RSU Vesting at IPO: Why Your Tax Withholding Is Probably Wrong

Your company is going public, or maybe it just did. Congratulations, that’s a genuinely big deal, and if this is your first time watching a company you work for go through an IPO, there’s a lot to take in at once. Years of RSUs that felt theoretical are about to become actual shares, actual income, and an actual tax bill with RSU vesting at IPO. During that process, your company will automatically withhold taxes on your behalf, usually by selling off a portion of your shares before you ever see them. It’s easy to assume that step means the tax side is already handled.

But it probably isn’t.

Not because anyone made a mistake, but because the withholding formula was never designed to match your real tax bill in the first place. If you don’t do anything about it before year-end, you could be looking at a surprisingly large check to the IRS next April, plus a penalty on top of it.

RSU Withholding is a Flat Rate, Not Your Rate

When your RSUs vest, the IRS treats that income as a supplemental wage, the same category as a bonus. Instead of running through the graduated tax brackets like your regular paycheck, supplemental wages get withheld at a flat rate set by the IRS. That rate is 22% up to $1 million in a calendar year, and 37% on anything above that, per IRS Publication 15.

That 22% rate isn’t based on your income, your filing status, or your actual tax bracket. It’s a flat number that applies whether you make $150,000 a year or $850,000 a year, which means it works out fine for some people and badly for others.

If your total income for the year, salary plus the value of your vested RSUs, keeps you in the 22% bracket or below, you’re probably fine. But if that vest pushes you into the 32%, 35% or 37% bracket, the withholding covers only part of what you actually owe. The rest is quietly accumulating as a bill with your name on it.

Let’s put real numbers on it. Say you’re single, earning $300,000 in salary, and your company’s IPO triggers a $400,000 RSU vest. Based on the 2026 federal tax brackets, much of that vest falls into the 35% bracket, with the highest portion taxed at 37%. Your company withholds federal income tax at 22% on the entire vest. Compare that with the tax you’ll actually owe on the additional income, and you could still end up around $50,000 short when you file your return.

Why IPOs Are Worse Than a Normal Vest

If your company had been public the whole time and your RSUs vested quarterly like a normal paycheck, you’d have this same withholding gap, but spread thin across the year. A few hundred dollars short here and there, which is manageable.

IPOs don’t work that way. Most private tech companies use what’s called double-trigger vesting for their RSUs. It’s been widely adopted since Facebook’s IPO. Under double-trigger vesting, your RSUs need two things to happen before they actually vest: you have to hit the time-based schedule (say, working there for a year, then vesting quarterly after that), and your company has to go through a liquidity event, typically an IPO or an acquisition.

This structure exists for a good reason. Private company stock can’t be sold on the open market. If your RSUs vested purely on the time-based schedule while the company was still private, you’d owe ordinary income tax on stock you had no way to sell to cover it. Double-trigger vesting solves that by holding everything back until there’s actually a market to sell into.

The tradeoff is that when the second trigger finally hits, years of accumulated vesting land all at once. Instead of a steady drip of shares and tax liability, you get one enormous vesting event and tax bill, concentrated into a single day. That’s what turns a manageable withholding gap into a genuinely large one.

Some employers allow employees to elect additional federal withholding beyond the default rate. If your company offers this election, it’s worth understanding what you’re actually choosing. Electing higher withholding means selling more shares at the IPO price to cover taxes now. Electing lower withholding means holding more shares and betting on where the stock goes next. That’s an investment decision that deserves to be thought through.

Other Pieces of the Puzzle

The federal withholding gap gets most of the attention, but there are other pieces of the puzzle to keep in mind. 

State taxes: If you live in a state with income tax, it likely applies its own flat supplemental withholding rate, and that rate is often lower than your top marginal state rate too. Same mismatch, smaller in dollar terms, but worth checking your specific state’s rate rather than assuming it’s handled.

Payroll taxes: Social Security withholding stops once you hit the annual wage base, but Medicare doesn’t cap out, and there’s an additional 0.9% Medicare surcharge once your wages cross $200,000 (or $250,000 if you’re married filing jointly).

The million-dollar cliff: If your vest, combined with other supplemental wages you’ve received during the year, pushes you over $1 million in cumulative supplemental income, the withholding rate jumps to 37% on the excess. That’s actually good news in one sense, since 37% is closer to your real marginal rate. But it can also catch people off guard if they weren’t expecting the jump.

Lock-up periods: Even when you know you’re short, you may not be able to sell additional shares to raise cash for weeks or months after the IPO, since most companies impose a lock-up period. That makes advance planning more important.

How to Find Out if You’re Actually Short

Add up your total taxable income for the year: salary, the fair market value of your vested RSUs on the vest date, and any other income you expect. Run that total through this year’s tax brackets to estimate your real liability. Then compare that to what’s actually been withheld so far, from your paycheck and from the RSU vest combined. The difference is your gap.

This is exactly the kind of calculation worth running with your tax advisor well before December 31st, not in the scramble leading up to April.

Closing The Tax Gap Without a Penalty

Once you know the size of the gap, you have two real ways to close it.

The first is increasing your W-4 withholding on your regular paycheck for the rest of the year. This is often the cleaner option, and here’s a genuinely useful detail: the IRS treats withholding as if it were paid evenly throughout the year, no matter when it actually happens. That means a withholding increase in November or December can effectively cover a shortfall from a vest that happened back in June. 

The second option is making an estimated tax payment directly to the IRS through Direct Pay.

Either way, the goal isn’t to predict your tax bill to the penny. It’s to clear the safe harbor rule. If you pay at least 90% of your current year’s tax liability, or 100% of last year’s tax liability (110% if your prior-year adjusted gross income was over $150,000), the IRS won’t charge you an underpayment penalty, even if you still owe a meaningful balance when you file.

Need Help with IPO Tax Withholding?

There’s nothing broken about your withholding. It’s working exactly as designed; it’s just designed around a flat statutory rate, not your actual tax situation. The discrepancy between those two things can be fixed if you catch it early enough.

Don’t wait too long. Once your return is filed, your options shrink down to writing a check and possibly a penalty. Checking the numbers mid-year, ideally not long after the vest, gives you months to address this, instead of days.

If your RSUs vested at IPO this year, or you’re expecting them to soon, it’s worth running the real numbers before year-end, rather than guessing.

That’s the kind of planning we do with clients at KB Financial Advisors every day, not just the tax withholding piece, but how it fits into the rest of your equity and investment portfolio. If you’d like a review of where you stand, we’re happy to talk it through.

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