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Child support guide

How child support is calculated — Income Shares, Percentage of Income, and the role of custody.

Child support is one of the most Googled legal topics in the U.S., yet the actual calculation formulas are buried in state statutes and law firm websites that require a contact form to get any real information. This guide explains the mechanics clearly.

The two main models

U.S. states use two primary formulas for calculating basic child support obligations:

Income Shares Model (~41 states). This model — used in California, New York, Florida, Ohio, and most others — combines both parents' gross incomes and consults a state-published schedule to determine the total amount a family at that income level would typically spend on a child. That total obligation is then split between the parents in proportion to their individual incomes. The custodial parent is presumed to spend their share directly. The non-custodial parent pays their share as a monthly transfer.

Percentage of Income Model (~9 states). Texas, Wisconsin, Alaska, and a handful of others apply a fixed percentage of the non-custodial parent's net or gross income, ignoring the other parent's earnings entirely. Texas net income percentages: 20% for 1 child, 25% for 2, 30% for 3, 35% for 4, and 40% for 5 or more. Simple, but widely criticized for creating perverse incentives and ignoring the custodial parent's ability to contribute.

How overnight stays change the calculation

Under the Income Shares Model, the basic obligation assumes the non-custodial parent has limited overnights. When the non-custodial parent has substantial parenting time — typically 40% or more in most states — courts apply a parenting time adjustment that reduces the cash payment. The rationale: when a parent has the child, they are directly spending on food, housing, transportation, and clothing. A cash transfer that doesn't account for this double-counts the obligation.

The adjustment formula varies by state. California uses a proportional reduction based on actual time percentages. Colorado, Georgia, and others use tiered adjustments above a 40% threshold. Some states (like New York) use a straight income shares model without a formal parenting time adjustment, instead relying on judicial discretion for custody deviations.

What “gross income” means here

Most states define gross income broadly for child support purposes. It includes wages, salary, bonuses, commissions, overtime (frequently averaged over multiple years), self-employment income net of reasonable business expenses, rental income, investment returns, and Social Security or disability benefits. Notably, it does not include a new spouse or partner's income in most states — only the two biological or legal parents' incomes are considered.

Courts also impute income: if a parent is voluntarily unemployed or underemployed, the court may calculate support based on what that parent could earn at their skill level rather than what they actually earn.

Add-ons beyond the basic obligation

The number shown by this calculator is only the basic child support obligation. Courts routinely add to this figure:

• Health insurance — the cost of adding the child to one parent's employer policy is allocated between parents by income share.
• Unreimbursed medical expenses — costs above a threshold (typically $250/year) are split by income share.
• Childcare costs — work-related daycare or after- school care is added to the basic obligation.
• Educational expenses — private school tuition, tutoring, or college contributions in some states.

In practice, add-ons often increase the effective child support obligation by 20–40% above the basic figure.

Modifying an existing order

Child support orders are not permanent. They can be modified by court petition when there is a substantial change in circumstances — typically defined as a change that would alter the support amount by at least 15% (varies by state). Common triggers include job loss, significant income change, change in custody arrangement, new major expenses for the child, or the child reaching majority or emancipation. Informal agreements between parents to pay a different amount are not legally enforceable and do not modify the court order — the legal obligation remains until a judge signs a modified order.