RSU Tax Guide 2026: The Withholding Gap That Catches Tech Employees Every April

How RSUs Are Taxed at Vest

RSUs become taxable on the vest date, not the grant date. When the vesting condition is met (typically a date passing, occasionally a milestone), the IRS treats the full fair market value of the shares as ordinary compensation income — exactly as if your employer paid you a cash bonus equal to the share value.

The taxable amount is straightforward: shares vesting × stock price on the vest date. If 100 shares of your employer's stock vest on a day when shares close at $150, you have $15,000 of ordinary income added to that pay period's W-2.

This income is subject to:

• Federal income tax at your marginal bracket (10% / 12% / 22% / 24% / 32% / 35% / 37% for 2026)
• State income tax at whatever rate your state applies, which varies from 0% (TX, FL, WA, NV, SD, AK, TN, WY) to 13.3% (CA top bracket)
• Social Security tax at 6.2% on income up to the wage base ($168,600 for 2024, indexed annually)
• Medicare tax at 1.45% on all wages, plus an additional 0.9% on wages above $200,000 (single) or $250,000 (married filing jointly)

There is no preferential rate, no holding-period advantage, and no deferral option for standard RSU vests — unlike incentive stock options (ISOs), where favorable tax treatment is available if specific holding requirements are met.

Why Your Employer Withholds Only 22%

The IRS classifies RSU vesting as supplemental wages, which sit in a different category from your regular paycheck. For supplemental wages under $1 million per year, employers may withhold federal income tax at a flat 22% rate. For supplemental wages above $1 million in a single calendar year, the rate jumps to 37% on the excess.

Almost every large employer uses the 22% flat rate as the default. It's administratively simple and avoids the company having to estimate each employee's marginal tax bracket. The 22% rate happens to match the actual federal bracket for single filers earning $48,475 to $103,350 in 2026 — a sweet spot for mid-career professionals at modest base salaries. Everyone else is mismatched.

Here's the size of the under-withholding for a $100,000 RSU vest, by federal bracket:

These are federal numbers only. State shortfalls layer on top and can match or exceed the federal gap in high-tax states.

Calculate Your Specific Withholding Gap

The math is straightforward enough to run by hand, but the calculator does it instantly across all the moving parts — federal bracket, state rate, FICA, and the additional Medicare surcharge for high earners.

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The "Federal Shortfall" row is your headline number. It tells you what to set aside between now and April. The remaining rows show state tax (a separate bill in most states) and FICA contributions (already withheld from your vest, included for completeness).

State Tax: The Layer Most People Forget

Federal is only half the story. State income tax adds a meaningful amount in most populated states.

The major employer states for RSU recipients:

• California — 9.3% to 13.3% marginal, plus 1.1% uncapped State Disability Insurance. California mandates a flat 10.23% supplemental withholding on RSU vests, which over-withholds slightly at the 9.3% bracket and under-withholds at higher brackets.
• New York — up to 10.9% state plus up to 3.876% NYC for residents. NYS mandates 11.7% supplemental withholding.
• Massachusetts — flat 5% (9% above $1M with the millionaire tax)
• Washington — 0% state income tax, but RSU income is included for the new 7% capital gains tax if you hold and sell
• Texas, Florida, Nevada, South Dakota, Alaska, Tennessee, Wyoming — 0% state income tax

The state of residency on the vest date generally determines which state taxes the income. If you moved during the vesting period, multi-state allocation rules apply — most states allocate based on workdays in each state during the period the RSUs were earned, not where you sit at vest. This gets complex fast; a CPA who specializes in equity comp pays for itself if you've moved across state lines.

What Selling Immediately Actually Does

Many tech employees hold their vested shares, often for emotional reasons (loyalty, belief in the company) or a vague sense that holding saves on tax. The math says otherwise.

When shares vest at $150, you pay full ordinary income tax on $150 per share regardless of whether you sell. Your cost basis in those shares becomes $150. If you sell immediately at $150, there's no additional gain or loss. If you hold for a year and the stock goes to $180, you have a $30/share long-term capital gain when you sell — taxed at 15% or 20% depending on income.

If you hold for a year and the stock drops to $120, you have a $30/share long-term capital loss — but you still paid ordinary income tax on the original $150. The shares were taxed at the high rate; the subsequent decline gives you only a small (capital-loss) offset.

The asymmetric risk is the case against holding. You bear concentrated single-stock risk for the chance of converting a portion of future gains from ordinary to capital-gains treatment — but only on the additional appreciation, not on the original vest value.

For most employees, the right policy is sell immediately, reinvest in a diversified portfolio, and treat RSUs as a recurring cash bonus. Some financial advisors recommend a 10% hold for employees who want exposure to the company's continued growth, but caution against larger concentrated positions.

What to Do About the Withholding Gap

Underpayment Penalty: When the IRS Charges Interest on the Gap

The IRS doesn't just want their money in April — they want it ratably through the year. Significant under-withholding triggers an underpayment penalty, currently 8% annualized (it floats with the federal short-term rate).

You avoid the penalty by satisfying any one of these conditions:

1. Total tax owed at filing is less than $1,000 (after withholding and credits)
2. Your year-round withholding and estimated payments equal at least 90% of the current year's tax liability
3. Your year-round withholding and estimated payments equal at least 100% of last year's total tax — or 110% if last year's AGI was over $150,000

The third option is the easiest for high earners with predictable income. If your prior-year tax bill was $80,000 and your AGI was over $150K, you need at least $88,000 in current-year withholding plus estimated payments to be safe — regardless of what your actual current-year liability turns out to be.

If you blow through the safe harbor on a year with unusually large RSU vests, the penalty is calculated quarter by quarter on the shortfall. You can pay the penalty when you file or wait for a notice from the IRS — both work, but waiting accrues additional interest.

A Few Common Mistakes

Mistake 1: Assuming the 22% withholding is "enough." It's enough only if you're in the 22% federal bracket — generally single filers earning $48K–$103K total. Everyone above is under-withheld.

Mistake 2: Ignoring the additional Medicare tax. The 0.9% additional Medicare tax kicks in above $200K single / $250K MFJ. Your employer is required to start withholding it once your wages cross $200K with that employer — but if you have multiple W-2 sources or significant non-wage income, you may owe the additional 0.9% on RSU vests even when no single employer crossed the threshold.

Mistake 3: Wrong cost basis on Form 1099-B. When you sell vested RSUs through a brokerage (typically the same broker your company uses for the grant), the broker may report cost basis as $0 — because they didn't pay anything for the shares. This is wrong. Your actual cost basis is the FMV at vest, which you already paid tax on. If you don't adjust this on Form 8949, you'll be double-taxed on the vest amount. Always verify cost basis matches your 1099-B and adjust on Form 8949 if it doesn't.

Mistake 4: Holding indefinitely "because the stock will go up." The decision to hold vested shares is functionally identical to the decision to buy that stock fresh today with the after-tax cash equivalent. Most people wouldn't make that second purchase — yet they hold the equivalent position by default.

What This Means for Your Total Take-Home

Across the full picture — federal at marginal rate, state at marginal rate, FICA contributions, and the additional Medicare tax for high earners — a senior tech employee in California typically nets around 48-52% of every RSU dollar at vest. The remaining 48-52% goes to taxes. In a no-state-tax state, that share jumps to roughly 65-70%.

This is not a reason to feel cheated; it's a planning input. RSU income that gets treated as ordinary wages is taxed at ordinary-wage rates. The fix isn't to find a tax dodge (there generally isn't one for standard RSUs); it's to plan for the bill, capture every dollar of qualifying pre-tax deductions (401(k), HSA, dependent care FSA), and avoid the surprise.

Related Calculators and Guides

• RSU Tax Calculator — the one referenced throughout this guide
• ESPP Tax Calculator — for employee stock purchase plan shares
• Bonus Tax Calculator — same withholding-gap dynamic for cash bonuses
• Tax Bracket Calculator — see where your total income lands
• Capital Gains Tax Calculator — for shares you hold and later sell

Sources: IRS Publication 15 (Employer's Tax Guide), IRS Revenue Procedure 2025-32 (2026 inflation adjustments), California EDD supplemental withholding tables. Tax law changes; verify current rates at irs.gov before making decisions. This guide is for educational purposes and does not constitute tax advice. Consult a CPA for your specific situation.