since September 1, 2025, the percentages apply to the first $11,700 of monthly net resources, up from $9,200, and many websites still show the old figure.</cite> <cite index="19-6">That puts maximum guideline support for one child at $2,340.</cite>
Whichever model sets the number, collection is a separate machine with its own rules — see how wage garnishment actually works if an order is already in enforcement.
Which states use which model?
| Model | How the number is built | States |
|---|---|---|
| Income shares | Combine both incomes, read the basic obligation off a state schedule, split it by income share | <cite index="2-3">Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, 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, plus Guam and the Virgin Islands</cite> |
| Percentage of income (flat) | One fixed percentage of the obligor’s income regardless of income level | <cite index="2-6">Alaska, Mississippi, Nevada, Wisconsin</cite> |
| Percentage of income (varying) | Percentage shifts with the obligor’s income level or other children | <cite index="2-6">North Dakota, Texas</cite> |
| Melson formula | Income shares plus a self-support reserve taken off the top first | <cite index="5-4">Delaware, Hawaii, Montana</cite> |
Treat this table as a map, not a deed. Classifications drift as legislatures amend statutes, and sources disagree at the margins: <cite index="8-10">some newer summaries place Montana and North Dakota in the Income Shares column, arguing both have moved away from the Melson and percentage groupings where older sources list them.</cite> The NCSL guideline model summary is the standard reference, but your state’s current worksheet is the only authoritative answer.
California is the clearest case of a state that fits no label cleanly. <cite index="30-1,30-2,30-5,30-6,30-7">Family Code section 4055 sets the statewide uniform guideline as CS = K[HN – (H%)(TN)], where CS is the support amount, K is the share of both parents’ income allocated to child support, HN is the high earner’s net monthly disposable income, and H% is the approximate percentage of time the high earner has primary physical responsibility for the children.</cite> <cite index="36-7">TN is the combined total net monthly disposable income of both parents.</cite> It is an income shares philosophy expressed as algebra, with parenting time built into the equation rather than bolted on afterward. <cite index="35-8">Senate Bill 343 updated the K-factor brackets for the first time since 1992</cite>, and DivorceNet’s walkthrough of the California guideline breaks the variables down in detail. Sources differ on the exact effective date of the K-factor change, citing September 1, 2024 and September 1, 2025 for different pieces of the bill, so check the current Judicial Council calculator rather than relying on a secondary summary.
How does custody time change the number?
Overnights are the currency. Most states count how many nights per year the child sleeps at each parent’s home, convert that to a percentage, and apply a shared-parenting adjustment once the lesser-time parent crosses a threshold.
The thresholds are specific and, until recently, brutally sharp. <cite index="44-3,44-4,44-5,44-6">Colorado used Worksheet A when a parent had 92 or fewer overnights and Worksheet B when both parents had 93 or more, and a parent with 92 overnights received no credit at all while a parent with 93 could receive a significant reduction.</cite> <cite index="42-11,42-12">A parent with 90 overnights paid the same support as a parent with zero.</cite> Virginia’s line sits at 91 days.
Here is the mechanism. The shared-parenting worksheet does not calculate one payment — it calculates two and nets them. <cite index="42-13">Colorado’s version applied a 1.5 multiplier to the total combined obligation, which is then divided between the parents.</cite> Run the earlier numbers through it, with Parent A at 35 percent of overnights:
- Combined obligation $1,095 x 1.5 = $1,642.50
- Parent A’s 62.5 percent share = $1,026.56; Parent B’s 37.5 percent share = $615.94
- Parent A owes for the 65 percent of time the child is with B: $1,026.56 x 0.65 = $667.26
- Parent B owes for the 35 percent of time the child is with A: $615.94 x 0.35 = $215.58
- Net: Parent A pays $451.68
At 10 percent of overnights under the old rule, Parent A paid the full $684.38 with no credit. Moving to 35 percent cuts roughly $233 a month, about a third, off the base order — before add-ons.
That cliff is now being legislated away in some states. <cite index="47-2,47-3">Colorado’s HB 25-1159 eliminated the 93-overnight threshold entirely as of March 1, 2026, so parenting time credit begins from the first overnight and scales proportionally.</cite> <cite index="42-6,42-8">The same bill also removed the $250 per child per year threshold for extraordinary medical expenses and expanded the definition.</cite> Expect more states to follow, since the cliff was a documented driver of custody litigation. Attorney commentary on the 2025 Colorado changes covers the reasoning.
Common questions about the calculation
Gross or net income? It depends entirely on the state, and this is the single most common source of wrong answers from online calculators. Georgia and Missouri schedules run on adjusted gross income; Washington and Illinois run on net; Texas uses "net resources," a statutory construct that allows only specific deductions. <cite index="13-4">Texas net resources include most income sources with only limited deductions allowed.</cite> None of these are the same as your take-home pay after 401(k) contributions and discretionary withholding.
Can the guideline number be changed? Courts can deviate, but the guideline figure carries a presumption of correctness and a judge who departs from it generally has to put the reason in writing. <cite index="37-15,37-16">California courts have authority to move away from guideline support when there is a strong reason, even though the formula is technically mandatory.</cite> Documented extraordinary expenses, a child’s special needs, and unusual income structures are typical grounds. Discussing whether your facts support a deviation is a conversation for a family law attorney in your state.
Does a new spouse’s income count? Generally not — the calculation looks at the two legal parents. What does change the math is the payer’s own additional children. <cite index="19-9">In Texas, an obligor with one child in the current case who already supports one child in another household pays 17.5 percent instead of 20 percent.</cite>
How often can it be recalculated? Federal rules set the floor. <cite index="49-7">States must provide notice at least once every three years to parents subject to an order, informing them of their right to request a review and, if appropriate, an adjustment.</cite> <cite index="49-6">A request outside the three-year cycle requires the requesting party to demonstrate a substantial change in circumstances.</cite> Notice of the right to a review is not the same thing as an automatic increase or decrease.
What about disputes over smaller shared costs? Unreimbursed medical bills, sports fees, and similar items are usually allocated inside the support order itself and enforced through family court rather than through the small claims court process used for ordinary debts between individuals.
A practical rule for walking into mediation: if you live in an income shares state, the two numbers worth arguing about are your parenting-time percentage and the add-on column, because those move the result far more than a few hundred dollars of disputed income. If you live in a percentage of income state, the entire fight is over what counts as your net resources, because the percentage itself is fixed by statute. Pull your state’s official guideline worksheet, fill in both scenarios, and bring the printout.