RSU Tax Withholding

RSU tax withholding is the collection of income and payroll tax that an employer performs when restricted stock units vest. Vesting delivers shares whose full market value is ordinary income that day, so withholding is owed at once, and because there is no cash payment to withhold from, the employer takes the tax out of the shares themselves. It typically does this by net share settlement, keeping back a number of shares equal to the tax and remitting the cash, or by a sell-to-cover, selling enough shares on the market to raise it. The distinctive problem of RSU withholding is not the mechanism but the amount: the federal income-tax piece is usually taken at a flat supplemental rate that can sit well below the employee's actual marginal rate, so the withholding systematically falls short for higher earners.

The flat rate is a feature of the regulation, and it explains why the number is wrong so predictably. An RSU vest is a supplemental wage payment, and Treasury Regulation Section 31.3402(g)-1 lets an employer withhold federal income tax on supplemental wages at a flat rate "without reference to any payment of regular wages and without regard to any entries on the Form W-4." That flat rate is 22% for supplemental wages up to $1 million in a calendar year, rising to a mandatory 37% on the portion above $1 million. Because the rate ignores the employee's salary and W-4, an employee already in the 32%, 35%, or 37% marginal bracket has 22% withheld on income that will actually be taxed at their higher rate. The gap is not an error by the employer; it is the design of the flat-rate method.

Payroll tax is separate, mandatory, and not part of that 22%. Social Security tax applies to the vest value up to the annual wage base, Medicare tax applies without limit, and the additional Medicare tax applies above a threshold. These are withheld in addition to the income-tax piece, so an employee reading "22% withholding" should understand that figure covers only federal income tax, and the total taken out is higher once Social Security and Medicare are added, and state income tax on top of that.

The mechanism decides how many shares disappear, not how much tax is owed. Under net share settlement the employer withholds shares and no market sale occurs; under sell-to-cover the shares are sold on the open market. Either way the employee ends up with fewer shares than vested and a cash tax payment made on their behalf. What neither mechanism changes is the total liability, which is fixed by the vest value and the employee's rates. So a vest that looks fully handled, with shares withheld and a clean pay stub, can still leave a four- or five-figure balance due months later.

The remedy is arithmetic done at each vest rather than at filing. After a vest, the employee can compare the tax actually withheld against the tax the income will really bear at their marginal rate, and cover the difference with a quarterly estimated payment or by raising withholding on regular salary through a new W-4. Doing this per vest, rather than discovering the cumulative shortfall in April, is what prevents both the surprise and the underpayment penalty that can attach when too little was paid in during the year. The published restricted stock unit page covers the broader taxation and the sell-at-vest decision; this page is about the withholding gap and how it is closed.

The employer computes the vest value, applies the flat supplemental rate to the federal income-tax piece, adds payroll and state tax, and takes the total out of the shares. The step the employee has to add is checking that flat rate against their real one.

A hypothetical example built around the gap, not the mechanics of a single vest. Owen has a large grant vest worth $200,000 in a year his marginal federal rate is 35%. His employer withholds federal income tax at the flat 22% supplemental rate, or $44,000, taking it by holding back shares. But the $200,000 will actually be taxed at 35% at the margin, roughly $70,000 of federal income tax, so the withholding is short by about $26,000 before state tax is even considered. Social Security tax (up to the wage base) and Medicare tax were withheld separately and do not close that income-tax gap.

If Owen does nothing, that $26,000 lands as a balance due at filing, possibly with an underpayment penalty for having paid in too little during the year. If instead he recognizes the shortfall in the quarter of the vest and makes a $26,000 estimated payment, or files a new W-4 that raises salary withholding by roughly that amount over the rest of the year, the liability is covered as it accrues. The numbers are illustrative, but the direction of the gap, flat 22% against a 35% margin, is the systematic part.

What RSU withholding does well

• It is automatic and requires no cash from the employee: the tax is taken out of the shares at vest.
• Some of the liability is genuinely prepaid, so the vest is not entirely unfunded at filing.
• Payroll tax on the vest is collected correctly and in full, separate from the flat income-tax rate.

Where it falls short

• The flat 22% income-tax rate ignores the employee's real bracket, so it under-withholds for anyone in the 32% band or above.
• The shortfall is invisible on the pay stub and surfaces only at filing, where it can carry an underpayment penalty.
• "22% withheld" covers only federal income tax; Social Security, Medicare, and state tax are additional, which employees often overlook when estimating.