Why are RSU vests under-withheld?

Because the IRS supplemental flat-rate withholding rule applies a uniform 22% federal rate to all supplemental wages — including RSU vests — regardless of your actual marginal bracket. If you're in the 24%, 32%, or 35% federal bracket (single filers above ~$103k, ~$202k, or ~$257k taxable income for 2026), the 22% under-withholds the federal portion, and you owe the gap at filing.

What's the cumulative supplemental threshold?

Once your YTD cumulative supplemental wages (bonuses + RSU vests + other supplemental) exceed $1,000,000, the federal withholding rate jumps from 22% to 37% on the excess. The calculator tracks YTD vests and applies this rule automatically — including the case where a single vest straddles the threshold (partial 22%, partial 37%).

How is the real tax calculated?

Real federal tax on this vest = federal_tax(salary + YTD vests + this vest) − federal_tax(salary + YTD vests). This isolates the marginal-rate slice attributable to THIS vest event, accounting for where in the bracket ladder this vest sits. FICA and state are flat (same withheld and owed).

Why does the cumulative YTD shortfall matter more than per-vest?

Because under-withholding compounds across multiple vests in a year. A single $50k vest at a 10pp rate gap is $5k under-withheld. Four quarterly vests of that size = $20k cumulative shortfall. By April, the surprise hits as one big balance owed. Tracking cumulative lets you set aside dollars throughout the year, not in a panic in March.

Should I file a new W-4 to fix this?

Yes, often the cleanest fix. Use Step 4c (additional withholding) to add a flat dollar amount per pay period equal to (annual cumulative shortfall ÷ remaining pay periods). Submit a new W-4 the day a vest hits — payroll usually applies it within 1-2 cycles. Alternative: pay quarterly estimated taxes via IRS Direct Pay (irs.gov/payments) to cover the gap.

What about FICA on RSU vests?

FICA (7.65% combined SS + Medicare) is flat — withheld and owed are identical, so it doesn't contribute to the shortfall. Note: SS portion is capped at $181k of wages for 2026; if your YTD wages already exceed this cap, only Medicare 1.45% applies to the vest. The calculator assumes you're below the SS cap unless YTD vests bring you over.

How does this differ from the Bonus Tax Calculator?

The math is essentially identical (RSUs are taxed as supplemental wages, same as bonuses). The DIFFERENCE is framing: the RSU calculator emphasizes the cumulative, multi-vest pattern that catches FAANG-tier employees. Most RSU recipients have 4-12 vests per year; the cumulative shortfall is what hurts at filing, not any single vest.

What if I sell the vested shares immediately to cover taxes?

Common at FAANG companies — the 'sell-to-cover' is the default. Your employer typically withholds shares equal to ~22% of the vest's FMV and remits the dollar value to the IRS as withholding. Your 1099-B shows the cost basis = FMV at vest (so no additional capital gain on the sold-to-cover portion). The under-withholding gap still applies — sell-to-cover doesn't fix the marginal-rate problem.

Can I avoid the gap by timing my vests?

Mostly no — RSU vest schedules are set by your grant. But you CAN time secondary moves: if a large vest is coming, increase your W-4 withholding (Step 4c) for a few months ahead of it to pre-cover the gap. Or sell the post-vest shares strategically (LTCG after 1+ year holding lowers the tax on any appreciation above FMV).

Does the calculator handle CA-specific RSU rules?

Partially. CA's supplemental rate (10.23%) is higher than most states; enter that as your state rate for accuracy. CA also has its own state-level AMT-like calculation that the calculator doesn't model — for ISO holders in CA, that's a separate tool. For pure RSU income (no ISO), the federal + flat state covers most of the tax.

What's the typical April surprise size for high earners?

$10-50k+ for FAANG-tier compensations (~$300k-$1M total comp). The 32% bracket starts at ~$202k single (~$262k MFJ); recipients with $200k+ in annual RSU vests at that bracket are under-withheld by ~$20k+ across the year. The 35% bracket is even worse — ~13pp gap. Without quarterly estimates or W-4 adjustments, this lands in April as a single bill.

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RSU Tax & Withholding Shortfall Calculator (US 2026) — The April Surprise, Quantified

Enter your salary + RSU vest + YTD prior vests. The calculator shows the 22% supplemental flat withholding vs your real marginal tax — and the cumulative shortfall across the year that triggers FAANG-tier April balances.

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Reviewed by CalcBold EditorialLast verified May 4, 2026Methodology

Annual vest schedule + cumulative shortfall

Project your full year of RSU vests. The grid shows each vest’s shortfall + the cumulative total — so you know how much to set aside before April.

FilingBase salary ($)Per vest ($)Vests / yrState (%)

Annual RSU total

$300,000

4 × per-vest

Total withheld (year)

$103,950

22% supplemental + FICA + state

Real tax (year)

$139,330

Marginal-rate on RSU portion

Annual shortfall

−$35,380

Vest-by-vest schedule4 vests this year

Vest #	Cumulative	Withheld	Real tax	Shortfall	Cum. shortfall	Marginal
#1	$75,000	$25,988	$32,117	−$6,130	−$6,130	35%
#2	$150,000	$25,988	$35,738	−$9,750	−$15,880	35%
#3	$225,000	$25,988	$35,738	−$9,750	−$25,630	35%
#4	$300,000	$25,988	$35,738	−$9,750	−$35,380	35%

Annual shortfall: $35,380

File a new W-4 (Step 4c, additional withholding) for $1,361 per biweekly paycheck (or quarterly estimated payments of $8,845/quarter via IRS Direct Pay) to cover the gap and avoid the underpayment penalty.

Rows shaded yellow show vests with under-withholding. The cumulative shortfall column is what actually matters at filing — it’s the dollar amount you need to set aside to settle in April without surprise.

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What This Calculator Does

The RSU Tax & Withholding Shortfall Calculator answers the FAANG-tier April surprise: “Why do I owe $30k+ at filing every year even though my RSUs were taxed at 22% on each vest?”The answer: because 22% is the IRS supplemental flat withholding rate, not your real marginal tax rate. If you’re in the 32% or 35% bracket, every vest is silently under-withheld by 10-13 percentage points, and the gap compounds across multiple vests per year into a five-figure April balance.

Where most online RSU tools just multiply your vest by 22% and call it done, this one tracks cumulative shortfall across multiple vests in a tax year — surfacing the true number you need to set aside (or add to your W-4) to avoid the surprise. Plus the calculator handles the $1M cumulative supplemental threshold (where withholding jumps from 22% to 37%) automatically.

Live recompute, share scenarios, save up to 5 named events. Privacy by default — all math runs in your browser.

The Math: Why RSU Vests Are Under-Withheld

RSUs vest as ordinary W-2 income at the vest date. Your employer applies the IRS supplemental flat-rate rule (Pub 15 § 7): 22% federal flat on the first $1M of cumulative supplemental wages, 37% on the excess. Plus 7.65% FICA + state at your rate.

But your real federal taxon the vest is not 22%. It’s your marginal bracket — the highest bracket the vest pushes you into:

That subtraction isolates the marginal-rate slice attributable to this single vest. FICA and state are flat (same withheld and owed), so the entire shortfall comes from the federal-marginal-vs-supplemental gap.

A Worked Example — “The $200k Engineer’s Quarterly RSU”

Suppose you’re a single filer earning $200,000 base salary. Your company vests RSUs quarterly at $75,000 per vest = $300,000 annual RSU. State 5%.

Total annual income (W-2 + RSUs) = $500,000 → top of the 32% bracket (single 2026), edges into 35% near year-end.
Per-vest withheld federal: 22% × $75k = $16,500.
Per-vest real federal (marginal): roughly $24,000 at the 32% bracket. Gap: $7,500 per vest.
Plus FICA + state: same withheld and owed → no shortfall there.
Per-vest shortfall: ~$7,500.
Across 4 vests/year: ~$30,000 cumulative shortfall — owed at filing.

That’s the FAANG-tier April surprise. The calculator tracks this cumulative gap explicitly and tells you how much to set aside per paycheck (or per quarter) to avoid being blindsided.

The Cumulative Tracker

The scenario panel below shows a vest-by-vest schedule with cumulative shortfall column:

Each row = one vest event (1, 2, 3, 4 by default).
Cumulative-after column = total RSU income through that vest.
Shortfall column = THIS vest’s gap (red if under-withheld).
Cumulative-shortfall column = year-to-date total shortfall — what you actually owe at filing.
Marginal column = your federal bracket at that point in the year.

Adjust per-vest amount + vests-per-year to match your grant’s vest schedule. The calculator handles cliff vests (1 large vest) and monthly vests (12 small vests) equally.

The $1M Cumulative Threshold

Once your YTD cumulative supplemental wages cross $1,000,000, federal withholding on the excess jumps from 22% to 37%. This is rare for most workers but routine at executive levels:

$250k bonus + $750k cliff vest = $1M cumulative → next vest starts at 37%.
IPO-related vests (“refreshes”) often push past $1M in a single year.

At the 37% supplemental flat, withholding usually MATCHES or slightly EXCEEDS your real marginal federal rate (the top federal bracket is also 37%). So vests above $1M are typically over- withheld — opposite of the standard problem. The calculator handles the $1M split correctly even when a single vest straddles the threshold.

Three Ways to Cover the Shortfall

1. W-4 adjustment (most efficient)

File a new W-4 with your employer using Step 4cto specify additional withholding per pay period. Set the additional amount to (cumulative annual shortfall ÷ remaining pay periods). The calculator’s annual-shortfall stat tells you the denominator number; divide by 26 (biweekly), 24 (semimonthly), or 12 (monthly) for your specific schedule.

The advantage: smooth cash flow, no penalty risk, no surprise. The cost: 5 minutes filing through your HR portal.

2. Quarterly estimated tax payments

Pay the gap as quarterly estimates via IRS Direct Pay (irs.gov/payments — free ACH, electronic confirmation). Due dates: April 15, June 15, September 15, January 15. Per-quarter amount = annual shortfall ÷ 4. Use the dedicated Quarterly Estimated Tax Calculator to model the safe-harbor rules + final per-quarter dollar amount.

3. Lump-sum cash on hand

Don’t adjust withholding; just save the gap into a separate savings account (high-yield, marked “tax savings”) and pay the balance at filing. Works for one-off vest events. Risky for high-shortfall recurring patterns: if you spend the dollars before April, you’re in trouble. The IRS may also assess an underpayment penalty if your safe harbor isn’t met (see the Quarterly Estimated Tax calc).

Common Mistakes

Assuming sell-to-cover fixes the problem. Most FAANG companies default to selling ~22% of the vested shares to cover withholding. The 22% gets remitted as federal withholding, but the marginal-rate gap remains. Sell-to-cover is a cash-flow convenience, not a tax solution.
Using the 22% rate as your “effective tax rate” mental model. RSUs are taxed at your marginal bracket like any other ordinary income. The 22% is just the withholding default. If your marginal is 32%, plan around 32% for the RSU portion.
Forgetting CA-specific rules. California has its own supplemental rate (10.23%) plus a state-level AMT-like calculation for certain equity events. The flat-state-rate input handles the supplemental piece reasonably; for full CA accuracy, consult a CA-licensed CPA.
Forgetting capital gains on appreciation.The vest-date FMV becomes your cost basis. Any sale ABOVE that FMV is short-term capital gain (if sold within 1 year of vest) or long-term capital gain (if held > 1 year). The calculator covers the at-vest tax; future appreciation is a separate calculation.
Not running the calc again after a promotion or bonus. Higher base salary changes your marginal bracket, which changes the per-vest shortfall. Re-run the calculator any time your base or bonus structure changes.

Save and Share

Click Saveto name each scenario (“Q1 vest 2026,” “Q2 vest after promo,” “Cliff vest”) and store in your browser. Up to 5 saves per calculator. Re-run after each vest with the updated YTD total to track cumulative shortfall in real time.

Click Share to copy a URL with your inputs encoded — useful for sending the exact scenario to a partner, accountant, or tax advisor. Print for a clean 1-page summary you can keep with your tax records.

Related Tools

Bonus Tax Calculator — same supplemental flat math applied to bonuses. Use for any one-off bonus event (annual performance, signing).
Stock Options ISO/NSO + AMT Calculator — for ISO/NSO option exercises (different mechanism — bargain element + AMT trap on ISOs).
Quarterly Estimated Tax Calculator — if you choose quarterly payments over W-4 adjustment, this calc computes the safe-harbor required + per-quarter amount.
Take-Home Pay Calculator — full annual take-home with 401(k), HSA, state — useful for the post-vest “what’s my real monthly?” view.

How to Read the Verdict

The headline number is the cumulative shortfall across all vests in the tax year — not any single vest. That figure is the dollar amount you need to either set aside or cover with extra W-4 withholding before April. Treat it as a liability already owed, not a forecast.

Shortfall under $1,000.Settle at filing — no underpayment penalty applies. Don’t bother changing your W-4.
Shortfall $1,000-$5,000. Move the cash into a HYSA earmarked for April. Skip the W-4 dance for one-time events (acquisition vest, refresher cliff).
Shortfall above $5,000 or recurring vests. File a new W-4 with extra Step 4c per pay period — divide the shortfall by remaining periods. Or pay an estimated quarterly to stop the IRS underpayment penalty clock.
YTD supplemental wages near $1M. Coordinate with payroll — withholding jumps from 22% to 37% on dollars above the threshold, and miscalculating the split is the single biggest cause of accidental over- or under-withholding at FAANG-tier comp.

Frequently Asked Questions

The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.

Why are RSU vests under-withheld?
Because the IRS supplemental flat-rate withholding rule applies a uniform 22% federal rate to all supplemental wages — including RSU vests — regardless of your actual marginal bracket. If you're in the 24%, 32%, or 35% federal bracket (single filers above ~$103k, ~$202k, or ~$257k taxable income for 2026), the 22% under-withholds the federal portion, and you owe the gap at filing.
What's the cumulative supplemental threshold?
Once your YTD cumulative supplemental wages (bonuses + RSU vests + other supplemental) exceed $1,000,000, the federal withholding rate jumps from 22% to 37% on the excess. The calculator tracks YTD vests and applies this rule automatically — including the case where a single vest straddles the threshold (partial 22%, partial 37%).
How is the real tax calculated?
Real federal tax on this vest = federal_tax(salary + YTD vests + this vest) − federal_tax(salary + YTD vests). This isolates the marginal-rate slice attributable to THIS vest event, accounting for where in the bracket ladder this vest sits. FICA and state are flat (same withheld and owed).
Why does the cumulative YTD shortfall matter more than per-vest?
Because under-withholding compounds across multiple vests in a year. A single $50k vest at a 10pp rate gap is $5k under-withheld. Four quarterly vests of that size = $20k cumulative shortfall. By April, the surprise hits as one big balance owed. Tracking cumulative lets you set aside dollars throughout the year, not in a panic in March.
Should I file a new W-4 to fix this?
Yes, often the cleanest fix. Use Step 4c (additional withholding) to add a flat dollar amount per pay period equal to (annual cumulative shortfall ÷ remaining pay periods). Submit a new W-4 the day a vest hits — payroll usually applies it within 1-2 cycles. Alternative: pay quarterly estimated taxes via IRS Direct Pay (irs.gov/payments) to cover the gap.
What about FICA on RSU vests?
FICA (7.65% combined SS + Medicare) is flat — withheld and owed are identical, so it doesn't contribute to the shortfall. Note: SS portion is capped at $181k of wages for 2026; if your YTD wages already exceed this cap, only Medicare 1.45% applies to the vest. The calculator assumes you're below the SS cap unless YTD vests bring you over.
How does this differ from the Bonus Tax Calculator?
The math is essentially identical (RSUs are taxed as supplemental wages, same as bonuses). The DIFFERENCE is framing: the RSU calculator emphasizes the cumulative, multi-vest pattern that catches FAANG-tier employees. Most RSU recipients have 4-12 vests per year; the cumulative shortfall is what hurts at filing, not any single vest.
What if I sell the vested shares immediately to cover taxes?
Common at FAANG companies — the 'sell-to-cover' is the default. Your employer typically withholds shares equal to ~22% of the vest's FMV and remits the dollar value to the IRS as withholding. Your 1099-B shows the cost basis = FMV at vest (so no additional capital gain on the sold-to-cover portion). The under-withholding gap still applies — sell-to-cover doesn't fix the marginal-rate problem.
Can I avoid the gap by timing my vests?
Mostly no — RSU vest schedules are set by your grant. But you CAN time secondary moves: if a large vest is coming, increase your W-4 withholding (Step 4c) for a few months ahead of it to pre-cover the gap. Or sell the post-vest shares strategically (LTCG after 1+ year holding lowers the tax on any appreciation above FMV).
Does the calculator handle CA-specific RSU rules?
Partially. CA's supplemental rate (10.23%) is higher than most states; enter that as your state rate for accuracy. CA also has its own state-level AMT-like calculation that the calculator doesn't model — for ISO holders in CA, that's a separate tool. For pure RSU income (no ISO), the federal + flat state covers most of the tax.
What's the typical April surprise size for high earners?
$10-50k+ for FAANG-tier compensations (~$300k-$1M total comp). The 32% bracket starts at ~$202k single (~$262k MFJ); recipients with $200k+ in annual RSU vests at that bracket are under-withheld by ~$20k+ across the year. The 35% bracket is even worse — ~13pp gap. Without quarterly estimates or W-4 adjustments, this lands in April as a single bill.
Can I save scenarios for each vest event?
Yes — click Save under the result, name each scenario ("Q1 vest 2026," "Q2 vest after promo," "Cliff vest") and store in your browser. Up to 5 saves per calculator. Re-run after each vest with the updated YTD total to track cumulative shortfall. Click Share to copy the scenario URL.