Child Support Formulas: Percentage of Income vs Income Shares
Guide · Child support · Reviewed September 20, 2026 · about 10 min read
Understanding how child support is calculated is a critical step for parents navigating separation or divorce. In the United States, states primarily use one of two dominant models: the Percentage-of-Income Model or the Income Shares Model. A less common but important variant, the combined-percentage model, also exists for higher income brackets in some states.
This guide explains these core methodologies, using specific examples from Alaska, Arizona, Colorado, and Florida, based on 2026 data. By the end of this guide, you will have a clearer understanding of the mechanisms behind child support calculations, including how income is defined, how the number of children impacts the amount, what income caps may apply, and the role of judicial discretion. This information can help you prepare for discussions with legal professionals, state agencies, or the other parent.
Understanding Child Support Calculation Models
Child support guidelines are established by state law to ensure that children receive financial support from both parents, reflecting the standard of living they would have enjoyed had their parents remained together. While the overarching goal is consistent, the methods states use to arrive at a support figure can differ significantly. These methods aim to provide a predictable and equitable framework for determining financial obligations, reducing the need for extensive litigation in every case.
The two primary models—Percentage of Income and Income Shares—each approach the calculation from a different philosophical and mathematical starting point. A third approach, often seen as a hybrid or variant, applies a percentage to combined parental income, particularly at higher income levels.
The Percentage-of-Income Model: Alaska's Approach
The Percentage-of-Income Model is a straightforward approach where child support is calculated as a fixed percentage of the non-custodial parent's income. This percentage typically increases with the number of children. The model focuses primarily on the income of the paying parent, with less emphasis on the custodial parent's income, though the latter may be considered for deviations.
Alaska utilizes this model, applying specific percentages to the paying parent's adjusted annual income. As outlined in Alaska Civil Rule 90.3, the percentages are:
- One Child: 20%
- Two Children: 27%
- Three Children: 33%
- Each Additional Child: An extra 3%
Alaska also implements an income cap for these calculations. The adjusted annual income subject to these percentages is capped at $138,000 per year, which translates to $11,500 per month. For incomes exceeding this cap, the guideline percentages do not automatically apply to the additional income. It is important to note that this is the primary-custody formula; shared custody, defined as each parent having 30% or more of overnights, uses a different calculation method.
The Income Shares Model: Arizona, Colorado, and Florida
The Income Shares Model is based on the principle that children should receive the same proportion of parental income as if their parents lived together in one household. This model considers the combined income of both parents, determines a basic support amount from a schedule, and then prorates that amount between the parents based on their respective shares of the combined income.
Arizona's Income Shares Calculation
Arizona employs the Income Shares Model, using the parents' Combined Adjusted Child Support Income, which is a monthly gross figure after specific guideline adjustments for spousal maintenance and support of other children. The 2022 Arizona Child Support Guidelines (effective January 1, 2022, adopted by Administrative Order 2021-56) include a Schedule of Basic Support Obligations. This schedule ranges from a combined monthly income of $750 up to $30,000, with amounts listed for up to six children. For incomes falling between rows in the schedule, the guideline states to use the income closest to the combined figure, rounding up when the income falls exactly halfway. For combined monthly incomes above $30,000, the amount for the $30,000 row is the presumptive obligation, unless a party can demonstrate that a higher amount is in the child's best interests. After determining the basic obligation, additional costs such as medical insurance premiums, child-care expenses, and any extra-education or older-child adjustments are added and split proportionally by each parent's income share. The calculation also credits the paying parent for parenting time (overnights) and includes a self-support reserve test, which is 80% of full-time minimum-wage earnings, to potentially reduce the final order.
Colorado's Income Shares Calculation
Colorado also uses the Income Shares Model, calculating support based on the parents' combined monthly adjusted gross income (gross income less pre-existing support and support for other children). The Schedule of Basic Child Support Obligations in C.R.S. § 14-10-115(7)(b), as amended by HB25-1159 for orders entered on or after March 1, 2026, covers combined monthly incomes from $50 to $40,000. For incomes between the specified rows, the statute requires interpolation to determine the precise amount. For very low incomes, if the paying parent's adjusted gross income is $650 or less, the order is a minimum of $10. Between $650 and the self-support reserve (29 hours a week at the state minimum wage), a reduced low-income amount of $50 to $150 applies. Just above the self-support reserve, a percentage-of-the-difference test can further lower the schedule figure. For combined incomes above $40,000, the court has discretion, but the presumptive amount cannot be less than the figure for the $40,000 row. Similar to Arizona, Colorado adds work-related and education-related child care, health insurance, and extraordinary medical expenses, splitting these by income share. Since March 2026, every overnight of parenting time also adjusts the obligation.
Florida's Income Shares Model with a Combined-Percentage Variant
Florida also operates under the Income Shares Model, using the parents' combined monthly net income. The basic child support obligation is determined from a schedule in Florida Statute § 61.30(6), which provides amounts for combined monthly incomes starting at $800 and extending up to $10,000. For combined monthly available income greater than $10,000, Florida employs a combined-percentage variant. The obligation is calculated as the amount for $10,000, plus a specific percentage of the income exceeding $10,000. These percentages are: 5% for one child, 7.5% for two, 9.5% for three, 11% for four, 12% for five, and 12.5% for six children. This hybrid approach ensures that higher-income families contribute a proportional amount beyond the fixed schedule. Florida also adds health insurance costs, non-covered medical expenses, and child care costs, which are then prorated between the parents. If a parent has the children for a substantial amount of time (20% or more of the overnights), the basic obligation may be adjusted.
Key Differences Between Models and the Combined-Percentage Variant
The fundamental distinction between the Percentage-of-Income and Income Shares models lies in how parental income is considered. The Percentage-of-Income Model, as seen in Alaska, primarily bases the support amount on the non-custodial parent's income alone. This can lead to simpler calculations but may not always fully reflect the combined financial resources available to the children.
Conversely, the Income Shares Model, used by Arizona and Colorado, aims for a more holistic view by considering both parents' incomes to determine a total support amount, which is then divided proportionally. This approach theoretically maintains the children's standard of living more closely to what it would have been if the parents had remained together.
Florida's system introduces a notable combined-percentage variant for higher incomes. While it starts with an Income Shares schedule, it transitions to applying a percentage to combined income above a certain threshold ($10,000 per month). This ensures that as combined income rises significantly, the support obligation continues to increase proportionally, rather than being capped at a fixed amount from a schedule or relying solely on judicial discretion for very high earners.
Income Caps and High-Income Considerations
States handle high parental incomes differently within their child support guidelines:
- Explicit Caps: Alaska's Percentage-of-Income Model includes an explicit income cap of $11,500 per month ($138,000 annually) for the paying parent's adjusted income. Income above this amount is not automatically subject to the guideline percentages.
- Schedule Limits with Discretion: Arizona's Income Shares schedule extends up to a combined monthly income of $30,000. For incomes above this, the amount from the $30,000 row is the presumptive obligation, but courts can order a higher amount if a party proves it is in the child's best interests. Similarly, Colorado's schedule goes up to $40,000 combined monthly income. Above this, courts have discretion, but the presumptive amount cannot be less than the top row figure.
- Combined-Percentage Extension: Florida's Income Shares model incorporates a unique combined-percentage variant for combined monthly incomes exceeding $10,000. Instead of a hard cap or pure discretion, a specific percentage (e.g., 5% for one child, 7.5% for two) is added to the base $10,000 amount for the income above that threshold. This provides a structured method for calculating support for higher-income families.
Court Discretion and Deviations
While child support calculations are guided by specific formulas and schedules, courts in all states retain the authority to deviate from the guideline amount if applying the formula would be unjust or inappropriate in a particular case. This means that the calculated amount is generally considered a presumptive figure, but it is not always the final order.
For instance, in Arizona, the amount derived from the schedule for combined incomes above $30,000 is presumptive, but a party can present evidence to prove that a higher amount is in the child's best interests. Colorado's guidelines similarly state that for combined incomes above $40,000, the court has discretion, though the presumptive amount cannot be less than the highest figure on the schedule. Florida law explicitly allows a court to deviate from the guidelines if the calculated amount is found to be unjust or inappropriate. Such deviations typically require specific findings by the court, detailing the reasons for departing from the standard calculation and how the adjusted amount serves the child's best interests.
Common reasons for deviations can include extraordinary medical expenses, special needs of a child, significant differences in living expenses between households, or other factors that make the guideline amount unfair. It is crucial for parents to understand that while the formulas provide a starting point, individual circumstances can influence the final child support order.
What This Guide Is Not
This guide provides general information about child support calculation models and is not a substitute for legal advice. The figures and rules presented are for illustrative purposes based on the provided 2026 data and may change. Child support calculations are complex and depend on individual circumstances, including specific income adjustments, deductions, and parenting time arrangements. Always consult the cited state statutes or official agency guidelines, or seek advice from a qualified legal professional in your state, for accurate and personalized information regarding your situation.
Frequently asked questions
What is the fundamental difference between the Percentage of Income and Income Shares models?
The Percentage of Income model calculates support as a fixed percentage of the paying parent's income, as seen in Alaska. The Income Shares model, however, estimates the amount parents would spend on children if they lived together, then divides this amount proportionally based on each parent's share of their combined income, as used by Arizona, Colorado, and Florida.
How do states handle very high or very low parental incomes?
States like Alaska apply an income cap, such as $11,500 per month, above which the percentage does not apply. Other states, like Arizona and Colorado, use a schedule up to a certain combined income (e.g., $30,000 or $40,000 monthly) and then allow judicial discretion, often with a presumptive minimum. Florida uses a combined-percentage variant for higher incomes.
Can a court order a child support amount different from the guideline calculation?
Yes, courts in all states generally retain the ability to deviate from the guideline amount if applying the formula would be unjust or inappropriate in a specific case. This requires a party to present evidence demonstrating why a different amount is in the child's best interests, as noted in the guidelines for Arizona, Colorado, and Florida.
What does 'adjusted income' mean in child support calculations?
'Adjusted income' refers to a parent's gross income after certain deductions are applied, which can vary by state. For example, Colorado defines it as gross income less pre-existing support obligations and support for other children. Alaska uses 'adjusted annual income' up to $138,000 a year.
How does shared parenting time affect child support obligations?
In states like Arizona and Colorado, the amount of time each parent spends with the children (e.g., number of overnights) can lead to adjustments in the basic child support obligation. Alaska also notes a different calculation for shared custody when each parent has 30% or more of overnights, indicating its importance in the final determination.