The RSU tax traps that hit in April, and how to dodge them

Why do so many high earners get blindsided by a tax bill in April, even though their company already withheld on every vest? Because the default withholding on RSU income is a flat rate, and a flat rate is almost always lower than the real marginal rate of someone who earns enough to have meaningful RSUs in the first place. That gap is the most common RSU trap, but it is not the only one. This guide is the whole field of them, start to finish: the withholding shortfall, the estimated-tax penalty, the double-taxed 1099-B, the short-term-gain slip, the blackout that locks you out, the wash sale your own vesting springs, the acceleration year, and the worst one, a cash tax bill on private shares you cannot sell.

Read it top to bottom the first time so you know which traps you are exposed to. Come back to any single section when you are about to act.

The withholding gap: the trap underneath all the others

When your RSUs vest, the full value is ordinary income, the same as salary. Your employer withholds tax on it, which feels like the system is handling it for you. The catch is how that withholding gets calculated. RSU income is treated as supplemental wages, and supplemental wages get withheld at a flat federal rate, not at your personal bracket.

For 2026 that flat rate is 22% on the first $1,000,000 of supplemental wages in the year, and 37% on anything above $1,000,000 2026.

Withheld · 22%

The gap you owe

0% up to 37% (your top rate)

+ 0.9% Medicare over $200k single / $250k joint + 3.8% net investment income tax over the same thresholds

Supplemental withholding itself jumps to 37% on anything above $1,000,000.

If your real top tax rate sits above that flat withholding rate, every dollar of vested stock is under-withheld by the difference. You do not feel it on any single vest. You feel all of it at once when you file.

Here is the second-order effect. The bigger your equity, the wider the gap, because the flat rate stays flat while your marginal rate climbs. Stack a strong vesting year on top of a strong salary and you can cross into your highest bracket while the withholding never moved off the flat number. The people most exposed to this surprise are exactly the people who think they are too sophisticated to be surprised. State tax can widen it further if you live somewhere with its own income tax, and a big year can also drag in the extra Medicare and net investment income taxes. For 2026 that is an extra 0.9% on wages above $200,000 for single filers and $250,000 for married filing jointly, plus the 3.8% net investment income tax once income clears those same thresholds 2026. None of that shows up in the flat supplemental withholding.

Size the shortfall and cover it

You cannot change how the flat rate works, but you can cover the gap before it becomes a penalty.

Why does this feel like a tax I already paid?

Because you did pay some of it. The flat withholding is real tax, it is just not enough tax. The difference between what was withheld and what you owe is the part that quietly accrues all year and shows up as a single uncomfortable number on your return.

Forgetting estimated taxes: the penalty on top of the bill

The withholding gap turns into a penalty when you forget that income tax is pay-as-you-go. What does it cost to forget about estimated taxes after a big vest? The tax you already owed, plus a penalty for paying it late. The IRS wants its money as you earn it, not all at once next April.

When payroll withholding does not cover what you owe, the IRS expects you to make up the difference in quarterly estimated payments during the year. Miss the payment and it charges an underpayment penalty, calculated as interest on the unpaid amount across the time it sat uncovered. It is not a flat fine. The longer the gap goes unpaid, the more it grows, which is why catching it early in the year costs less than catching it in April.

Can I just bump my W-4 instead of paying quarterly?

Often, yes, and it can be cleaner. Payroll withholding counts as paid evenly across the year even if you raise it late, while an estimated payment only counts for the quarter you make it. If you spot a shortfall mid-year, raising your W-4 can smooth the timing in a way a late estimated payment cannot.

What if the vest was a one-time event like an IPO?

A single large settlement can blow past your safe harbor in one quarter. Size the gap right after the event and make the estimated payment for that quarter rather than waiting. The private-company version of this, where you may not even be able to sell to pay, is its own section below.

The double-taxed 1099-B: the most expensive trap that costs nothing to make

What is the most expensive RSU mistake that costs you nothing to make? Letting your broker’s 1099-B report a $0 cost basis, which makes you pay tax twice on the same shares. The money is overpaid silently, the form looks official, and the IRS will happily accept the larger number. This is the single most common RSU overpayment I see, and it hides in plain sight on a document most people forward to their accountant without reading.

You already paid full ordinary income tax once, at vesting, on the entire value of the shares. That vesting value is supposed to become your cost basis, the part of a future sale that is not gain and does not get taxed again. The double-tax happens when that basis goes missing on your 1099-B. If the form shows your basis as $0, it reports your whole sale price as a capital gain. So the vesting value gets taxed a second time, now as a gain, on top of the income tax you already paid on it at vesting. Same dollars, two taxes, one because of a blank field.

This is not your broker making an error. A reporting rule generally bars brokers from including the compensation income, the vesting value, in the basis they report to the IRS for this kind of equity. So the official 1099-B basis often comes through as $0 or as only what you paid out of pocket, which for RSUs is usually nothing. The real basis still exists, almost always on a supplemental statement that does not go to the IRS.

Selling into short-term gains: the calendar mistake

Can the day you click sell change your tax rate on the same shares? Yes, and selling a few days too early can hand the IRS a much bigger slice for nothing. After your RSUs vest, the shares behave like any stock you own at the vesting price, and anything they gain after that is a capital gain. How long you hold past vesting decides the rate.

Hold more than a year past vesting and the gain is long-term, taxed at the lower long-term rate. Sell within a year and it is short-term, taxed at your ordinary rate, the same high rate as your salary. For 2026 long-term capital gains are taxed at 0%, 15%, or 20% depending on income, while short-term gains are taxed at your ordinary rate, where the top is 37% 2026. For a high earner, that gap is wide enough that a few days near the line genuinely matters.

The clock starts at vesting, not at grant. That is the detail that catches people. Your shares vested on a specific date, and the one-year line runs from that date, share by share, lot by lot. A grant that vested monthly has a different clock for every batch. So you look at a position, decide to sell, and sell the lot that happens to be eleven months and three weeks old. You just turned a long-term gain into a short-term one by a margin of days.

Watching the clock is worth a few days of patience when the shares are right at the line and the only thing standing between you and the lower rate is the calendar. It is not a reason to hold a concentrated position you wanted to sell. This is where the tax tail starts wagging the dog: people hold a stock they should have trimmed, telling themselves they are waiting for the long-term rate, and the position drops more than the tax would ever have cost. The rate is a tiebreaker, not the whole decision, and the full sell-or-hold logic is in sell at vesting or hold.

The blackout window: when you cannot sell at all

Why can’t you sell your own vested shares whenever you want? Because if you work at a public company, a blackout window can lock you out for weeks at a stretch, and it tends to slam shut right when you have a reason to sell. A blackout window is a stretch of time when employees who might know material non-public information cannot trade the company’s stock, usually the weeks leading up to an earnings release. The point is to keep you from trading on information the market does not have yet.

Here is the part people learn the hard way. Earnings is exactly when the stock moves most, and earnings is exactly when you are locked out. You watch the price run up, you decide you want to take some off the table, and the window is already closed. Or you are waiting for the window to open so you can sell, the company pre-announces bad news, and now you are selling into a hole you saw coming and could not act on. A blackout does not just delay a sale. It strips your timing control during the only weeks the price was going to swing enough to matter.

The clean way out is to take the decision out of the window entirely with a 10b5-1 plan, a selling schedule you set up in advance while the window is open and you have no inside information. Once it is in place, the broker sells on the schedule you wrote, even during a blackout, because you made the decision back when you were allowed to. The full mechanics, and how to build one to match a diversification target, are in sell at vesting or hold.

Do blackout windows apply to me if I am not an executive?

Often, yes. Many companies apply the trading window to a wide group of employees, not only the C-suite, because rank-and-file engineers and managers can hold material information too. Check your company’s insider trading policy for who is covered and when the windows open and close. Do not assume you are exempt because you are not an officer.

The wash sale your own vesting springs

Can selling your company stock at a loss leave you with no loss to deduct? Yes, if RSUs or ESPP shares are landing around the same time. A wash sale happens when you sell a security at a loss and buy the same security within a 30-day window on either side of the sale. When that happens, the IRS disallows the loss for now. The loss is not gone forever, it gets added to the cost basis of the replacement shares, so you recover it later when you sell those. The catch is timing: you wanted the loss this year, and the rule pushes it down the road.

Here is the part that catches people with RSUs and ESPPs. A wash sale needs a purchase of the same stock near the loss sale, and with equity comp those purchases happen on their own. RSUs vesting counts as acquiring shares. An ESPP buying shares on the purchase date counts too. So you can sell your company stock at a loss to harvest it, and a routine vest or an ESPP purchase three weeks later quietly becomes the matching buy that disallows your loss. You did not place a trade. The grant did it for you.

The simple defense is to know your vesting and ESPP purchase dates before you sell at a loss. If you want the loss to count this year, keep the loss sale outside the 30-day window around any vest, purchase, or reinvestment of the same stock. Check the calendar first, give yourself room on both sides, and the harvest counts when you need it.

Acceleration on acquisition: the windfall with a tax tail

Your company just got acquired. Do your unvested RSUs pay out, carry over, or vanish? Two clauses in your grant decide, and most people read them for the first time the week the deal is announced, which is the worst possible time.

Your unvested RSUs accelerate the moment the deal closes, no matter what happens to your job. Change of control is the only event needed. Great for you, which is exactly why it is rare for rank-and-file employees.

You need two things: the change of control and a qualifying termination, usually getting laid off or pushed out within a set window after the deal. Survive both and your shares vest. Keep your job and they just keep vesting on the old schedule. This is the common one.

When the acquirer takes over, your unvested RSUs usually meet one of four fates, and you have to read your plan to find out which.

Acceleration sounds like pure upside, and it can sting anyway. When a chunk of unvested RSUs vests all at once, the full value is ordinary income in a single year, taxed like salary, just compressed and oversized. A large acceleration can push you into a higher bracket and trip the income surtaxes for that one year, and the flat 22% supplemental withholding 2026 your employer applies usually trails a high earner’s true marginal rate, which reopens the withholding gap from the top of this guide. Pull your grant agreement and your equity plan now, not when a deal is rumored. Learn whether you are single-trigger or double-trigger, and what counts as a qualifying termination, because that one definition can be the difference between keeping your shares and losing them.

The worst trap: a tax bill on private shares you cannot sell

If there is one RSU danger worth losing sleep over, it is this one. What is the worst thing that can happen with RSUs? Owing a real cash tax bill on shares you are not allowed to sell. The tax is real money and the asset you would use to pay it is locked in a drawer you cannot open.

Most private-company RSUs are double-trigger. They vest only when two things happen: you put in the time, and the company has a liquidity event like an IPO or an acquisition. The design is meant to protect you, so you do not owe tax on illiquid paper every year along the way. It works until the second trigger fires. The moment the company goes public, every share that already cleared its time-based vesting settles at once. All of it becomes ordinary income in that one tax year, valued at the price on the settlement date. Years of vesting compress into one bracket.

Shares settle as they vest. You can usually sell immediately to cover the tax. The income and the liquidity arrive together, so the bill is annoying but payable.

Years of vested shares settle in one event. A lockup can block selling for months. The income arrives now; the liquidity to pay for it arrives later, at a price that may have dropped.

Here is where it turns dangerous. The income lands, but a post-IPO lockup often blocks you from selling for months. So you have a large ordinary-income bill due, and the only asset that could pay it is frozen. Withholding on this income is the flat supplemental rate 2026, which usually sits below a high earner’s real marginal rate, so even after your company sells shares to cover withholding, you can still owe a large balance at filing.

You cannot stop the income from landing. You can refuse to be caught without the cash. Before any liquidity event, get a rough count of how many time-vested shares are sitting behind the second trigger, estimate the income they would throw off at a plausible price, set aside reserves outside the stock, and understand your lockup terms before the event, not after. A tax bill you saw coming is a logistics problem. A tax bill on shares you cannot sell, that you never planned for, is the thing that turns a windfall into a crisis.

What this means for you

The pattern across every one of these traps is the same: RSU income arrives faster and gets taxed harder than the withholding suggests, and the surprise is always the part nobody owned in advance. Size the withholding gap on vesting day and set the cash aside. Clear the safe harbor so a balance due is not a penalty. Supply the real basis on every 1099-B so you do not pay twice. Watch the one-year clock, the blackout calendar, and the vesting dates that can spring a wash sale. Read your acceleration clauses before a deal, and model the private-company bill before a liquidity event, not at filing.

None of these is hard once you can see it coming. The mistake is treating a predictable event as a surprise. If your vesting is large or lumpy, or a liquidity event is on your horizon, a fit check is a cheap way to find out which of these you are walking into before it costs you.