California taxes RSU income at withholding rates structurally lower than actual marginal rates. The federal 22% supplemental plus California 10.23% supplemental adds to roughly 32% withholding. A high earner’s actual combined marginal rate, with SDI, MHST, and additional Medicare, can run closer to 50%. The shortfall hides in plain sight until April.
A client (illustrative, not real) had a $500,000 RSU vest in late 2024. The shares were sold automatically to cover taxes, as is standard. The brokerage statement showed taxes withheld. The W-2 in January reflected the full vest amount as income. The client filed his return in April expecting a small refund.
He owed an additional $77,000.
Not because anything was wrong with his return. Because the standard withholding on RSU income in California — federal supplemental rate of 22%, California supplemental rate of 10.23% — adds up to roughly 32% withholding on someone whose actual combined marginal rate, with all the layered California-specific taxes, runs closer to 50%.
For California RSU recipients in tech, biotech, finance, or any equity-compensated profession, this is one of the most consistent and most expensive surprises in the tax year.
Why the withholding doesn’t match what you owe
When an RSU vests, the IRS treats the value as ordinary wage income. Your employer is required to withhold federal and state income tax on that income — but at the supplemental wage rate, not your actual marginal rate.
Federal supplemental withholding is 22% on RSU income below $1 million per year (37% above). For someone whose actual federal marginal rate is 32%, 35%, or 37%, the 22% withholding is structurally short by 10–15 percentage points.
California supplemental withholding is 10.23%. For California earners actually in the 9.3%, 10.3%, 11.3%, 12.3%, or 13.3% bracket, the 10.23% withholding may approximate the actual rate at moderate incomes — but it doesn’t account for the additional California-specific taxes that stack on top.
That gap between withholding rate and actual marginal rate is the source of the surprise tax bill. And for high earners, the gap compounds with several other California-specific factors most withholding calculations don’t surface.
The California layers most people don’t see
The SDI wage cap was removed in 2024. California previously capped State Disability Insurance contributions at a specific wage threshold. Starting in 2024, the cap was eliminated entirely. SDI now applies at 0.9% on every dollar of wage income, with no ceiling. For someone with $1 million in RSU income, that’s $9,000 in SDI alone — and it’s typically not modeled in standard withholding projections.
The Mental Health Services Tax (MHST) adds 1% above $1 million. Combined wage and RSU income above $1 million triggers a 1% surcharge on income over the threshold. For executives or founders with seven-figure vest years, this is real money.
The 3.8% Net Investment Income Tax (NIIT) doesn’t apply to RSU wages directly — but it applies to the capital gains on shares held past vesting and later sold at a higher price. For RSU recipients who hold their vested shares, NIIT becomes part of the eventual tax picture.
California does not conform to federal Qualified Small Business Stock (QSBS) exclusions. This is the tactical wrinkle that catches even sophisticated founders and early employees. Federal law (under OBBBA) excludes a significant portion of QSBS gains from federal capital gains tax. California taxes those same gains at up to 13.3%. For someone whose federal QSBS exclusion eliminates $10 million of federal tax, the California portion can still run $1.3 million or more. (See the earlier post in this series on QSBS for the federal mechanics.)
What the gap looks like in practice
Consider an illustrative tech employee with $500,000 of RSU income in 2026, on top of a $250,000 salary. Total wage income: $750,000.
What’s withheld on the $500,000 RSU portion:
• Federal supplemental: 22% → $110,000.
• California supplemental: 10.23% → $51,150.
• Medicare: 1.45% → $7,250.
• Total withheld on RSU income: approximately $168,400.
What’s actually owed on the $500,000 RSU portion (at the household’s marginal rates):
• Federal marginal: 35% → $175,000.
• California marginal: 12.3% → $61,500.
• Additional Medicare (0.9% over $250K): $4,500.
• SDI (0.9% uncapped): $4,500.
• Total owed on RSU income: approximately $245,500.
Shortfall: roughly $77,000 — owed at tax filing, often with underpayment penalties if the household didn’t make estimated tax payments to close the gap.
This is for someone well under $1 million of total income. For founders or executives crossing the $1 million threshold, MHST adds 1% to the gap. For households with NIIT exposure on retained shares, the gap grows further.
The moving-while-vesting trap
For California residents considering a move to Texas, Nevada, Florida, or any other state, the tax-exit decision interacts with RSU timing in ways that often surprise people.
California uses a workday-allocation formula for RSU income when the recipient relocates mid-vesting. The formula is straightforward: (California workdays during the vesting period ÷ total vesting workdays) × vest value = California-taxable income.
A four-year RSU grant where you worked in California for the first two years and Texas for the next two will have 50% of each vest allocated to California — even after you’ve moved. You’ll file a California non-resident return (Form 540NR) and owe California tax on the allocated portion.
For executives with large pre-IPO grants who relocate before a liquidity event, the timing matters significantly. Moving before a grant date eliminates California’s claim on that specific grant. Moving during a vesting period only reduces California’s claim proportionally. The planning math is specific to each grant’s vesting schedule.
What to do
Estimated tax payments to close the withholding gap. The simplest fix. Calculate your projected annual liability, subtract employer withholding, and pay the difference through quarterly estimated payments (Form 1040-ES federal, Form 540-ES California). This avoids underpayment penalties and the April shock.
Additional voluntary withholding. Most employers allow you to request additional federal and state withholding via your W-4 and DE-4 forms. Setting this higher than the supplemental defaults closes the gap automatically — no quarterly check-writing required.
Sell-to-cover at the actual rate. Some employers and brokerages allow you to elect a higher sell-to-cover percentage at vest — selling enough shares to cover the actual marginal rate rather than the supplemental rate. Worth confirming with your equity administrator.
Roth conversion suppression in heavy RSU years. If you’re doing multi-year Roth conversions (covered in an earlier post in this series), large RSU years are typically wrong years to convert. The combined ordinary income from RSU vesting and conversion can push you into higher brackets, trigger NIIT, push you toward IRMAA two years later (covered in the prior post on the IRMAA cliff), and create cumulative tax inefficiency.
Relocation timing if planning a California exit. For executives with significant unvested RSU grants who are considering a move, the timing relative to grant and vest dates determines how much income remains California-taxable. The decision is rarely just “when do I want to move” — it’s also “what state allocation am I willing to accept on the equity I haven’t yet earned.”
The pattern
The recurring theme of this series: the work that determines whether a plan succeeds happens before the moment the plan is supposed to deliver. Estate planning isn’t the document. Roth conversion isn’t the conversion. SALT cap isn’t the headline. Business value isn’t the gap. IRMAA isn’t the bill.
RSU planning fits the same pattern. The W-2 in January is the visible event. The actual tax position is what gets settled in April — and what could have been planned for in October. The households that pay the least in surprise taxes are the ones modeling the real numbers, not the withheld numbers.
California withholds 10.23% on your RSUs. Your actual rate is usually higher. The shortfall hides until April.
The plan is the residue. The planning is the work.
Key takeaways
• The federal supplemental withholding (22%) and California supplemental withholding (10.23%) on RSUs typically under-withhold for high-earning households. The combined withholding rate is approximately 32%, while the actual combined marginal rate can run 50% or higher.
• California removed the SDI wage cap in 2024 — 0.9% now applies to all wage income with no ceiling. For RSU recipients with significant vesting income, this adds thousands per year in unmodeled tax.
• The 1% Mental Health Services Tax (MHST) applies above $1 million of California taxable income. For executives, founders, and senior tech workers crossing the threshold, this layer compounds the under-withholding gap.
• California does not conform to federal QSBS exclusions. Even when federal QSBS qualifies for partial or full federal capital gains exclusion, California still taxes the gain at up to 13.3%.
• Relocating mid-vesting doesn’t eliminate California’s claim on RSUs. California uses a workday-allocation formula that proportionally taxes vests based on time worked in the state during the vesting period.
• The planning fixes are operational: additional voluntary withholding, quarterly estimated tax payments, coordinated Roth conversion timing, and relocation planning that accounts for unvested equity allocation.
Common questions about California RSU taxation
How are RSUs taxed in California?
When RSUs vest in California, the vest value is taxed as ordinary wage income at both the federal and California state level. Federal employers withhold at the 22% supplemental wage rate on RSU income below $1 million (37% above). California withholds at the 10.23% supplemental rate regardless of income level. Additional layers — uncapped SDI at 0.9%, additional Medicare at 0.9% over $250K, the Mental Health Services Tax at 1% over $1 million, and federal NIIT at 3.8% on retained-share gains — stack on top. For high earners, the actual combined marginal rate on RSU income can run 50% or higher, while withholding typically lands closer to 32%.
Why is my RSU withholding not enough?
The federal supplemental withholding rate (22% under $1 million) and California supplemental rate (10.23%) are both structurally lower than the actual marginal rates of high-earning households. A household whose actual federal marginal rate is 35% gets under-withheld by 13 percentage points on RSU income. The California supplemental rate may approximate moderate-income California rates but doesn’t account for SDI, MHST, additional Medicare, or NIIT. The gap is settled at tax filing — often with underpayment penalties if estimated tax payments weren’t made to close the gap.
Does California tax QSBS gains?
Yes. California does not conform to federal Qualified Small Business Stock (QSBS) exclusions. While federal law allows a significant exclusion of QSBS capital gains (recently expanded under OBBBA), California taxes those same gains at up to 13.3% as ordinary capital gain. For founders and early employees expecting a federal QSBS exclusion to substantially reduce their tax on a sale, the California layer often runs into the seven figures and is one of the most consistent surprises in the QSBS planning conversation.
What is the California SDI wage cap?
California removed the State Disability Insurance (SDI) wage cap in 2024. Previously, SDI contributions were capped at a specific wage threshold. Starting in 2024, the cap was eliminated entirely. SDI now applies at 0.9% on every dollar of wage income, with no ceiling. For RSU recipients with significant vest income, this adds substantial unmodeled tax — $9,000 on $1 million of wage income, scaling proportionally.
What is the Mental Health Services Tax in California?
The Mental Health Services Tax (MHST), enacted under Proposition 63 in 2004, imposes a 1% surcharge on California taxable income above $1 million. Combined with California’s top marginal income tax rate of 12.3%, MHST brings the effective top rate to 13.3% — among the highest state income tax rates in the country. The threshold is not indexed for inflation, so more taxpayers cross it over time. For founders, executives, and equity-compensated professionals with seven-figure vest years, MHST is a meaningful layer to plan for.
How does relocating from California affect RSU taxation?
For California residents who relocate mid-vesting (to Texas, Nevada, Florida, or any other state), California uses a workday-allocation formula to determine how much of each subsequent vest remains California-taxable. The formula is: (California workdays during the vesting period ÷ total vesting workdays) × vest value = California taxable income. A four-year RSU grant where you worked in California for two years and Texas for two years will have 50% of each vest allocated to California — even after you’ve moved. You’ll file a California non-resident return (Form 540NR) and owe California tax on the allocated portion. Moving before a grant date eliminates California’s claim on that specific grant; moving during a vesting period reduces California’s claim proportionally.
This article is for informational and educational purposes only and is not intended as tax, legal, or financial planning advice. Tax rates, thresholds, and withholding rules can change. Confirm current rates with the IRS, the California Franchise Tax Board, and the California Employment Development Department at the time of any planning decision. Consult your CPA, tax advisor, and financial advisor about your specific situation.
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