The RSU Withholding Gap: Why Your Vested Shares Leave You Owing Tax

Your RSUs vested, your employer withheld tax, and you still owe five figures in April. It isn't a mistake. It's the predictable result of a withholding rule that under-collects for high earners, and it's entirely plannable.

Few tax surprises are as common, or as avoidable, as the RSU balance due. An executive watches shares vest, sees a block automatically sold or withheld "for taxes," and reasonably assumes the tax is handled. Then the return is prepared and a large amount is owed on income that already had tax taken out of it. The reaction is always the same: how do I owe more when they already withheld?

The answer is a mismatch between how RSUs are withheld and how they're actually taxed. It's built into the rules, and it widens as your income rises.

Key points

• RSU vesting is ordinary wage income on the full value, whether or not you sell.

• Employers commonly withhold federal tax at a flat 22% (37% above $1 million of supplemental wages).

• For a household in the 35% bracket, that's 13 cents short on every dollar: $26,000 on $200,000 of vesting.

• The shortfall can also trigger an underpayment penalty.

• The fix is simple once it's sized: estimated payments, a W-4 adjustment, or both.

How RSUs are taxed: the part that works

When RSUs vest, the full market value of the shares becomes ordinary compensation, reported on your W-2 exactly like salary. There's no favorable rate and no deferral: vesting is the taxable event, whether or not you sell. If you later sell, only the gain or loss after vesting is a capital item.

That part is straightforward. The problem isn't how RSUs are taxed. It's how they're withheld.

The 22% trap

RSU vesting is treated as supplemental wages, and employers may withhold federal tax on supplemental wages at a flat 22%, rising to 37% only on supplemental wages above $1 million in a year. For most executives with meaningful grants, 22% is far below their actual marginal rate of 32%, 35% or 37%.

Your federal bracket	Withheld	Shortfall per $100,000 of vesting
32%	22%	$10,000
35%	22%	$13,000
37%	22%	$15,000

The shares that were sold or withheld covered 22%. Your bracket demands more. The difference doesn't disappear. It waits for you on the return.

California, and the taxes nobody withholds

California withholds on bonuses and stock compensation at a flat 10.23%. Whether that's enough depends on your actual California rate, which ranges from 9.3% to 13.3% at higher incomes. For many households it's close; at the top brackets, it's short.

Two other taxes aren't withheld at all. The 0.9% Additional Medicare Tax is withheld only on wages above $200,000 from a single employer, so two-earner households are often under-withheld on it. And if you sell vested shares at a gain, the 3.8% Net Investment Income Tax applies once modified AGI exceeds $250,000 (joint) or $200,000 (single), with no withholding at all.

Closing the gap before April

None of this is a reason to fear RSUs. They're cash-equivalent compensation. It's a reason to plan for the true tax rather than trusting the default withholding.

Calculate the real rate, then pay the difference. The moment RSUs vest, the true tax is knowable: your marginal federal and California rate on the vesting value, minus what was withheld. Paying that difference in as an estimated payment turns the April surprise into a non-event.

Use the safe harbors. If last year's AGI was over $150,000, paying 110% of last year's tax through the year protects you from the federal penalty even if this year's tax is higher. California's rule is similar, but at California AGI of $1 million or more, estimates must be based on the current year's tax.

Adjust your W-4. For predictable, recurring vesting, extra withholding on regular paychecks closes the gap through payroll. Federal withholding counts as paid evenly through the year, so even a late-year W-4 change helps with the penalty.

Separate the tax question from the hold question. Selling at vest locks in the value already taxed and creates little additional tax. Holding is a separate decision with its own consequences. Conflating "should I hold?" with "how do I cover the tax?" is a common and costly error.

Size it against the full year. RSU income stacks on salary, bonus, ISO exercises and a spouse's wages to set your actual marginal rate. The gap can only be sized correctly against the whole year, not the vesting event alone.

Treat the 22% withholding as a deposit, not the tax. The tax is your real marginal rate on the vesting value. The difference is yours to pay, on your schedule if you plan, or all at once in April if you don't.

The engineer and the chess player

Sizing the gap is engineering: true marginal rates, safe-harbor tests, California's schedule. But RSUs also matter on the chess board in a way people overlook. Their ordinary income lands on a fixed schedule you don't control, and it fills your brackets and shrinks your AMT room before you make any other move. In a year with a large vest, an ISO exercise has less room, a Roth conversion costs more, and the SALT phase-down may already be in play. Planning around the vest calendar, rather than reacting to it, is what turns RSUs from a recurring surprise into a predictable part of the plan.

What to take from this

RSU planning is mostly a matter of getting ahead of a knowable number. Calculated before each vest, and coordinated with the rest of the year, vesting stops producing surprises and becomes just another planned, funded part of your tax picture.

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Discuss your situation with Natalie C. Papagni, CPA

If your RSUs have been leaving you with an unexpected balance due, the fix starts before the next vest. A complimentary 30-minute consultation is the natural first step.

Natalie C. Papagni, CPA — Tax, Planning & Advisory Services 4275 Executive Square, Suite 200, La Jolla, CA 92037 (858) 754-8277 | service@lajollataxcpa.com | lajollataxcpa.com

Disclaimer: This material is provided for general educational purposes only. It is not tax, legal or accounting advice, and it does not create a CPA–client relationship. Tax outcomes depend on individual facts and circumstances, and figures and rules change from year to year. Consult a qualified professional about your specific situation before acting. Figures reflect 2026 federal and California rules; examples are illustrative.