A Nvidia employee with $300,000 in RSUs vesting this year watches her employer withhold $66,000 in federal tax, 22% of the vest value. Her actual federal bill at the 35% marginal bracket is $105,000. The gap is $39,000. California withholds separately at 10.23%, covering $30,690. Her actual California bill is $36,900. The combined under-withholding on one $300,000 vest: over $49,000. She discovers this in April, after the shares have been sold, spent, or reinvested, when the options for avoiding penalties have narrowed significantly.
This is not a payroll error. Both the IRS and the California Franchise Tax Board require employers to apply mandatory flat rates to what the tax code calls supplemental wages, the category that covers RSUs, bonuses, and commissions. Those flat rates are designed for moderate earners. For California tech employees with combined income above $200,000, the rates consistently under-collect.
What is the California RSU tax gap?
The California RSU tax gap is the difference between the flat supplemental withholding rates an employer applies when RSUs vest and the employee’s actual marginal tax rates. Federal law requires employers to withhold at 22% on RSU income below $1,000,000 per employer per year, regardless of the employee’s true bracket. California requires employers to withhold at 10.23%. For high-income California tech employees, these flat rates leave $40,000 to $100,000 or more uncollected annually.
Who Has This Problem
This gap does not affect every employee equally. It concentrates among:
Single filers whose combined salary and RSU vests exceed $100,000 annually, the point at which the 22% federal supplemental rate begins under-withholding meaningfully.
Dual-income households where both spouses earn W-2 income. Each employer withholds based only on the wages it pays, with no knowledge of the other spouse’s income. The household’s true marginal rate on combined income can be 10 to 15 percentage points above what either employer’s withholding assumes.
Employees at Nvidia, Apple, Google, Meta, Tesla, Databricks, Anthropic, SpaceX, Amazon, Airbnb, Salesforce, AMD, Microsoft, Cisco, Robinhood, DoorDash, and Kraken. The gap appears at every company on this list because it is structural, a function of tax law, not payroll, for employees who have not elected a higher supplemental withholding rate.
Why Employers Withhold Less Than You Owe
The Federal Gap
Supplemental wages, including RSUs, bonuses, and commissions, are withheld at a mandatory flat rate of 22% on amounts up to $1,000,000 per employer per year, jumping to 37% only on amounts above that threshold. The 22% rate corresponds to the 22% federal tax bracket, which applies to single filers with taxable income between $50,401 and $105,700 in 2026. Any salary above $105,700 puts you in the 24%, 32%, or 35% bracket before a single RSU share vests. When those shares vest, every dollar is taxed at your actual marginal rate, not 22%.
The gap at each bracket level:
| Your Marginal Rate | Employer Withholds | Shortfall per Dollar | On $300K Vest |
| 24% | 22% | 2% | $6,000 |
| 32% | 22% | 10% | $30,000 |
| 35% | 22% | 13% | $39,000 |
| 37% | 22% | 15% | $45,000 |
The California Gap
California mandates a 10.23% flat supplemental withholding rate on RSU vests regardless of income. California’s brackets reach 11.3% at $66,296, 12.3% at $338,639, and 13.3% above $1,000,000 (the 12.3% bracket plus the 1% Mental Health Services Tax). An employee whose combined income exceeds $338,639 is under-withheld at the California level. An employee crossing $1,000,000 faces a 3.07% California gap, $15,350 owed beyond the 10.23% withheld on every $500,000 in RSU income.
A Note on Sell-to-Cover
When RSUs vest, your employer automatically sells enough shares to fund the statutory withholding obligation. This is called sell-to-cover. It satisfies the 22% federal and 10.23% California withholding requirement. It does not satisfy your actual tax liability. Sell-to-cover handles the employer’s legal obligation; the remaining gap is yours.
The Married Filing Jointly Difference
Federal bracket thresholds for MFJ filers are roughly double the single filer thresholds. The 35% bracket begins at $512,451 MFJ versus $256,226 single in 2026. A married employee filing jointly with a non-working spouse hits a lower marginal rate than a single filer at the same income, which narrows the gap slightly.
The gap compounds when both spouses have W-2 income. A household with $200,000 in Spouse A’s salary and $250,000 in Spouse B’s salary plus $300,000 in Spouse B’s RSU vests has $750,000 in combined income, well into the 35% federal bracket. Neither employer accounts for the other spouse’s income. The household under-withholding can exceed $80,000 in a single year.
California’s community property rules treat RSU income earned during marriage as belonging equally to both spouses for California tax purposes, regardless of whose name is on the grant. This affects the California return filing but does not change how employers withhold.
The SDI Layer
California State Disability Insurance applies to all RSU income at 1.3% in 2026 with no wage cap, a rule in place since January 1, 2024. On a $300,000 vest, SDI adds $3,900 in California tax. It is withheld separately from the 10.23% supplemental rate and is routinely omitted from year-end tax projections.
The Four Situations Where the Gap Is Largest
Large cliff vests
An employee with a one-year cliff receives all shares from that tranche in a single quarter. A $500,000 cliff vest concentrates the full under-withholding in one payroll event.
Dual-income households
Each employer withholds based only on its own wages. A dual-income tech household where both spouses receive RSUs is among the most systematically under-withheld situations in California.
Income near the $1,000,000 threshold
California’s Mental Health Services Tax creates a step increase at $1,000,000. An employee whose salary is $800,000 and who vests $400,000 in RSUs crosses into the 13.3% bracket on $200,000 of that vest income. The 10.23% withholding rate does not adjust for this crossing.
Year-end vests after the Social Security wage base is met
Social Security tax applies at 6.2% on the first $184,500 in combined wages in 2026. Once annual salary has cleared the $184,500 Social Security wage base, RSU vests later in the year owe no additional Social Security tax, but employers may model withholding as if it still applied, temporarily masking the income tax under-withholding on the same check.
The Cost Basis Problem That Doubles the Damage
A second problem emerges when you sell your vested shares. Many brokers report $0 or an incorrect cost basis on Form 1099-B. Your correct cost basis is the fair market value of the shares on the vest date, the same amount already included as W-2 income. When the basis is wrong, tax software calculates a capital gain equal to the full sale proceeds, generating a second tax charge on income already fully taxed at vesting.
ALERT: RSUs do not generate Form 3922. That form applies exclusively to ESPP purchases under IRC §423. For RSU cost basis documentation, use your brokerage’s Stock Plan Transaction Supplement, Vesting Confirmation, or Confirmation of Release, the document from Schwab Equity Awards, Fidelity NetBenefits, Morgan Stanley at Work, or E*TRADE that records the vest-date fair market value. If your tax software or preparer references Form 3922 for RSU basis, that is an error.
On a $200,000 sale of RSU shares with $0 reported basis and $200,000 of correct basis, the uncorrected error costs approximately $66,600 in combined federal and California tax on income already fully taxed. The correction requires Form 8949. Prior-year errors can be recovered through a Form 1040-X amended federal return and Form 540X for California. The three-year statute of limitations on refund claims means a 2023 error can be corrected through approximately April 2027.
Some brokers do report the correct basis, this error is common, not universal. If your Form 1099-B shows a basis that matches the vest-date FMV shown in your vesting documentation, no adjustment is needed.
How to Close the Gap Before April
Adjust your W-4. Request additional withholding from your employer on Form W-4, Line 4(c). Divide the projected annual shortfall by remaining pay periods and withhold that amount from each paycheck. Update after every material income event.
Make quarterly estimated payments. Pay the shortfall directly to the IRS and California FTB quarterly. Federal due dates: April 15, June 15, September 15, January 15. California’s due dates are identical for 2026.
California’s safe harbor calculation differs from the federal rule: California requires payment of 90% of current-year tax or 100% of the prior-year California tax liability. The federal 110% rule for taxpayers with prior-year AGI above $150,000 does not apply in California. At high income levels, model both safe harbors independently. Satisfying the federal safe harbor does not guarantee California penalty avoidance. Missing a quarterly deadline triggers California underpayment penalties at 7% annually from the original due date, even if you pay in full by April 15. The penalty is avoided by satisfying the California safe harbor, 90% of current-year tax or 100% of the prior-year California liability, regardless of whether the full April balance is paid on time.
Request a higher federal supplemental rate. Some employers and equity plan platforms (Schwab Equity Awards, Fidelity NetBenefits) allow employees to elect a higher federal supplemental withholding rate than 22%. Where available, electing 35% or 37% eliminates the federal gap without quarterly payments.
Sell additional shares at vest. Beyond the shares sold in sell-to-cover, selling additional shares at vest creates cash to fund the true liability. This concentrates the stock diversification decision at the vest date, a separate but related planning consideration.
If You Are Also Considering Leaving California
California taxes RSU income attributable to work performed in California even after you move to Nevada, Texas, or another state. The taxable portion is calculated as the ratio of California workdays to total workdays during the vesting period. RSUs that began vesting while you were a California resident carry a California tax obligation that follows you to your new state. See our guide to California exit tax planning for the full analysis of what the FTB requires and what it will find.
Two Scenarios in Practice
Scenario A: The Senior Engineer
Salary: $180,000. Quarterly RSU vests: $75,000 each, $300,000 annually. Total income: $480,000. Federal marginal rate on RSU income: 35% (income above $256,226 single filer threshold). California marginal rate: 12.3%.
Federal withheld at 22%: $66,000
Federal owed at 35%: $105,000
Federal gap: $39,000
California withheld at 10.23%: $30,690
California owed at 12.3%: $36,900
California gap: $6,210
SDI at 1.3%: $3,900
Combined annual gap: ~$49,110
Correct action: Increase W-4 additional withholding by approximately $1,200 per pay period on a semi-monthly payroll, or make quarterly estimated payments of approximately $12,278 combined to the IRS and FTB.
Scenario B: The Tech Executive
Salary: $450,000. Annual RSU vest: $600,000. Total income: $1,050,000. Federal marginal rate: 37%. California marginal rate: 13.3%, income crosses the $1,000,000 MHST threshold.
Federal withheld at 22%: $132,000
Federal owed at 37%: $222,000
Federal gap: $90,000
California withheld at 10.23%: $61,380
California owed at 13.3%: $79,800
California gap: $18,420
Combined gap before SDI and Medicare surtax: ~$108,420
Correct action: Combine a W-4 adjustment with quarterly estimated payments. Ignoring this gap until April means a $108,420 tax bill plus California underpayment penalties at 7% annually from each quarterly due date.
What Capital Tax Does
Capital Tax models the combined federal and California withholding gap for each client’s specific vesting schedule, income level, and filing status, then designs a W-4 adjustment or estimated payment calendar to eliminate the April shortfall before the first vest date of the year. We recalculate after every material income event: a promotion, a new grant, an accelerated vest, or a company acquisition.
We also reconcile each client’s RSU vesting history against their Form 1099-B reported basis at year-end. Where brokers have reported incorrect basis, we correct Form 8949 using the vest-date fair market value from the brokerage’s vesting documentation and file amended returns for prior years where the error occurred.
We serve RSU recipients at Nvidia, Apple, Google, Meta, Tesla, Databricks, Anthropic, SpaceX, Amazon, Airbnb, Salesforce, AMD, Microsoft, Cisco, Robinhood, DoorDash, and Kraken. The withholding gap appears at every company on this list because it is structural, not company-specific. Our equity and high-net-worth tax planning services support executives and households managing RSUs, concentrated stock, and other complex equity compensation.
Begin Your California RSU Tax Assessment
If your combined salary and RSU income exceeds $200,000 in 2026 and you have a vest date in the next 90 days, calculate your specific withholding exposure before that date. Begin your California RSU Tax Assessment at capitaltax.com/tax-planning-for-executives.
Do not wait until April. The penalty clock starts at each quarterly estimated payment deadline, not at filing.
Frequently Asked Questions About California RSU Tax
Are RSUs taxable when they vest?
Generally, yes. The fair market value of RSUs is usually treated as wage income when the shares vest and are delivered, subject to the applicable plan terms and tax rules.
Why can I owe additional taxes after RSUs vest?
Employer withholding may not cover your final tax liability. Total income, filing status, other compensation, and applicable federal and state tax rules determine the final amount.
Does sell-to-cover pay all my RSU taxes?
Not necessarily. Sell-to-cover generally funds required withholding, but the amount withheld may be lower than the tax ultimately owed on your return.
Can RSU sales result in double taxation?
Incorrect cost basis reporting can make it appear that a larger capital gain occurred than actually did. Reviewing the vest-date value and making any required reporting adjustments can help prevent this error.
Can California tax RSUs after I move to another state?
Potentially. California may tax compensation attributable to services performed in the state, depending on the circumstances and applicable sourcing rules. Residency changes and equity grant details should be reviewed carefully.
How can I reduce the risk of an RSU tax shortfall?
Review projected annual income, check federal and California withholding, consider additional withholding or estimated payments where appropriate, and reconcile equity compensation records before filing.