RSU Supplemental Withholding: Why Most Employees Owe More at Filing

Summary — Key Points at a Glance

What this post covers

• How RSU vesting creates ordinary income and why the withholding at that event often does not match the employee's actual tax bracket
• The federal supplemental withholding rate and where it diverges from marginal rates for high-income employees
• FICA taxes and the Additional Medicare Tax, and how they interact with RSU vesting
• California's supplemental withholding rate and the parallel state-level gap
• How employees may address the withholding gap proactively to manage estimated tax obligations

The core concept

• Employers are required to withhold income tax on RSU vesting, but they use a flat supplemental rate rather than the employee's actual marginal rate
• For employees in the 32%, 35%, or 37% federal bracket, a 22% withholding rate leaves a gap that compounds across every vesting event in the year
• FICA taxes, the Additional Medicare Tax, and California's own supplemental rate can further widen the total shortfall
• The gap often surfaces only at filing, when a large balance due arrives alongside potential underpayment penalties

For the full detail, continue reading.

RSU Basics: Vesting Is the Tax Event

A restricted stock unit (RSU) is an employer promise to deliver shares of company stock once an employee satisfies a vesting schedule — typically based on continued employment over a period of years, sometimes combined with performance conditions. Unlike stock options, RSUs carry no exercise decision. When shares vest, they are delivered automatically, and that delivery is the tax event.

At vesting, the fair market value of the shares received is ordinary income. This income is reportable on Form W-2 as compensation, subject to federal income tax, state income tax, and FICA taxes in the same manner as salary. No special tax rates apply at vesting. RSU income is earned income, not capital gains.

After vesting, the employee holds shares with a cost basis equal to the FMV recognized as income. Any subsequent appreciation or depreciation is a capital gain or loss measured from that vesting-date basis.

This two-stage structure — ordinary income at vesting, capital gain treatment afterward — is important context for understanding the withholding discussion that follows. The capital gain portion is entirely separate from the withholding issue, which concerns only the income recognized at vesting.

The Supplemental Withholding Rate

When RSUs vest, employers are required to withhold income tax. The IRS permits two methods for withholding on supplemental wages:

1. The aggregate method: Calculate withholding as if the supplemental wages were part of the employee's regular pay, using the employee's W-4 instructions. This method is more accurate but administratively complex for most payroll systems.
2. The flat rate method (the default for most employers): Apply a flat supplemental withholding rate to the entire supplemental wage amount.

Many employers use the flat supplemental wage withholding method for RSU vesting income. As of the date this article was written:

Federal supplemental withholding rates are set by IRS guidance and subject to change. Verify current rates with the IRS or a qualified tax professional.

The 22% rate applies to the vast majority of RSU recipients in a given year. The 37% rate engages only after cumulative supplemental wages in the calendar year surpass $1 million.

Where the Gap Originates

The structural mismatch arises because the flat supplemental rate does not adjust based on the employee's actual tax bracket. Federal income tax brackets for 2025:

Brackets reflect federal income tax rates in effect for 2025 as of the date this article was written. Tax brackets are adjusted annually for inflation.

Depending on filing status and total household income, some or all of the incremental RSU vesting income may fall into tax brackets above the 22% supplemental withholding rate. Every dollar withheld at 22% that falls into a higher bracket leaves a gap to be settled at filing.

A Hypothetical Illustration

The following is a hypothetical illustration for educational purposes only. It does not represent any actual investor's situation.

Assumptions: California resident; married filing jointly; $350,000 in combined W-2 salary; $200,000 in RSU vesting income; no other equity events. For purposes of this illustration, all RSU vesting income is assumed to fall within a 35% marginal federal bracket.

This illustration does not account for other income sources, deductions, or credits. Actual tax owed depends on the complete return. It illustrates only the mechanical gap produced by the supplemental rate.

FICA Taxes: The Often-Overlooked Component

Federal income tax withholding is only part of the picture. RSU vesting income is also subject to FICA taxes — Social Security and Medicare — which operate on separate rules.

Social Security Tax

Social Security tax is imposed at a rate of 6.2% on wages subject to Social Security tax, up to the annual Social Security wage base. For 2025, the Social Security wage base is $176,100. Earnings above that amount are generally not subject to additional Social Security tax, although Medicare tax continues to apply without a wage cap. Once total wages for the year — including RSU income — exceed the wage base, no further Social Security tax applies for that employee for the remainder of the calendar year.

The Social Security wage base is indexed for inflation and increases annually. Verify the current year's wage base with the Social Security Administration or a qualified tax professional.

For employees whose base salary already exceeds the wage base, RSU vesting in the same year creates no additional Social Security tax. For employees whose salary falls below the wage base, RSU income that pushes total compensation above it may trigger additional Social Security withholding on the vesting event.

Medicare Tax

Medicare tax is assessed at 1.45% on all earned income, with no wage base ceiling. This applies to every dollar of RSU vesting income regardless of total annual compensation.

Additional Medicare Tax

The Additional Medicare Tax of 0.9% applies to earned income above the following thresholds:

Thresholds reflect current law as of the date this article was written. Verify current figures with the IRS or a qualified tax professional.

The mechanics of the Additional Medicare Tax create a systematic withholding gap for married couples. Employers begin withholding the Additional Medicare Tax once cumulative wages paid to that employee exceed $200,000 in the calendar year — regardless of the employee's filing status or spousal income. An employer does not have visibility into whether the employee is married, whether a spouse also earns income, or whether other sources of earned income exist.

This produces a predictable under-withholding pattern for dual-income households. If both spouses earn $180,000 individually, neither employer will withhold the Additional Medicare Tax, since neither employee's wages cross the $200,000 per-employee threshold. But at the household level, $360,000 in combined income exceeds the $250,000 MFJ threshold by $110,000, triggering an Additional Medicare Tax liability of approximately $990 owed at filing. RSU vesting on top of salary income in that household compounds the exposure.

California Treatment: A Parallel Gap

California taxes RSU vesting as ordinary income at the state level, treating it identically to salary for California personal income tax purposes. California does not provide a preferential tax rate for capital gains. Although gain recognized after the sale of vested shares is generally capital gain for federal tax purposes, California taxes that gain using the same rates that apply to ordinary income.

When RSUs vest, California generally requires employers to withhold California personal income tax on RSU income treated as wages. Employers often withhold California income tax using the state's supplemental wage withholding rates:

California supplemental withholding rates are set by the Franchise Tax Board and are subject to change. Verify current rates with the FTB or a qualified tax professional.

California's top marginal income tax rate is 13.3% for income above $1 million; the brackets below that level range from 1% to 12.3%. For California residents in the upper state brackets, the gap between the 6.6% supplemental rate and the applicable California marginal rate can add several additional percentage points to the total shortfall.

Using the hypothetical from the earlier illustration — $200,000 in RSU vesting income for a California resident facing a 12.3% California marginal rate — the California withholding gap would be approximately 5.7 percentage points, or roughly $11,400 on that income amount. Combined with the federal gap, the hypothetical combined withholding shortfall would exceed $37,000 before any FICA adjustments.

This is one reason that California residents with RSU income frequently receive a large combined federal and state bill in April rather than just a federal bill.

The Tax Bill at Filing

Because neither the federal nor the California withholding system adjusts for individual circumstances at the time of each vesting event, the cumulative gap between what was withheld and what is owed often surfaces only when the tax return is prepared.

For employees with multiple vesting events across the year, each one adds to the running shortfall. A quarterly vesting schedule with four vesting events, each withheld at 22% federal while the marginal rate is 35%, produces a compounding shortfall by year-end.

The result is typically one or more of the following:

• A large balance due when the return is filed in the spring
• An underpayment penalty if quarterly estimated payments were not made or withholding was not otherwise adjusted
• Interest charges on any underpayment

The IRS imposes an underpayment penalty when a taxpayer fails to pay enough tax during the year through withholding or estimated payments. Under federal underpayment rules, penalties may generally be avoided if taxpayers satisfy one of the applicable safe-harbor tests, including paying sufficient tax through withholding and estimated payments based on either the current year's liability or the prior year's liability. For taxpayers with RSU income that significantly exceeds prior-year levels, relying solely on the prior-year safe harbor may limit penalties but will not prevent a large balance due.

Addressing the Gap

There are two primary mechanisms for closing the withholding gap on RSU income. The appropriate approach depends on cash flow, income predictability, and the broader financial picture.

Adjusting Salary Withholding

Employees may submit a revised Form W-4 to their employer to increase the additional dollar amount withheld from each regular paycheck. By specifying an additional flat dollar amount, it is possible to spread the anticipated RSU tax shortfall across the remaining pay periods in the year.

California residents may also submit a revised California Form DE-4 to increase state withholding from regular wages.

This approach works well when vesting events are predictable and the projected shortfall can be estimated in advance. The tradeoff is that increasing paycheck withholding reduces take-home pay throughout the year, affecting cash flow planning.

Quarterly Estimated Tax Payments

Employees may also make quarterly estimated tax payments directly to the IRS (and the California FTB) to cover the gap not addressed through paycheck withholding. Under the standard estimated tax schedule, payments are due in April, June, September, and January for income earned in the prior quarter.

This approach provides more flexibility over cash flow timing and allows payments to be calibrated to actual vesting events rather than distributed evenly across paychecks.

Practical Considerations

• Project total annual income early in the year — RSU vesting schedules are known in advance; modeling expected vesting income enables a more accurate shortfall estimate before the first event occurs
• Include all equity events — ISO exercises, ESPP purchases, NSO exercises, and bonus income in the same year interact with RSU vesting income in determining the final marginal rates
• Account for spousal income — For married households, combined income determines marginal rates and the Additional Medicare Tax threshold; per-employee payroll withholding does not capture the household-level picture
• Revisit after major changes — A job change, promotion, or significant shift in expected equity income mid-year may change the safe harbor calculation and the appropriate adjustment amount

Interaction With Other Equity Events

The RSU withholding gap does not occur in isolation. For employees with multiple forms of equity compensation, the same calendar year may include:

• ESPP purchase and sale: Ordinary income from a disqualifying ESPP disposition stacks with RSU vesting income in the same bracket calculation
• ISO exercises: ISO exercises do not create ordinary income under the regular federal income tax at the point of exercise, but they generate an AMT adjustment in the year of exercise — and high RSU income in the same year affects the income level at which that AMT adjustment matters
• NSO exercises: Non-qualified stock option exercises produce ordinary income at exercise, adding to the same income stack as RSU vesting
• Year-end bonuses: A performance bonus also subject to supplemental withholding at 22% can compound the total gap further if the marginal rate is above that level

The annual income-stacking effect is one reason equity-heavy compensation packages require year-round tax attention rather than a single review at filing time.

Questions Worth Raising With a Qualified Professional

The RSU withholding gap is mechanical and predictable — which means it is largely addressable with advance planning. The following questions are commonly relevant:

• Based on the projected RSU vesting schedule and current income level, what is the estimated federal and California withholding shortfall for the year?
• Would adjusting W-4 withholding on regular paychecks, making quarterly estimated payments, or some combination be the most practical approach given current cash flow?
• Does the projected combined household income trigger the Additional Medicare Tax, and if so, is the employer withholding for it correctly?
• Does the prior-year tax liability safe harbor apply, and if so, what is the minimum required payment to reduce or eliminate the underpayment penalty?
• Are there other equity events — ISO exercises, ESPP dispositions, NSO exercises, or bonuses — anticipated in the same calendar year that would affect the marginal rates applied to RSU vesting income?
• For California residents: how does the combination of the California withholding gap and the federal withholding gap affect the projected balance due, and what is the recommended payment approach for both jurisdictions?

Answering these questions requires projecting total income across all sources, modeling applicable brackets and thresholds, and integrating the withholding picture across payroll, equity events, and estimated payments. Many taxpayers find it easier to estimate withholding needs when projections are reviewed earlier in the year, before vesting events occur, rather than after the fact at filing.

Related Reading

• Employee Stock Purchase Plans: The Tax Complexity Behind the DiscountRead article →
• Incentive Stock Options (ISOs) and the Alternative Minimum TaxRead article →
• Estimated Taxes and the Safe Harbor RuleRead article →

This post is for general educational purposes only and does not constitute tax or investment advice. Individual tax situations vary; consult a qualified tax professional or financial advisor before making planning decisions. The examples in this post are hypothetical and simplified for educational purposes only and do not represent the experience or results of any actual individual. Federal supplemental withholding rates, tax brackets, FICA rates, and thresholds reflect rules in effect as of the date this article was written and are subject to change; verify current figures with the IRS or a qualified tax professional. California supplemental withholding rates are set by the Franchise Tax Board and are subject to change; verify current rates with the FTB. The California top marginal rate of 13.3% referenced in this post applies to income above $1 million; actual California rates vary by income level and filing status. The Social Security wage base and Additional Medicare Tax thresholds are subject to periodic adjustment. Advisory services offered through Trusted Path Wealth Management, LLC, an investment adviser registered with California. Registration does not imply a certain level of skill or training.