You had a strong year. Your big, new RSU grant from your promotion started vesting. Maybe your spouse had a big year too. Or perhaps your private company RSUs were finally delivered and settled upon completing an IPO. Everything felt fine in December. Then you opened your finished return in April and the number at the bottom was far larger than you expected, with a Massachusetts surtax you either did not know existed or did not think applied to you.
Here is the straight answer. Taxes were withheld, but never against your full picture. And in Massachusetts, crossing a certain income line adds a tax on top that nobody showed you in December.
Why do I owe so much in taxes after my RSUs vest?
Because the shares that vested were taxed at a flat withholding rate that often falls short of what a high earner actually owes, and no single withholding event (or employer) knows what your whole household's income will be by year-end.
Why your withholding never saw the whole picture
When your shares vest, the company sets aside taxes at a flat federal rate reserved for one-time payments like bonuses and equity — call it supplemental withholding. For most of the year it looks reasonable. But that flat rate is frequently lower than the rate a high earner actually pays on their top dollars. State tax gets withheld at a standard rate too. Neither calculation knows what your total income will be.
That is the trap. Each piece can be withheld correctly on its own while the household as a whole lands in brackets no single event accounted for. A large equity vest in the same year as a big bonus and a spouse's strong income stacks up. Withholding was never looking at the full picture of all your sources of income.
Then there is Massachusetts. Once your household's Massachusetts taxable income crosses the surtax threshold ($1,107,750 for tax year 2026; originally $1 million when the surtax began in 2023, and indexed annually for inflation), the state applies an added 4% surtax on every dollar above that line — the so-called millionaires surtax. Your withholding did not account for it. So you owe an extra charge on income you believed was already covered. By April, it is too late to adjust withholding for money you earned last year.
A bigger-than-expected tax bill is unpleasant. A bill you have no cash to cover is a much bigger issue
The difference between what you owe and what was withheld has to come from somewhere. If you did not set it aside, the money may already be spent, invested, or committed for a specific use. Some people end up selling positions to cover a tax bill — possibly at a loss during a short-term market decline, or even triggering more taxes in the process. Other households may resort to carrying the shortfall into the next year, and end up already behind on the quarterly payments the IRS expects from people whose withholding does not cover their liability.
That is how an April surprise becomes a rolling one. You are still cleaning up last year while the current year's cash planning gets harder. Smart people make these moves all the time — not because they miss the cost, but because by April the costly options are the only ones left. Covering one tax bill by creating another is when a financial win starts to feel like it is being taken back. With advice and planning ahead, that scramble is avoidable.
This gap was knowable in December, not just in April
The mechanics are predictable. Review your total household income against the rates that actually apply — federal, state, the 3.8% net investment income tax that hits investment earnings at higher incomes, and the Massachusetts surtax where it applies. Do that before year-end and the shortfall shows up while you can still do something about it. The exact figure is your CPA's calculation. The decision to look before December 31 is yours.
Knowing early is what lets you set money aside and make an estimated payment while it still counts. An unexamined shortfall becomes a scramble for cash plus an underpayment penalty. The specific next step is to have your CPA confirm the actual number before December 31, so you can set the money aside and make that payment. If you need to make an adjustment, a financial advisor can determine the best way to produce the cash to cover your tax bill, without unexpected consequences. This might be trimming a position in your portfolio that has done really well and grown too large — done deliberately, on your timeline, rather than under deadline pressure. It could also be harvesting tax losses if you have any.
Pro-Tip: After a big vest in a strong income year, do not assume that tax withholding covered all that you owe — flat supplemental withholding routinely falls short at high brackets, and in Massachusetts a household above the millionaires surtax threshold owes an added surtax on top. Treat a shortfall as the default, set cash aside for it now rather than in April, and confirm the actual number with your CPA before year-end, while there is still time to make an estimated payment.
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Common Questions
Why is RSU withholding so inaccurate?
When your shares vest, taxes are set aside at a flat rate meant for one-time payments, not at your real top rate. For a high earner, that flat rate often falls short.
On top of that, the withholding on any single vest has no idea what the rest of your household income will be — your bonus, a spouse's income, other equity. Each piece can look fine alone while the total pushes you into higher brackets nothing accounted for. The gap is largest once your RSU (or PSU) income exceeds your salary and bonus — sometimes by a wide margin. This also shows up during the year after a promotion when RSUs become a much larger part of your compensation package and begin to vest.
What is the Massachusetts millionaires surtax and how does it affect my RSU income?
Massachusetts applies an extra 4% tax on taxable income above the surtax threshold ($1,107,750 for tax year 2026; originally $1 million when the surtax began in 2023, and indexed annually for inflation), in addition to the regular state rate. A strong year with a large vest can push you over that line, sometimes for the first time. When it does, every dollar above the threshold carries the added charge — and your vest withholding did not account for it. So you can owe a surtax on income you assumed was already covered, which is a common reason the April bill lands larger than expected.
How do I avoid a large unexpected tax bill from RSU vesting?
The reliable move is to look throughout the year, not after. Taxes are filed annually, but the IRS expects payment throughout the year — through withholding or quarterly estimated payments. Review your total household income against the rates that actually apply and have your CPA confirm the real number. That gives you time to set money aside and make estimated payments, if necessary, while that can still reduce or avoid an underpayment penalty. Once April arrives, the levers to adjust withholding or spread income are gone — the planning window is the back half of the year, not tax season.
This blog was written by Jeremy Bohne, Principal & Founder of Paceline Wealth Management. Paceline is a fee-only investment advisor serving clients in the Boston area, and on a remote basis throughout the country. Paceline specializes in helping tech and biotech leaders, business owners, physicians, and those seeking financial planning services.