The Hidden Tax

The January moment

The conversation often begins in January. A W-2 arrives; the numbers, for once, don't resolve. Salary looks right. Bonus looks right. Then the column for restricted stock — the grants that vested quietly over the year, the ones whose value had already been mentally spent — shows something that doesn't line up. The tax withheld against them is smaller than it should be. Sometimes noticeably smaller. Sometimes by eighty thousand dollars, or a hundred and fifty.

There is a reason for this, and it is not a mistake. The Internal Revenue Code treats supplemental wages — a category that includes restricted stock, bonuses, and commissions — under a different withholding rule than ordinary salary. The default rate is 22 percent. For anyone whose real marginal bracket sits well above that, the gap is arithmetic, not error. And by the time the W-2 lands, the money is already spent.

Where the number comes from

The 22 percent figure is set by Treasury regulation, not by employer policy. Supplemental wages — a category that has come to mean, in practice, almost everything that isn't a regular paycheck — are withheld at a flat federal rate, separate from the salary calculation on the employee's W-4. For aggregate supplemental payments of $1 million or less to a single employee in a calendar year, the rate is 22 percent. Above that threshold, the law requires a mandatory 37 percent rate on the excess.

Seen one way, the mechanism is tidy. The payroll system does not have to guess at an employee's marginal bracket on the day stock vests; it applies a uniform rate and moves on. Seen another way, it is bluntness masquerading as precision. An engineer earning $180,000 is withheld at the same 22 percent as a senior executive earning $1.8 million on the dollar of supplemental income below the $1 million line. One of those two people is reasonably served by the default. The other is not.

Why 22 percent rarely matches the real rate

Consider three cases that are ordinary rather than rare.

A senior manager earning a $280,000 base salary, a $70,000 bonus, and another $150,000 in vesting restricted stock over the year sits, in 2026, somewhere in the 32 percent federal marginal bracket. The supplemental withholding on the RSU portion comes in at 22 percent. The gap — ten points on $150,000 — is roughly $15,000, before any state tax and before any Medicare surcharge.

A vice president earning $500,000 plus another $400,000 in vested restricted stock lands in the 35 percent bracket, or close to it. The same 22 percent withholding produces a gap closer to thirteen points. On $400,000 of supplemental income, that is roughly $52,000 of federal shortfall alone.

An executive with a larger equity stake — $250,000 in salary and $900,000 in restricted stock vesting over the year — sits in the 37 percent bracket on the last dollar of income. Because the aggregate supplemental total stays just under the $1 million trigger, the employer's withholding remains flat at 22 percent. The federal shortfall on the stock alone is fifteen points, or roughly $135,000.

None of these are unusual situations. They are the ordinary fact pattern of a mid-career professional at a publicly traded technology, biotech, or financial-services employer. And in every one of them, the withholding is legal, the employer has followed the rules, and the employee is still going to owe a surprising amount in April.

The $1 million tripwire — and why it isn't the fix it looks like

The law does acknowledge the problem, after a fashion. Once an employee's cumulative supplemental wages cross $1 million within a single calendar year, every dollar above that line is withheld at 37 percent — the top federal bracket. A senior executive with a large vest can reasonably expect that the last tranche of restricted stock will be withheld at a rate that approximately matches the rate at which it will be taxed.

Approximately. Not exactly. The 37 percent rule addresses federal income tax and only federal income tax. It does not solve for state withholding, which in California, New York, or New Jersey may add another ten or eleven points. It does not address the Additional Medicare Tax of 0.9 percent that applies to wages above $200,000 for a single filer or $250,000 for a married couple filing jointly — a surcharge the employer begins withholding only when its own wage payments to an employee cross $200,000, and which therefore tends to be underwithheld in two-earner households. It says nothing about the 3.8 percent net investment income tax that may apply later, when the shares are eventually sold above basis.

Some employers — many large technology firms among them — allow employees to elect 37 percent withholding on supplemental wages even below the $1 million threshold. The election, where available, narrows the federal gap considerably for anyone already in the top bracket. It does nothing for the rest of the stack.

If the rate being withheld against your largest form of variable compensation doesn't match the rate at which it will be taxed, the difference has to come from somewhere. The only real question is when.

The safe harbor nobody told you about

The federal tax system does not, in fact, require that every dollar owed in April be withheld by December 31. It requires only that the taxpayer satisfy one of two tests under §6654. For most high earners, the relevant one is 110 percent of the prior year's federal tax liability. If total withholding plus estimated payments for the current year equals at least 110 percent of what the taxpayer owed the year before, no underpayment penalty attaches, regardless of how large the balance due ultimately is.

The practical implication is subtle and often missed. The balance due in April can still be a substantial number — the total tax liability for the year is not reduced by meeting the safe harbor — but the penalty does not apply. That converts the problem from "interest and penalty on an underpayment" into "cash-flow planning for an April payment." These are different problems with different remedies.

A second feature of the withholding rules matters here. Estimated tax payments are tested quarterly: a payment made in December cannot cure an underpayment that occurred in April. But wage withholding, by statute, is treated as paid evenly throughout the year, regardless of when it was actually collected. That asymmetry is the basis of a useful maneuver. Extra withholding applied in November or December, through the salary paycheck, counts for the whole year. An employee who realizes in October that the numbers are going to be ugly may still have room to move. An employee who waits until February does not.

Three moves available before December 31

There is no universal answer here, and the useful framing is less "what to do" than "what is available." Three options are worth understanding.

The first is an employer election, where the employer permits one. Some payroll systems allow an employee to elect 37 percent federal withholding on supplemental wages in advance of a vesting event; others do not. The distinction is often buried in the stock plan administrator's documentation, not in the company's HR portal. The election, when available, tends to be all-or-nothing — it applies to every vest in the year, not just the one the employee happens to be worried about.

The second is additional flat-dollar withholding against salary. The current Form W-4 has a field, at Step 4(c), that allows the employee to specify an extra dollar amount to be withheld from each paycheck. Because wage withholding is treated as evenly paid across the year, this mechanism can rescue a shortfall identified in the final quarter. It is often the lightest-touch intervention.

The third is an estimated payment, made quarterly on Form 1040-ES. This is the bluntest tool and the one with the least flexibility; a missed installment cannot be recovered by a larger payment later. But for employees whose compensation arrives in a single mid-year lump — a double-trigger vest at an initial public offering, for example — it may be the only mechanism that moves cash at the right time.

None of these choices exists in isolation. The correct combination depends on where the rest of the tax picture sits: state of residence, household filing status, whether capital gains realized earlier in the year have already consumed the safe harbor cushion, and whether the new SALT deduction cap meaningfully interacts with the state tax owed. Under the law signed in July 2025, the cap rose to $40,000 but phases down for income above $500,000 of modified adjusted gross income — which means, for many of the households this article describes, the federal deduction for state tax paid is partial rather than full.

The pattern underneath

The 22 percent problem is visible because it produces a bill. Many of its cousins do not. Mutual funds that distribute capital gains in years the investor did not sell; rebalancing that realizes gains in a taxable account when a retirement account would have done the same work; losses that sit unused in one position while gains are harvested in another — each is a version of the same pattern, a rate mismatch between what the tax code applies and what the investor notices. The restricted-stock version is simply the one that arrives in an envelope. The rest is quieter, and over a portfolio's lifetime, usually larger.

A short diagnostic — six questions, no figures required — is available at /check. It asks about the places where rate mismatches tend to live. What it finds may or may not be news. But the stock-vest surprise, in our experience, is usually the easiest one to see.