If a big batch of your RSUs vests in October, November, or December, the tax withheld on that vest is probably not the tax you'll actually owe on it. That gap is quiet — no one calls to warn you — and it usually surfaces in April, as either a smaller refund than expected or a balance due with an underpayment penalty attached. The good news is the mismatch is fixable, but only if you catch it before December 31.
Table of Contents
- Key Takeaways
- How RSU Vesting Is Taxed
- The 22%/37% Withholding Trap
- Why a Q4 Vest Is Especially Risky
- The Catch
- How to True Up Before Year-End
- Putting It All Together
- Frequently Asked Questions
Key Takeaways
- RSUs are taxed as ordinary income the moment they vest, not when you sell. Under IRC 83(a), the full fair market value of the shares on the vest date lands on your W-2 as wages, whether or not you sell a single share.
- The default withholding rate is a flat 22% (37% above $1 million in supplemental wages for the year), not your real marginal rate. For anyone in the 32%, 35%, or 37% bracket, that flat rate under-withholds — sometimes by thousands of dollars per vest.
- A Q4 vest compounds the problem because there's no time left to recover through payroll. A January or June shortfall can be absorbed by adjusting withholding on the rest of the year's paychecks. A November or December vest gives you weeks, not months.
- You have two levers to fix it before year-end: your W-4, or a Q4 estimated payment. Both routes exist specifically because the IRS assumes flat-rate withholding won't match everyone's actual bracket.
- The estimated tax deadline for Q4 2026 income is January 15, 2027. Miss it, and the underpayment penalty calculation doesn't care that your employer, not you, chose the withholding rate.
How RSU vesting is taxed
Restricted stock units don't get taxed like actual stock ownership until they vest. Before that, they're a promise — you own nothing the IRS cares about. The moment the vesting condition is satisfied, though, IRC 83(a) treats the fair market value of the shares as compensation income, taxed at ordinary rates, on that exact date.
Your employer reports this as W-2 wages. If you want the mechanics of how that shows up on your pay stub and tax forms, our guide to restricted stock units walks through the full cycle from grant to vest to sale.
Most companies handle the tax bill through a sell-to-cover: the plan administrator automatically sells a portion of the newly vested shares to cover withholding, and you receive the rest. That's convenient, but "covers withholding" is doing a lot of quiet work in that sentence — it covers the statutory withholding rate, which is a different number than your actual tax rate. We cover the reporting side of that transaction in how to report sell-to-cover RSU sales on your taxes.
The 22%/37% withholding trap
Here's where the gap opens up. Federal law lets employers withhold on supplemental wages — bonuses, commissions, and RSU vests all qualify — at a flat optional rate instead of running them through your regular W-4 withholding tables. Per IRS Publication 15 (Circular E) for 2026, that flat rate is 22% on supplemental wages up to $1 million in the calendar year, and a mandatory 37% on the portion above $1 million.
Twenty-two percent is a reasonable proxy if your total income puts you in the 22% or 24% bracket. It is not a reasonable proxy if your total income — salary plus equity — puts you in the 32%, 35%, or 37% bracket, which describes a lot of RSU holders at growth-stage and public tech companies.
Illustrative example. Say you're a single filer with a $295,000 base salary, and on November 15, 2026, 412 shares vest at $164.85 per share — a $67,918 vest. Your employer withholds the flat 22%: $14,942. But between your salary and the equity, your income for the year pushes you into the 35% bracket, meaning the real federal tax on that $67,918 chunk of income is closer to $23,771. That's an $8,829 shortfall from this one vest alone — before you account for state tax or any other vests earlier in the year.
Why a Q4 vest is especially risky
A shortfall from a March or June vest is annoying but manageable — you have two, three, or four remaining paychecks' worth of regular withholding to adjust upward and quietly close the gap by December 31. A vest in November or December doesn't give you that runway.
Rhetorical question worth asking every Q4: if this vest were withheld correctly, would your refund or balance due look the same? For most people who vest in Q4 and don't check, the honest answer is no — and they find out in April, when it's too late to do anything but pay.
This is also the point in the year when people have the least attention on tax planning. Q4 is bonus season, open enrollment, year-end deal closing, holidays. The RSU vest lands, the sell-to-cover happens automatically, and the shortfall sits invisible until a preparer runs the numbers months later.
The catch
None of this is a loophole or a strategy failure — it's simply how supplemental wage withholding is built, and there are real constraints to know about before you act:
The flat rate isn't wrong, it's just generic. It was designed as an administrable default for employers, not a personalized calculation, so it was never going to match every taxpayer's bracket.
Adjusting withholding late in the year has limited runway. With only one or two paychecks left in 2026, even a large increase in withholding percentage may not pull in enough dollars to close a big gap.
Multiple vests compound the mismatch. Vests in March, June, September, and November each under-withhold a little, and the cumulative shortfall is bigger than any single vest suggests.
State withholding has its own rules. Some states mirror federal supplemental-wage treatment; others don't, so a federal fix doesn't automatically fix a state shortfall.
How to true up before year-end
You have two practical levers, and they aren't mutually exclusive.
1. Adjust your Form W-4 for the rest of the year. You can increase withholding from your regular paychecks using the extra-withholding line on Form W-4, pulling in additional federal tax before December 31 to offset an under-withheld vest. Our step-by-step W-4 guide covers how to calculate the right extra amount. The advantage: withholding is treated as paid evenly throughout the year for penalty purposes, regardless of which paycheck it came from, which can help even if you make the adjustment late in Q4.
2. Make a Q4 estimated tax payment. If W-4 adjustments can't close the gap in time, you can send a direct payment to the IRS using the estimated tax system. For the 2026 tax year, the fourth and final installment is due January 15, 2027. Our guide to how federal quarterly tax payments work explains how to calculate the payment and where to send it.
Either route can help you avoid or reduce an underpayment penalty under IRC 6654, which generally applies if you owe $1,000 or more at filing and haven't met a safe harbor (commonly 90% of the current year's tax or 110% of the prior year's tax for higher earners). A tax advisor can model which safe harbor applies to your specific income mix and confirm the payment amount that gets you there.
How Harness helps
Timing a W-4 adjustment or an estimated payment around an RSU vest requires knowing your full-year income picture — salary, bonus, every vest date, and your actual marginal bracket — not just the number on your latest pay stub. This is exactly the kind of calculation where a specialist advisor earns their fee: modeling your remaining 2026 vests and total liability and naming the dollar amount to true up. Harness connects equity holders with tax advisors experienced in RSU compensation and year-end planning.
Putting it all together
Before year-end, confirm three things:
- What's your actual marginal bracket once salary, bonus, and all 2026 vests are added together — not just the 22% assumption your employer used?
- How much of a gap exists between what's been withheld on your vests so far and what you'll actually owe on that income?
- Which lever closes it in time — a W-4 adjustment on remaining paychecks, a Q4 estimated payment by January 15, 2027, or both?
Answer those three questions before December 31, and the "surprise" tax bill stops being a surprise.
Frequently Asked Questions
Is RSU income taxed when it vests or when I sell the shares? RSU income is taxed as ordinary wages on the vest date, based on the fair market value of the shares that day, under IRC 83(a). Any gain or loss after vesting is a separate capital gains event when you eventually sell.
Why did my employer only withhold 22% on my RSU vest? Federal rules allow employers to withhold on supplemental wages, including RSU vests, at a flat 22% rate (37% above $1 million in cumulative supplemental wages for the year) instead of using your personal W-4 withholding tables. It's an administrative default, not a calculation of your actual tax rate.
How do I know if I'm under-withheld on my RSUs? Add your salary, bonus, and the value of all 2026 vests to estimate your total taxable income and marginal bracket, then compare that to the 22% (or 37%) actually withheld on each vest. A tax advisor or a detailed year-end projection can quantify the exact gap.
What's the deadline to fix an RSU withholding shortfall for 2026? You can adjust your W-4 for the rest of the year, or make an estimated tax payment. The fourth-quarter 2026 estimated payment deadline is January 15, 2027.
Can I just wait and pay whatever I owe when I file? You can, but if the shortfall is large enough, you may owe an underpayment penalty under IRC 6654 in addition to the tax itself. Closing the gap before year-end, through withholding or an estimated payment, can reduce or eliminate that penalty.
Does selling shares from a sell-to-cover transaction create additional tax? Potentially, if the shares are sold for more or less than their value on the vest date. That's a separate, usually small, capital gain or loss. See our guide on reporting sell-to-cover RSU sales for the mechanics.
Disclaimer:
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