California Supplemental Tax Regulations for Amazon RSUs
California Franchise Tax Board (FTB) mandates a 10.23% flat supplemental wage withholding rate for stock options and RSU vests under Cal. Rev. & Tax. Code § 18663. However, California enforces a steeply progressive bracket system reaching 11.3% above $418,961, 12.3% above $698,271, and 13.3% above $1,000,000 (via the 1% Mental Health Services Tax), plus an uncapped 1.1% California State Disability Insurance (CASDI) payroll tax on all W-2 wages and RSU vests.
California Bracket Architecture: 9 progressive brackets from 1.0% to 12.3%, plus a 1.0% Mental Health Services surtax over $1,000,000 (13.3% top income rate) and 1.1% uncapped SDI payroll tax.
California Safe Harbor & Estimated Tax Vouchers (FTB Form 540-ES (Estimated Tax for Individuals))
California Form 540-ES follows an uneven 30% (Q1), 40% (Q2), 0% (Q3), and 30% (Q4) payment schedule. Critically, once California AGI reaches $1,000,000, the prior-year safe harbor (110%) is completely revoked—forcing taxpayers to remit at least 90% of current-year California tax liability.
Multi-State Relocation & Nonresident Sourcing Rules in California
Under the California FTB Equity-Based Compensation Guidelines (FTB Pub. 1004), if you work in California for part of the RSU vesting period and relocate to a zero-tax state (such as WA or TX) before the vest date, California still taxes the fraction of the RSU vest corresponding to California workdays divided by total workdays from grant date to vest date.
Employer Stock Plan Specifics: Amazon (Morgan Stanley at Work (Shareworks) / Fidelity)
Amazon equity packages are heavily backloaded into years 3 and 4 (40% of initial grant each year), bridged in years 1 and 2 by monthly prorated cash sign-on bonuses. While cash sign-on bonuses are often withheld via the aggregate payroll method or flat 22% supplemental rate, transitioning to Year 3 replaces monthly cash with lump-sum May and November RSU vests. When $80,000–$200,000+ of AMZN vests in a single tranche, Morgan Stanley or Fidelity withholds only 22% federal tax even though your top marginal rate is 32%, 35%, or 37%.
Withholding Election Guidance: Inside the Amazon Stock Portal (via Morgan Stanley at Work or Fidelity NetBenefits), navigate to Tax Withholding Elections prior to the open trading window preceding your May 15 or November 15 vest. If your plan permits supplemental rate overrides, increase federal withholding from 22% to 32% or 35%. Note that withholding rate changes cannot be modified once an insider trading blackout window closes or within 10 business days of a scheduled release.
2026 Statutory Parameters Quick Reference
2026 Federal & California Statutory RSU Tax Parameters (AMZN)
| Statutory Parameter | 2026 Rate / Rule |
| Federal Supplemental Withholding Rate | 22.00% (≤ $1M) • 37.00% (> $1M) |
| California Supplemental Withholding | 10.23% |
| California Top Marginal Tax Bracket | 13.30% |
| California Estimated Tax Voucher | FTB Form 540-ES (Estimated Tax for Individuals) |
| 2026 Social Security Limit | $184,500 (6.2% FICA Cap) |
| Medicare & Additional Medicare | 1.45% Uncapped + 0.9% over $200k/$250k |
| Amazon Stock Plan Brokerage | Morgan Stanley at Work (Shareworks) / Fidelity |