Methodology

This calculator implements California's statewide uniform child-support guideline as defined in California Family Code §§ 4055 & 4059, operative September 1, 2024. The formula itself is CS = K[HN − (H%)·TN], with a per-child multiplier for multiple children per §4055(b)(4). See the glossary for definitions of HN, H%, TN, K, and every other term used throughout the calculator.

What we include

• Net disposable income (§4059): gross income minus federal tax liability, California state tax liability, FICA + Medicare + Additional Medicare, California SDI, and mandatory deductions for union dues, retirement, health insurance, other support paid, and job-related expenses.
• Federal tax (TY 2025 + 2026): statutory brackets from IRS Rev. Proc. 2024-40 (2025) and the 2026 revenue procedure, post-OBBBA (PL 119-21) standard deductions, CTC with refundable ACTC cap, Earned Income Credit, and Alternative Minimum Tax (§55).
• OBBBA 2025–2028 deductions: qualifying tips, qualifying overtime, senior bonus ($6,000 age 65+), and auto-loan interest — with MAGI-based phase-outs per PL 119-21.
• Self-employment tax (§1401): 92.35% SE base × (12.4% OASDI to the wage base + 2.9% Medicare + 0.9% Additional Medicare above threshold), with the ½-SE-tax deduction above-the-line per §164(f).
• Net Investment Income Tax (§1411): 3.8% surtax on net investment income above filing-status MAGI thresholds.
• Capital gains & qualified dividends: stacked-rate calculation (0% / 15% / 20%) for long-term gains and qualified dividends; short-term gains taxed as ordinary.
• California tax (TY 2025 + 2026): Schedules X/Y/Z from FTB Tax Rate Schedules, indexed standard deduction and personal exemption credit, CA itemized deduction (R&TC §17072 — mortgage + property tax + charity + medical, no SALT cap), renter's credit (opt-in per R&TC §17053.5, phased at AGI), and Behavioral Health Services Tax on income over $1M.
• California refundable credits: CalEITC, Young Child Tax Credit (YCTC), and Foster Youth Tax Credit (FYTC). TY 2026 uses 2025 parameters verbatim per FTB-540-ES instructions, pending the 2026 540 booklet.
• FICA (TY 2025 + 2026): 6.2% SS on wages up to the annual SSA Contribution Base, 1.45% Medicare on all wages, 0.9% Additional Medicare above filing-status thresholds.
• California SDI: 1.2% on all wages for TY 2025; 1.3% for TY 2026 (no cap since SB 951 removed the ceiling effective 2024) per EDD.
• §4055(b)(7) Low-Income Adjustment: rebuttable presumption applies when the monthly net of the parent paying support is below the full-time California minimum-wage gross.
• §4062 add-ons: mandatory (childcare for work, uninsured medical) and discretionary (education, travel) add-ons are allocated between parents by income share per §4061(b).
• §4070–§4073 hardship deductions: extraordinary health expenses, minimum basic living expenses, and natural/adopted children from other relationships — with §4071(b) "hardship for other children" capped by the hypothetical-guideline amount (matching the AOC methodology).

Income is taken as provided. The calculator does not impute earning capacity or apply voluntary-unemployment/underemployment adjustments — those are §4058(b) judicial determinations, not guideline computations. Entering $0 for a parent computes the guideline as if that parent has no §4058 gross income.

Current limitations

• TY 2026 CA tax parameters use 2025 values verbatim where FTB has not yet published 2026 figures (brackets, standard deduction, exemption credit, CalEITC / YCTC / FYTC). This matches FTB Form 540-ES instructions for 2026 estimators. SDI is updated to the EDD-published 2026 rate (1.3%). Values will be replaced with 2026-final numbers when the FTB Form 540 booklet publishes.
• QBI (§199A) above-threshold W-2 / UBIA limit uses a conservative default of 0.5 × W-2 wages with no UBIA. The simplified 20%-of-QBI deduction (the common case for sole-proprietor filers below the §199A taxable-income threshold) is modeled in full. SSTB phase-out is now exposed in the wizard (advanced step); UBIA is not, and the no-UBIA branch is conservative for SE filers without depreciable business property.

Validation

Every calculation component is unit-tested against the authoritative source's own published values (IRS cumulative "Tax Is" column, FTB cumulative schedules, SSA contribution-and-benefit base, EDD SDI tables). End-to-end scenarios are compared against the California DCSS Guideline Calculator via a Playwright-driven scraping harness; 191 parametric scenarios spanning baseline cartesian (children × incomes × timeshares), HoH custodial filing, self-employment, itemized deductions, aged/blind filers, mixed-source income, and statutory edge cases are captured as fixtures and rerun against our calculator on each build.