Employers withhold a flat 22% on RSU vests, but many RSU earners sit in the 32-37% brackets. Here's the gap math and three ways to close it before April.
Here's the short answer to why you owed a painful amount in April despite your employer "withholding taxes" on every RSU vest: RSU income is withheld at a flat 22% federal rate, but the income itself is probably being taxed at 32%, 35%, or 37% on your return. The withholding isn't wrong — it's just a default that has nothing to do with your actual bracket. The gap between the two rates, multiplied by your vest income, is the surprise bill.
This catches almost everyone the first year their equity comp gets meaningful. Salary withholding adjusts to your W-4; RSU withholding mostly doesn't. If RSUs are new to you, start with our complete guide to RSU, ESPP, and stock option taxes for the full picture — this post focuses on the withholding gap and how to close it before it becomes a penalty problem.
RSU vest income is treated as supplemental wages — the same category as bonuses and commissions. Federal rules let employers withhold on supplemental wages at a flat 22%, and nearly all of them do, because it's simple and automatic. Only once your supplemental wages for the year pass $1 million does the mandatory rate jump to 37% on the excess.
So the system is calibrated for someone whose marginal rate is around 22%. A tech employee with a $200,000 salary and $150,000 of annual vests is nowhere near that. Every dollar of RSU income stacks on top of salary and gets taxed at the top of your bracket — while the withholding stays parked at 22%. Add state income tax under-withholding in high-tax states and the extra Medicare tax that applies at higher incomes, and the hole gets deeper.
Take a single filer with a $220,000 salary and $180,000 of RSU vests during the year:
Scale the vests up or down and the gap moves with them, but the structure is constant: each percentage point of difference between your marginal rate and 22% costs you that percent of your total vest income. At $300,000 of vests and a 37% marginal rate, the gap is a 15-point spread — $45,000.
And that's assuming your basis is reported correctly when you sell. Plenty of people get hit twice: once by the withholding gap, and again by the $0 cost-basis error on their 1099-B that overstates their capital gains. Check for both.
The IRS expects tax to be paid as income arrives, not settled up in April. If your withholding falls too far short during the year, you owe an underpayment penalty on top of the balance — computed at the IRS interest rate, set quarterly. With today's rates, the penalty on a five-figure shortfall is real money, not a rounding error.
The escape hatch is the safe harbor: no underpayment penalty if your withholding and timely estimated payments reach 90% of the current year's tax, or 100% of last year's total tax — 110% of last year's tax if your prior-year AGI was over $150,000, which describes most people reading this. For high-income filers, the 110% prior-year number is the target to plan around, because it's fixed and knowable in January while your current-year tax is still a moving target.
The simplest fix. On Form W-4, line 4(c), you can direct your employer to withhold an extra flat amount from each paycheck. Estimate your annual gap, divide by remaining pay periods, and set it. Withholding has a quiet superpower: the IRS treats it as paid evenly through the year no matter when it actually comes out, so a big catch-up in November can still erase an underpayment that built up in the spring.
If you'd rather not touch your paycheck, make estimated payments on the quarterly schedule — April 15, June 15, September 15, and January 15. This works best when vests are lumpy: a big vest in August can be matched with a September 15 payment. The mechanics, vouchers, and timing rules are covered in our complete guide to estimated taxes.
Some equity platforms let you elect a higher withholding rate on vests, or you can simply sell additional shares at vest and route the proceeds to an estimated payment. Selling at vest has minimal capital gains consequence — the shares' basis is the vest-date value, so a same-day sale produces little or no gain. Holding the shares while owing the tax on them is the risky combination: if the stock drops, you still owe tax on the higher vest-date value.
One habit worth building either way: recheck the gap after every grant refresh and every meaningful move in the stock price. Vest income is share count times price, so a rising stock quietly widens a gap you sized in January — a mid-year recalculation in June or July catches it while there's still time to adjust withholding gradually instead of scrambling in December.
The flat-22% problem is chronic for public-company employees, but it turns acute at a liquidity event. If you hold double-trigger RSUs at a startup, years of accumulated grants can vest at once when the company IPOs — a one-time income spike that blows far past what 22% withholding can cover, sometimes into the mandatory 37% territory and usually with a lockup preventing you from selling. That scenario has its own playbook — see our post on double-trigger RSUs and IPO taxes.
The 22% is a default, not your tax rate. If your bracket is higher, the difference is a bill with your name on it — the only question is whether you pay it on schedule or with penalties.
RSU withholding at 22% is a systemic mismatch for anyone in the upper brackets, and it never fixes itself — every vest widens the gap. The move is to quantify your gap early in the year, pick a mechanism (W-4 extra withholding, quarterly estimates, or heavier sell-to-cover), and aim at the 110% safe harbor so penalties are off the table regardless of how the year ends. Taxagon's CPAs and EAs build exactly these projections for equity-comp clients every year — if you'd rather have the planning done for you, reach out.
This article is general information, not tax advice for your specific situation. Tax law changes; figures are for the years stated.
Talk it through with a licensed US tax professional — we'll tell you honestly whether it applies to your situation.
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