How is child support calculated in California?

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California   child support   is governed by the   statewide uniform guideline   in   Family Code §§ 4050-4076  , which produces a   presumptively correct   amount that courts may deviate from only in narrow circumstances.    1. The Guideline Formula (Fam. Code § 4055)   The basic formula is:  >   CS = K [HN − (H%)(TN)]    Where   CS   is child support,   K   is a fraction based on combined net income,   HN   is the higher earner's net monthly disposable income,   H%   is the higher earner's approximate timeshare with the child, and   TN   is the parties' total net monthly disposable income. Because the algebra is complex, courts and practitioners use   certified guideline calculators   —   DissoMaster   and   X-Spouse   — required by   Cal. Rules of Court 5.275  .    2. Income Shares Logic   Although technically formula-based, California is functionally an   Income Shares model  : it estimates what the parents would spend on the child if intact, then allocates that amount in proportion to each parent's income, adjusted for custodial timeshare.    3. Add-Ons (Fam. Code §§ 4061-4063)   On top of guideline support, courts add: -   Mandatory add-ons   — childcare costs for employment/training and uninsured health care (split equally absent good cause) -   Discretionary add-ons   — educational/special needs and travel for visitation    4. Duration (Fam. Code § 3901)   Support continues until the child: - Turns   18   and is no longer a full-time high school student, OR - Turns   19  , OR - Marries, dies, becomes self-supporting, or is emancipated — whichever first.  Adult disabled children may receive lifetime support under   Fam. Code § 3910  .    5. Modification   Either parent may seek modification on a   material change of circumstances   (Fam. Code § 3651). Courts commonly find a material change when guideline recomputation produces a difference of   $50 or 20%  , whichever is less (local rule of thumb; see  In re Marriage of Williams , 150 Cal. App. 4th 1221 (2007)).    6. Deviation from Guideline (Fam. Code § 4057)   Rebuttable presumption may be overcome only by specific findings — e.g., extraordinarily high earner, deferred sale of home, special-needs child.  This is legal information, not legal advice.