California RSU Tax Planning 2026

If you had $200,000 of RSUs vest at a Bay Area tech company this year, your employer almost certainly withheld federal income tax at 22%. You are probably in the 35% federal bracket. The state of California, where you live, withheld at 10.23% on supplemental wages while your actual marginal rate is closer to 13.3%. By the time you add the 0.9% Additional Medicare tax, the 3.8% Net Investment Income Tax on any investment income, and the 1.1% California mental health services surcharge, your real combined rate on that vest is north of 50%. You were withheld at roughly 32%. That gap is your problem to solve before April 15, 2027, with penalties accruing if you do not.

RSU under-withholding is the single most common surprise tax bill we see at our San Jose office. It is not a mistake by your employer. It is the IRS supplemental wage rule combined with a default California rate that simply does not contemplate the income levels common in Silicon Valley. This guide explains why the gap exists, exactly how big it gets, and the five specific moves California RSU recipients should make for the 2026 tax year. We work through these scenarios constantly with employees at our tech-employee tax practice.

Why the Default 22% Withholding Is Wrong for You

RSUs are taxed as ordinary W-2 compensation at vest, on the fair market value of the shares on the vesting date. Your employer reports this on your W-2 as wages and withholds federal and state tax. Here is the catch: the IRS treats RSU vests as supplemental wages under Treas. Reg. §31.3402(g)-1, and supplemental wages are withheld at a flat statutory rate, not at your actual marginal rate.

For 2026, the federal supplemental rate is 22% on aggregate supplemental wages under $1 million in the calendar year, and 37% on the portion above $1 million. California uses 10.23% for stock-based supplemental wages and 6.6% for other supplemental wages. These are not your tax rates. They are mechanical withholding rates that Congress set decades ago and never indexed to high-income earners.

If you are a California-resident software engineer earning $250,000 in base salary plus $300,000 in RSU vests, your actual federal marginal rate is 35% and your actual California marginal rate is 11.3% (plus the 1.1% mental health surcharge on income over $1M, which kicks in for many staff and principal engineers at the larger firms). Your effective combined rate on the RSU income alone is closer to 47-50% once you layer in Social Security wage base, Medicare, and Additional Medicare. Withholding at 22% federal + 10.23% state is approximately 32.23%. The shortfall: 15-18 percentage points on every dollar of RSU income.

Worked Example: $300K RSU Vest, California Resident

You are a Senior Software Engineer at a public Bay Area tech company. 2026 compensation: $260,000 base, $300,000 of RSUs vesting quarterly throughout the year. Filing status: married filing jointly with a working spouse earning $190,000 (no equity). California resident the entire year.

Total household W-2 income: $750,000.

RSU withholding (default): Federal supplemental 22% on $300K = $66,000. California 10.23% on $300K = $30,690. Medicare 1.45% + Additional Medicare 0.9% on the portion above $250K MFJ threshold = approximately $5,250. Total RSU withholding: roughly $101,940, or ~34% of the gross vest value.

Actual tax liability on that $300K slice: At the household's marginal federal rate of 35% and California marginal rate of 11.3% (plus 1.45% Medicare, 0.9% Additional Medicare on the over-threshold portion, and assuming no AMT exposure), the marginal combined effective rate on the RSU income is approximately 48.65%. Tax actually owed on the RSU slice: ~$146,000.

Shortfall: ~$44,000 due with the April 2027 return. If the household did not pay quarterly estimates or withhold-up by year end, the IRS underpayment penalty under IRC §6654 applies, currently ~8% annualized on the underpaid amount, plus the equivalent California penalty under R&TC §19136. On a $44K shortfall sitting from each quarterly vest date to April 15, the penalty alone runs roughly $1,800-$2,400.

The fix: File a revised Form W-4 in January 2026 with additional federal withholding of $11,000 spread across remaining paychecks, or make quarterly estimated payments of approximately $11,000 federal + $3,800 California per quarter (Form 1040-ES and Form 540-ES). Either path eliminates the penalty and avoids the April liquidity scramble.

The Five Moves Every California RSU Recipient Should Make

1. Run the gap analysis in January, not December. Project your full-year W-2 income (base + bonus + RSU vest schedule at expected stock price) and compute your actual marginal rate. Compare that to your aggregate withholding rate. If the gap is more than 5 percentage points, you have an under-withholding problem to solve in Q1, not in October.
2. Use Form W-4 Step 4(c) to add a flat additional withholding amount per paycheck. This is the cleanest fix because it is automatic and does not require quarterly estimated payment paperwork. Divide your projected shortfall by remaining paychecks and add it as "extra withholding."
3. If you cannot adjust W-4, make quarterly estimated payments. Federal Form 1040-ES and California Form 540-ES, due April 15, June 15, September 15, and January 15. We cover the full mechanics in our estimated tax payments guide.
4. Sell-to-cover is not enough at high income levels. Most employers offer sell-to-cover for RSUs (selling enough shares at vest to cover the 22%/10.23% withholding). At your real marginal rate, those proceeds cover roughly two-thirds of the actual tax bill. The remaining third has to come from your other cash or from selling additional shares — which is itself a taxable event for the gain or loss between vest and sale.
5. Coordinate with your spouse's withholding. If your spouse also receives equity or has variable income, the household marginal rate gets pulled even higher. The W-4 default assumes one wage earner; dual-equity households almost always need both spouses to add extra withholding or split estimated payments.

Sell-to-Cover vs Hold: The Sale Timing Decision

The day your RSUs vest, the full FMV gets added to your W-2 as ordinary income and your cost basis in the resulting shares equals that FMV. If you sell on the vest date, there is no additional gain or loss beyond rounding. If you hold the shares, anything you eventually sell them for above (or below) the vest-day FMV is a capital gain or loss, with the holding period starting on the vest date.

The decision is not primarily a tax decision. It is a concentration and liquidity decision dressed up as a tax decision. Three frameworks:

• Concentration test. If your employer's stock represents more than 10-15% of your investable net worth at any point, the prudent move is to sell vested shares on or near vest and diversify. Tax rate on the modest gain or loss between vest and sale is far less expensive than carrying single-stock concentration risk through a bad year.
• Capital gains holding period. Long-term capital gains rates (0%, 15%, or 20% federal plus 3.8% NIIT for higher earners) apply only after one year + one day from vest. For California, there is no preferential capital gains rate — all gains are taxed as ordinary income up to 13.3% (plus the 1.1% mental health surcharge over $1M). Holding for LTCG saves federal tax, not California tax.
• Tax-loss harvesting opportunity. If your employer's stock drops below your vest-day FMV, selling crystallizes a capital loss you can use to offset other gains (or up to $3,000 of ordinary income annually). Wash-sale rules apply within 30 days, but they apply to your purchase of substantially identical stock — they do not apply to future RSU vests in most fact patterns. Talk to your advisor before relying on this.

RSU + ISO Stacking in the Same Year

Many Bay Area tech employees hold both RSUs and ISOs, particularly those who joined a startup early and survived to IPO. Exercising ISOs in the same year as a large RSU vest is one of the highest-risk tax maneuvers in the playbook. Two compounding effects:

1. Your AMT exposure spikes. The bargain element on ISO exercise (FMV at exercise minus strike) is an AMT preference item under IRC §56(b)(3). When stacked on top of $300K of ordinary RSU income that has already pushed you into the top brackets, the AMT calculation can produce a tentative minimum tax that exceeds your regular tax by tens or hundreds of thousands of dollars. Read our AMT in 2026 guide before any ISO exercise.
2. California conformity gap. California has its own AMT system that does not conform to all federal changes. California AMT rates are 7% for individuals and 6.65% for corporations, with different exemption phaseouts. Your federal AMT calculation does not tell you your California AMT exposure; both must be run.

The right move in an RSU + ISO year is usually to model the AMT before deciding when (or whether) to exercise. Sometimes the answer is: exercise in a low-income year, or exercise only enough to use up the AMT exemption headroom, or wait. Our ISO vs NSO breakdown covers the structural mechanics; the year-specific planning is what we do during a planning meeting.

California Tax Quirks You Need to Know

California taxes RSU income differently from the federal system in several ways that catch transplants and remote workers off guard:

• No preferential capital gains rate. California taxes long-term capital gains as ordinary income, up to 13.3% (or 14.4% with the mental health surcharge over $1M). The federal LTCG discount does not apply at the state level.
• 1.1% Mental Health Services Tax surcharge. Imposed under R&TC §17043 on the portion of California taxable income above $1M. Easy to trigger in a big RSU year, even for households whose typical income is well below that threshold.
• Multi-state RSU sourcing. RSUs are sourced to the state(s) where you worked during the grant-to-vest period, on a workday-allocation basis. If you were granted RSUs while a California employee but moved to Texas before vest, California still claims the portion attributable to your California workdays during the vesting period. This is governed by CCR §18662-6 and prior FTB guidance. Audits in this area are common.
• No conformity to certain federal equity provisions. California has historically not conformed to the §83(i) qualified equity grant deferral, has its own AMT system, and treats certain incentive stock option transactions differently. Always run both federal and California calculations.
• EDD treats RSU vests as wages for SDI purposes. California State Disability Insurance withholding (1.2% for 2026, with no wage cap as of 2024 changes) applies to the full RSU vest value. On a $300K vest, that is another $3,600 of payroll-side withholding.

For a broader 2026 California tax outlook including rate changes, surtax thresholds, and conformity updates, see our California tax changes 2026 brief.

Multi-State RSU Sourcing for Remote Workers

If you live in California but work some days from a vacation home in another state, or if you moved into or out of California during the vesting period, your RSU income gets sourced across states based on workdays during the relevant period. The general rule:

For each RSU tranche, California sources the portion of the vest equal to (California workdays during the grant-to-vest period) ÷ (total workdays during the grant-to-vest period). Your employer typically only withholds for your current state of residence, which means in a multi-state year, you owe non-resident returns and can face double-withholding or under-withholding depending on direction.

Common pitfall: an employee who moves from California to Washington (no state income tax) mid-year still owes California tax on the California-source portion of every RSU tranche that vests post-move, until the underlying grant has fully vested. We see at least one move-mistake audit per year. Plan the move with this in mind.

The Penalty Math: What Under-Withholding Actually Costs

The federal underpayment penalty under IRC §6654 is computed as the IRS short-term applicable federal rate plus 3 percentage points, applied to the underpaid amount for each quarter the deficiency existed. As of Q1 2026, this is approximately 8% annualized. California uses a similar formula and is currently at approximately 7-8% annualized.

Safe harbors that eliminate the penalty:

1. You owe less than $1,000 federal ($500 California) after withholding.
2. You paid in at least 90% of the current year's actual tax through withholding + estimates.
3. You paid in at least 100% of the prior year's actual tax (110% if your prior year AGI exceeded $150,000) through withholding + estimates.

The prior-year safe harbor is the workhorse for RSU recipients. If your 2025 tax bill was $180,000 and your AGI was over $150K, paying in $198,000 (110%) during 2026 through withholding plus estimated payments eliminates the federal penalty entirely, regardless of how much your 2026 RSU vest spikes your actual liability. Same idea applies for California with its own version of the rule.

Quick Reference: 2026 California RSU Rates

Item	2026 Rate / Amount	Citation
Federal supplemental wage rate (under $1M)	22%	Treas. Reg. §31.3402(g)-1
Federal supplemental wage rate (over $1M)	37%	IRC §1(j)
CA stock-supplemental withholding	10.23%	CA EDD DE 44
CA marginal top rate	13.3%	R&TC §17041
CA Mental Health Services Tax (over $1M)	+1.1%	R&TC §17043
CA SDI (no wage cap)	1.2%	CA UIC §984
Federal Additional Medicare (over $200K single / $250K MFJ)	0.9%	IRC §3101(b)(2)
Federal NIIT on investment income	3.8%	IRC §1411
Underpayment penalty rate (approx Q1 2026)	~8% annualized	IRC §6654, AFR + 3%

FAQ: California RSU Tax Questions We Hear Most

Why does my employer only withhold 22% on my RSUs if I am in the 35% bracket?

Because IRS regulations under Treas. Reg. §31.3402(g)-1 require employers to use a flat statutory supplemental wage rate for stock-based compensation, not your actual marginal rate. The 22% figure is set by Congress and does not adjust to your income level until you cross $1M in aggregate supplemental wages for the year, at which point the rate above $1M jumps to 37%. The fix is on your side: revise your W-4 to add extra withholding, or make quarterly estimated payments.

Is it better to sell RSUs at vest or hold them?

This is a portfolio concentration question, not primarily a tax question. The tax cost between vest and sale is the gain or loss on the difference in price, taxed at ordinary income rates if held under a year and at LTCG rates (federally) if held over a year. The real risk is concentration: if your employer stock is more than 10-15% of your net worth, holding adds material single-stock risk for an LTCG discount that, in California, does not exist at the state level. Most planners default to sell-at-vest unless there is a specific reason to hold.

Does the California 1.1% mental health surcharge apply to RSU vests?

Yes, if your California taxable income exceeds $1,000,000 in the year. The surcharge under R&TC §17043 applies to all income types above the threshold, including RSU compensation income. For a household with significant base salary plus a large RSU year, this is more common than people expect. Run the projection in January.

If I move from California to Texas before my RSUs vest, do I avoid California tax?

No, not on the portion of the RSU income attributable to your California workdays during the grant-to-vest period. California uses a workday-based sourcing rule. If a tranche was granted while you worked in California for 12 months and vests after you moved to Texas, the California portion is generally the ratio of California workdays during the vesting period to total workdays. The FTB actively audits move-out situations involving large equity events.

What happens if I exercise ISOs in the same year as a large RSU vest?

You stack the ISO bargain element on top of an already-high ordinary income year, which significantly increases AMT exposure for both federal (28% top AMT rate) and California (7% AMT). Many people accidentally pay six-figure AMT bills this way. Model the AMT before you exercise, not after. Our AMT in 2026 guide covers the structure.

Can I avoid the underpayment penalty by paying everything on April 15?

No. The IRS penalty under IRC §6654 looks at whether you paid enough each quarter, not just the total by April 15. Even if you write a check that covers everything by the April deadline, you owe penalty on any quarter where withholding plus estimated payments fell short. The simplest safe harbor: pay in 110% of last year's tax (if your AGI exceeded $150K) through withholding + estimates across all four quarters. California has a parallel rule under R&TC §19136.

When RSU Tax Planning Pays for Itself

The break-even on professional RSU planning is roughly one avoided penalty or one prevented over-payment. For a household with $200K+ of annual RSU vests, the typical first-year value of getting the withholding mechanics right, the sale timing right, and the multi-year stack right (across RSU + ISO + ESPP + bonus + capital gains) is somewhere between $8,000 and $40,000 in tax and penalty avoided. Our planning fees are a fraction of that. Book a free consultation and we will model your specific RSU schedule against your full compensation picture before the next quarterly vest hits.