RSU withholding: what it is and why it often falls short

For a lot of employees, the moment RSUs vest feels like a win. Shares hit your account, your net worth ticks up, and the hard work behind that grant feels like it paid off. What is less visible at that moment is the tax bill that may already be forming, and whether the withholding that just happened will be enough to cover it.

For many high earners, it won't. The gap between what was withheld and what is actually owed tends to surface months later at filing, by which point the options for addressing it are limited.

Understanding how RSU withholding works, where it commonly falls short, and what options exist for closing the gap is one of the more practical things people with meaningful equity can do for their after-tax returns.

What is RSU withholding?

RSU withholding is the process by which your employer collects taxes on RSU income at the time of vesting and remits them to the IRS on your behalf. When your shares vest, the fair market value of the shares delivered on that date is treated as ordinary income, and your employer is required to withhold taxes on that amount before the shares reach your account.

The mechanics are similar to payroll withholding on your salary, but the amounts involved can be significantly larger, particularly during a vesting cliff or a year with multiple tranches vesting at once.

How RSU withholding works

The two most common withholding methods are share withholding and same-day sale, both of which produce the same tax event at vesting. The difference between the two is operational rather than tax-related.

With share withholding, your employer sells enough of your vested shares to cover the tax obligation and delivers the remaining shares to you. With a same-day sale, all vested shares are sold immediately and you receive the after-tax cash proceeds.

In either case, the withheld amount goes directly to the IRS, and the transaction is reflected on your W-2 at year-end. The RSU income shows up as wages, and the withholding appears alongside it.

Unfortunately, those two numbers do not always reflect the full tax liability.

Why RSU withholding often falls short

The IRS requires employers to withhold taxes on supplemental wages, which includes RSU income, at a flat 22% rate for most employees, regardless of their actual marginal tax bracket. For supplemental wages above $1 million, the rate increases to 37%, but the 22% flat rate applies to the vast majority of RSU recipients below that threshold.

For someone in the 32%, 35%, or 37% federal bracket, the math creates an immediate gap. The employer withholds at 22%, but the actual federal liability is higher. That gap does not disappear; it accumulates and settles when you file.

Here is a simplified example: a $200,000 RSU vest with 22% withholding generates $44,000 withheld. For someone with a 35% marginal rate, the federal liability on that income is closer to $70,000, leaving a $26,000 shortfall before state taxes are even considered.

High-tax states add meaningfully to that gap. California and New York, for example, can add 9% to 13% on top of the federal shortfall, leaving you with a much higher tax balance than the federal number alone suggests.

What taxes may apply to RSUs?

Federal income tax is the largest component, but several other taxes can apply to RSU income at vesting, including:

• FICA taxes, including Social Security up to the annual wage base and Medicare at 1.45%, apply to RSU income as ordinary wages.
• The Additional Medicare Tax of 0.9% applies to wages above $200,000 for single filers and $250,000 for married filing jointly.
• State income tax applies in most states and is calculated based on your state of residence and, in some cases, the state where you worked when the shares vested. For employees who have changed states between grant and vesting, state tax treatment can be particularly complex.

The Net Investment Income Tax of 3.8% may also apply later, when shares that have appreciated since vesting are sold. While that event is separate from the vesting tax, it is worth factoring into the full picture of what RSU income costs across both events.

What happens after RSUs vest?

Vesting is the first tax event. Selling the shares, whether you do so immediately or later, is the second.

If you sell your RSUs immediately at vesting, the gain or loss is minimal because your sale price and cost basis are nearly identical. The vest-date fair market value becomes your cost basis, so what you pay and what you receive will be very close in value.

If you hold the shares after vesting and sell later, any appreciation above your vest-date basis is treated as a capital gain. If you wait a year before selling, the gain will qualify for long-term capital gains rates, which are lower than ordinary income rates. If you sell within a year of vesting, the gain will be taxed as a short-term gain at ordinary income rates.

That holding decision is where after-tax returns can diverge meaningfully depending on what the stock does and how the gain is eventually taxed. It is also where concentration risk enters the picture, since holding employer stock means your investment portfolio and your income are tied to the same company.

Common RSU withholding mistakes

Several patterns come up regularly among employees with RSU grants.

• Assuming withholding equals the final tax bill. Withholding is a starting point, not a final accounting. The difference settles at filing, and for high earners, that difference can be substantial.
• Ignoring state taxes. Federal withholding gets most of the attention, but state taxes can add substantially to the total bill.
• Holding too much company stock after vesting. Shares received through RSU vesting will add to an existing concentration in employer equity, and many employees hold those shares longer than their portfolio strategy would otherwise justify. Concentration risk is always present, even when the stock is that of your employer.
• Large vesting years pushing you into a higher bracket. When multiple tranches vest in a single year, or when a vesting event coincides with a large bonus or other income event, the combined effect can push taxable income into a higher bracket than expected.
• Not setting aside cash for the potential tax balance. Without proactive planning, the tax bill that arrives in April can create cash flow pressure, particularly if the shares have declined in value since vesting.

How to close the withholding gap before tax time

If you expect your withholding to fall short, a few options can help.

Adjusting W-4 withholding from regular paychecks is one approach. Requesting additional flat dollar withholding throughout the year can offset the RSU gap incrementally.

This works best when vesting events are predictable enough to estimate the shortfall in advance. The IRS has a Tax Withholding Estimator that can help you try to predict your withholding.

You can also make estimated quarterly tax payments using a Form 1040-ES, which are due in April, June, September, and January. Underpayment penalties apply if you owe more than $1,000 at filing and have not paid at least 90% of the current year liability or 100% of the prior year liability.

Some employers also allow supplemental withholding elections specifically for equity compensation events. Reviewing equity plan documents or checking with HR or a stock plan administrator can clarify whether that option is available.

Some practical questions worth working through before your shares vest include: how much is vesting, what tax bracket you are likely in for the full year, whether the 22% rate will cover that liability, and whether any adjustments to withholding or estimated payments are needed before year-end. Getting ahead of the gap before December costs far less than discovering it in April.

RSU withholding is automatic, but it rarely captures the full tax liability. For high earners, the gap compounds across federal, state, and additional taxes. The earlier you plan for it, the more options you have to close it before filing.