California doesn’t use a simple percentage or a lookup table on its own — it runs both parents’ numbers through an algebraic formula written directly into state law. Here’s what goes into it.
California’s Guideline Formula
Family Code §4055 sets California’s statewide guideline formula:
CS = K [HN – (H%)(TN)]
In plain language, the formula estimates how much of the higher earner’s income should go toward the child, then adjusts for how much time that parent already spends with them.
- CS — the guideline child support amount
- K — a factor based on both parents’ combined income and the higher earner’s parenting time
- HN — the higher earner’s net monthly disposable income
- H% — the higher earner’s percentage of time with the child
- TN — both parents’ combined net monthly disposable income
Net Disposable Income, Not Gross
Courts start with gross income, then subtract state and federal taxes, mandatory union dues, mandatory retirement contributions, health insurance premiums, and other required deductions to reach each parent’s “net disposable income.” Effective September 1, 2025, Senate Bill 343 updated the formula’s K-factor and income bands to run on net income and reflect current costs — the first change of this kind since the guideline was adopted in 1992.
Parenting Time Is Built Into the Math
Unlike states that only nudge the number for shared custody, California builds the parenting-time percentage directly into the formula through the H% term — every additional overnight changes the result.
When Courts Can Deviate
The formula’s result is presumed correct, but Family Code §4057(b) lets a judge adjust it for situations such as:
- A written agreement between the parents that the court finds fair
- Extraordinarily high income where the guideline amount would exceed the child’s reasonable needs
- A parent’s income near the low-income adjustment threshold
- Custody or timeshare arrangements the standard formula doesn’t capture well
Estimate Your California Child Support
California’s certified DCSS Guideline Calculator applies the exact §4055 formula courts use. For a faster ballpark figure before working through the full worksheet, try the Child Support Calculator.
Official Resources
- California Department of Child Support Services
- California Family Code §4055 (Guideline Formula)
- Child Support Calculator (childpaycalc.com)
This guide is for general information only and is not legal advice. Formulas, percentages, and dollar caps change over time and courts can deviate from the guideline result — confirm current figures with the official resources above or a family law attorney before relying on any number.