Author: Hazel Secco, CFP®, CDFA®
Estimated reading time: 10 minutes
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Table of contents
- The Supplemental Wage Withholding Gap: 22% In, 35% Owed
- Why Nobody Catches This RSU Tax Mistake
- Move 1: Close the Gap with Deliberate RSU Tax Planning
- Move 2: Sell RSUs on Vest, Unless There’s a Written Reason Not To
- The 1099-B Double-Tax Trap When You Sell
- What to Do This Week
- Want the full framework?
- Listen to This Episode
- Frequently Asked Questions
- Sources
Your RSUs vested. Taxes came out of every single vest. And you still owe the IRS $18,000 in April. Nothing went wrong. Your employer under-withheld on purpose, by IRS default, and nobody at HR is going to call and explain it. That is the RSU tax withholding gap, and it is built into how your employer pays you.
Here is the scale of it. For every $200,000 in RSU vests this year, if you’re in the 35 percent bracket, you’re quietly short about $26,000 in withheld tax. That’s the check you’ll write in April unless you close the gap now. And because grants tend to grow with seniority, the check gets bigger every year you ignore it.
I’m Hazel Secco, CFP® and fee-only fiduciary. In this post I’ll show you exactly where the gap comes from, why nobody on your payroll, your brokerage, or your tax return is going to catch it for you, and the two moves that defuse it, including how to know what you’re already on the hook for this year.
The Supplemental Wage Withholding Gap: 22% In, 35% Owed
Here’s what actually happens on every vest. When restricted stock units vest, the IRS treats the value as supplemental wages, the same category as a bonus. Your employer withholds federal tax at a flat 22 percent. That’s the default rate for supplemental wages under $1 million, written into IRS Publication 15. Above $1 million in supplemental wages, the mandatory rate jumps to 37 percent.
The problem: 22 percent is not your tax rate. If your marginal federal rate is 32 percent, you’re short 10 cents on every dollar of vested stock. In the 35 percent bracket, 13 cents. In the top 37 percent bracket, 15 cents. On $200,000 of vests, that’s $20,000 to $30,000 in under-withheld tax. Every year. And it stacks with each new grant.
It compounds from there. If your modified adjusted gross income crosses $200,000 single or $250,000 married filing jointly, you also owe the 3.8 percent Net Investment Income Tax on your investment income. Those thresholds have been frozen since 2013. They never adjust for inflation. So every year, more high-earning women quietly pay the NIIT without ever hearing the term, often for the first time in the same April they discover the withholding gap. And MAGI doesn’t stop mattering when you retire: the same number later determines your Medicare IRMAA surcharges, so the habits you build managing it now pay off twice.
Why Nobody Catches This RSU Tax Mistake
Because it’s nobody’s job. Your HR team runs the equity program. Ask them about withholding and they’ll tell you, truthfully, that taxes are coming out. What they won’t tell you is that the rate is a floor, not a match for your bracket. They don’t know your bracket, and it’s not their job to.
Your equity platform (Fidelity, Schwab, E-Trade) isn’t going to explain it either. They execute the vests, they report the transactions, they mail you a 1099-B in January. Job done.
And your CPA isn’t going to prevent it. They’re paid to file your return accurately. When they see the shortfall in March, they report it, attach the underpayment penalty, and move on. That’s tax filing. Telling you in July that you’re going to owe $24,000 next April, and fixing it in August, is tax planning, which is a different job entirely. For most women with equity compensation, nobody is doing it.
That’s why your pay stub is so misleading here. Taxes are visibly coming out of every vest, so your brain files it as handled. It’s not handled. It’s under-handled by design.
Move 1: Close the Gap with Deliberate RSU Tax Planning
If you don’t fix the withholding gap, nothing else in your equity plan matters. You close it one of three ways, or a combination. This is the first move in the bracket-planning framework I lay out in The Executive Woman’s Tax Playbook, and it’s the one with a deadline attached.
Option A: bump your W-4. File a new W-4 with your employer and request additional federal withholding per paycheck. It’s the simplest fix, and it shows up automatically every payday.
Option B: quarterly estimated payments. Pay the IRS directly using Form 1040-ES on the four quarterly deadlines. This is the cleanest fix for big quarterly vests, because you size each payment to the vest that just happened. I’ve written a full walkthrough on how to pay estimated federal taxes with equity compensation if you want the mechanics.
Option C: a supplemental sell-to-cover. Most employers already sell a slice of each vest to cover the 22 percent. You instruct the broker to sell an additional tranche to cover the gap between 22 percent and your actual marginal rate. Because you’re selling at the vest price, the extra sale shows up as a short-term gain of essentially zero.
Here’s a hypothetical composite. A VP at a biotech company has $280,000 in RSUs vesting across four quarters. Her federal marginal rate is 35 percent. Her employer withholds the standard 22 percent: $61,600. Her actual federal liability on those vests is about $98,000. Gap: $36,400. Add state tax and the NIIT she hadn’t budgeted for, and her April shortfall is roughly $48,000. We moved her onto quarterly estimates starting in Q2. When her return was filed, she owed about $1,200. Not $48,000. Same income. Same vests. Different structure.
Move 2: Sell RSUs on Vest, Unless There’s a Written Reason Not To
The second mistake is treating each vest as a decision. The shares hit your brokerage, you look at the price, you think maybe it’ll go up, and you hold. Multiply that by every vest over three years and you’ve accidentally built a half-million-dollar concentrated position in the one company that also pays your salary. Paycheck and portfolio riding on the same employer. That’s double exposure, what I call home stock syndrome.
Try this question. If someone handed you $280,000 in after-tax cash this year, would you use every dollar to buy shares of your employer? Of course not. But by holding your vests, that’s exactly what you’re doing. You just never consciously made the decision.
The rule: sell on vest unless one of three written reasons applies. One, you’re in a trading blackout and legally cannot sell. Two, a tax-favored holding period actually applies and you’re weeks from it. Three, you have a written concentration limit (say, no more than 10 percent of my net worth in employer stock) and you’re still under it. Otherwise, you sell on vest and diversify. The sale creates almost no gain, because your basis is the vest price and you’re selling immediately. If you’re weighing the exceptions for shares you already hold, I’ve laid out that framework in Should You Sell or Hold Your Vested RSUs.
One reason holding feels safe: it feels like patience. It feels like discipline. Your brain has spent thirty years learning to buy and hold, and now it refuses to sell shares your employer is essentially giving you. But the comfort of holding is exactly what builds the concentration problem. When the stock drops 20 or 30 percent, the shares that felt like patience become the shares that cost you the most. You don’t have to sell everything. You have to stop letting vests pile up on autopilot.
The 1099-B Double-Tax Trap When You Sell
One warning when you do sell. Brokerages sometimes report your cost basis as zero on the 1099-B, leaving off the value you already paid tax on at vesting. If nobody catches it, you get taxed twice on the same money: once as wages at vest, and again as a phantom capital gain at sale.
The fix is record-keeping, not strategy. Keep your vest confirmations: date, share count, vest-date price. At tax time, make sure the basis on the 1099-B reflects the vest-date value, not zero, and correct it on your return if it doesn’t. This one line item quietly costs people thousands, and it’s entirely avoidable.
The RSU tax mistake, then, is two problems stacked on top of each other. The withholding gap is structural, and fixable today. The hold-for-patience trap is psychological, and fixable with a written rule. Do both, and the surprise stops being a surprise: you know what you owe, you’ve already paid it, and your portfolio isn’t riding on your employer. And once withholding is handled, the same bracket awareness opens up bigger opportunities, like knowing when a Roth conversion window is worth using. Do neither, and you’ll write a surprise check to the IRS every April for as long as you have equity comp.
What to Do This Week
Find your number today. Open your equity portal, find your year-to-date RSU vest total, and multiply by 0.13 if you’re in the 35 percent bracket, or 0.15 if you’re in the 37. That’s the approximate check you’re writing next April 15 if you change nothing between now and December. Ten minutes.
Pick one of the three withholding fixes and start it this week. New W-4, quarterly estimates via Form 1040-ES, or a supplemental sell-to-cover. Any one of them is dramatically better than doing nothing. Don’t wait for your CPA to tell you in March.
Write your sell-on-vest rule down. Not in your head. On paper. Include the three exceptions: blackout, imminent tax-favored holding period, written concentration limit. The next time a vest hits and your brain says “maybe it’ll go up,” you read the rule, not the price.
Pull last year’s 1099-B and check the basis on any RSU sales. If it shows zero for shares that vested as taxed wages, flag it. You may have paid tax twice, and amended returns can recover it.
Want the full framework?
The tax moves behind this episode (the bracket planning, Roth sequencing, and account-location decisions I walk through with clients) are in my free guide, The Executive Woman’s Tax Playbook. It’s a free PDF you can read in one sitting. Get the playbook here.
Hazel Secco, CFP®, CDFA®, is the founder of Align Financial Solutions, a fee-only fiduciary firm that works with high-earning women and female executives on retirement planning, equity compensation, and tax strategy.
Already past the research phase? Book a free 15-minute Align Call: https://alignfinancialsolutions.com/book-a-call/
🌐 https://alignfinancialsolutions.com 📺 https://www.youtube.com/@AlignYourRetirement 💼 https://linkedin.com/in/hazel-secco
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Frequently Asked Questions
How much tax is withheld on RSUs when they vest?
Employers withhold federal tax at a flat 22 percent on RSU vests, the IRS default rate for supplemental wages under $1 million. Above $1 million in supplemental wages, the mandatory rate jumps to 37 percent. If your marginal rate is 32, 35, or 37 percent, that default leaves you under-withheld on every single vest.
Why do I still owe taxes on my RSUs if my employer already withheld?
Because the 22 percent default withholding is a floor, not a match for your bracket. In the 35 percent bracket, you are short 13 cents on every dollar of vested stock, which on $200,000 of vests is roughly $26,000 still due in April. Close the gap with a new W-4, quarterly estimated payments, or a supplemental sell-to-cover.
Should I sell my RSUs as soon as they vest?
In most cases, yes. Sell on vest unless one of three written reasons applies: a trading blackout, a tax-favored holding period you are weeks from reaching, or a written concentration limit you are still under. Selling immediately creates almost no gain because your basis is the vest price, and it keeps your portfolio from riding on the same employer that pays your salary.
How do I avoid getting taxed twice on RSU sales?
Check the cost basis on your 1099-B. Brokerages sometimes report it as zero, leaving off the value you already paid tax on as wages at vesting, which creates a phantom capital gain. Keep your vest confirmations showing date, share count, and vest-date price, then correct the basis on your return if the 1099-B is wrong.
Disclaimer: Advisory services are offered through Align Financial Solutions LLC (“AFS”), an Investment Advisor in the State of New Jersey. This article is for educational purposes only and does not constitute personalized tax, legal, investment, or financial planning advice. Rules and figures are current for 2026 and subject to change. All client scenarios are hypothetical composites for illustration and do not represent any specific client outcome. Consult a qualified professional about your specific situation.
Sources
- IRS Publication 15 (Circular E), Employer’s Tax Guide (supplemental wage withholding rates): https://www.irs.gov/publications/p15
- IRS, About Form 1040-ES, Estimated Tax for Individuals: https://www.irs.gov/forms-pubs/about-form-1040-es
- IRS, Net Investment Income Tax: https://www.irs.gov/individuals/net-investment-income-tax