RSU Tax Calculator: Estimate Vesting Tax & Withholding Gap

An RSU Vest Creates Compensation Before It Creates an Investment

A Restricted Stock Unit starts as a promise from an employer rather than as stock you already own.

In a conventional stock-settled arrangement, shares are delivered when the vesting and settlement conditions are satisfied. The fair market value transferred at that point is generally treated as employee compensation.

That distinction matters because the first tax question is not whether the stock later rises or falls.

It is:
How much compensation value was delivered at the vest?

For this calculator:
Gross RSU Vest Value = Shares Vesting × Fair Market Value per Share at Vest

If 1,000 shares vest at a hypothetical fair market value of $50, the modeled compensation value is:
1,000 × $50 = $50,000

That $50,000 is the starting point for the vest-event tax estimate.

The example is purely mechanical. It does not say anything about what the stock will be worth later.

Shares Withheld for Taxes Do Not Reduce the Gross Vest Value

Many RSU plans do not deliver every vested share into the employee's brokerage account.

An employer may withhold shares or arrange a sale to help cover payroll taxes.

Suppose 1,000 shares vest at $50 and 300 shares are withheld for taxes.

The gross value of the vest is still:
1,000 × $50 = $50,000

The approximate value of the withheld shares is:
300 × $50 = $15,000

And the employee receives 700 net shares.

Withholding 300 shares does not turn the event into a 700-share taxable vest.

The withheld shares are part of how the employer funds or settles withholding on compensation that arose from the full vest.

That is why this RSU tax calculator keeps three figures separate:
gross shares vesting
shares withheld or sold to cover
net shares delivered

Tax Liability and Employer Withholding Are Different Calculations

This difference is the central purpose of the calculator.

A payroll system may withhold federal income tax from the RSU event.

That withholding is not necessarily the amount of federal income tax the vest ultimately creates.

Think of withholding as a payment toward tax rather than the final tax calculation itself.

The calculator therefore estimates both sides separately.

First it estimates the incremental tax attributable to the vest.

Then it estimates or accepts the employer's withholding.

The difference becomes either:
Potential Withholding Shortfall

or:
Potential Withholding Surplus

A shortfall does not mean that exact amount will definitely be due when the tax return is filed.

A surplus does not mean the employee will receive that amount as a refund.

The rest of the tax return still matters.

Why a 22% RSU Withholding Rate Can Differ From Your Actual Federal Tax

When an employer treats separately identified RSU compensation as supplemental wages and the IRS flat-rate method is available, federal income-tax withholding may use a flat supplemental percentage.

For qualifying supplemental wages within the applicable annual threshold, the 2026 optional flat rate is 22%.

But the employee's actual federal income tax is calculated under progressive tax brackets.

Those are two different systems.

Imagine a taxpayer whose existing taxable income already places additional income in a bracket above 22%.

A 22% withholding rate can then be lower than the incremental federal income tax produced by adding the RSU compensation to taxable income.

The reverse can also occur in another tax situation.

That is why the calculator does not take the employer withholding rate and label it the employee's tax rate.

The Federal Income-Tax Estimate Uses a Before-and-After Calculation

For its standard federal income-tax estimate, the tool starts with the projected federal taxable income entered before the current RSU vest.

It calculates ordinary federal income tax on that amount using the 2026 tax brackets.

Then it adds the RSU vest value and calculates the tax again.

Conceptually:
Incremental Federal Tax = Tax After RSU Vest − Tax Before RSU Vest

This matters when the vest crosses tax brackets.

Suppose part of an RSU vest falls in one bracket and the rest falls in the next bracket.

Multiplying the entire vest by one marginal percentage would miss that transition.

The before-and-after approach follows the layered structure of the federal ordinary income-tax brackets.

It is still a simplified estimate. Qualified dividends, long-term capital gains, tax credits and other special tax calculations can change the actual result.

Social Security Tax Can Stop Partway Through a Vest

Employee Social Security tax has an annual wage base.

For 2026, this calculator uses a $184,500 Social Security wage base and a 6.2% employee rate.

That means an RSU vest may be:
fully subject to Social Security tax,
partly subject to Social Security tax,
or above the remaining wage base entirely.

Consider an employee with $180,000 of Social Security wages before a hypothetical $20,000 RSU vest.

Only the remaining $4,500 below the $184,500 wage base is subject to employee Social Security tax under this simplified scenario.

The calculator does not apply 6.2% to the entire $20,000 vest.

This is why the YTD Social Security wage input matters.

Multiple Employers Can Make Social Security Withholding Look Different

The annual employee Social Security tax limit and an individual employer's withholding process can produce different-looking numbers when someone changes jobs or has more than one employer.

The calculator therefore distinguishes:
total YTD Social Security wages across employers
current-employer YTD Social Security wages

The first helps estimate the employee's annual Social Security tax attributable to the vest.

The second helps model what the current employer may withhold.

If those numbers differ, employer withholding can be higher than the employee's annual Social Security tax attributable to the current vest.

This tool shows the difference rather than silently assuming one employer.

Medicare Does Not Stop at the Social Security Wage Base

Regular Medicare tax works differently.

The employee Medicare rate is 1.45%, and there is no Social Security-style wage ceiling for regular Medicare tax.

So reaching the Social Security wage base does not make regular Medicare withholding disappear.

A large RSU vest can therefore have no remaining Social Security tax while still being subject to regular Medicare tax.

That difference is easy to miss when payroll taxes are combined into one percentage.

This calculator keeps them separate.

Additional Medicare Tax Has Two Different Threshold Concepts

Additional Medicare Tax is one of the clearest examples of tax liability and payroll withholding not always matching.

The employee's tax threshold depends on filing status.

For example, the threshold is higher for Married Filing Jointly than for Married Filing Separately.

The employer does not use those filing-status thresholds for withholding.

Instead, an employer begins Additional Medicare Tax withholding after wages it pays the employee exceed $200,000 during the calendar year.

That difference can create both underwithholding and overwithholding relative to the vest's final Additional Medicare Tax effect.

A Married Filing Jointly taxpayer may have household wages that create Additional Medicare Tax liability even though one employer has not crossed its own withholding threshold.

A Married Filing Separately taxpayer can reach the individual's tax threshold before the employer reaches $200,000.

That is why this calculator shows:
Estimated Additional Medicare Tax
and
Employer Additional Medicare Withholding
as separate lines.

The $1 Million Supplemental-Wage Threshold Matters for Very Large Vests

The flat supplemental-wage rules change when annual supplemental wages paid by the employer exceed $1 million.

For 2026, the amount within the applicable threshold can use the 22% flat method when the method is otherwise permitted.

The portion above the $1 million annual supplemental-wage threshold is subject to the mandatory 37% rate under the current federal rule.

If prior supplemental wages are $950,000 and the current modeled RSU vest is $100,000, the calculator does not apply 37% to all $100,000.

Instead, under the flat-method model:
the first $50,000 reaches the $1 million threshold
the remaining $50,000 is above it

That is why the tool asks for prior YTD supplemental wages when the flat federal withholding method is selected.

A Hypothetical RSU Withholding-Gap Example

Consider a simplified Single filer.

Assume:
projected taxable income before the RSU vest: $150,000
gross RSU vest value: $50,000
total YTD Social Security wages before vest: $180,000
YTD Medicare wages before vest: $190,000
current employer wage amounts are the same
employer uses the 22% flat supplemental federal-withholding method
no state tax is included

Under the 2026 ordinary federal brackets, the simplified federal income-tax calculation rises from $28,598 before the vest to $40,598 after adding the $50,000 vest.

The modeled incremental federal income tax is therefore:
$40,598 − $28,598 = $12,000

Only $4,500 of the vest remains below the 2026 Social Security wage base:
$4,500 × 6.2% = $279

Regular Medicare tax on the $50,000 vest is:
$50,000 × 1.45% = $725

The vest also moves Medicare wages from $190,000 to $240,000. For a Single filer, $40,000 of the modeled vest falls above the $200,000 Additional Medicare threshold:
$40,000 × 0.9% = $360

Total modeled tax attributable to the vest is:
$12,000 + $279 + $725 + $360 = $13,364

Modeled employer federal withholding at 22% is:
$50,000 × 22% = $11,000

Adding the same payroll-tax withholding produces estimated total withholding of $12,364.

The modeled difference is:
$13,364 − $12,364 = $1,000

The calculator would label that difference a Potential Withholding Shortfall, not an amount definitely owed.

The example is deliberately simplified and excludes state tax and other tax-return items.

Economic After-Tax Value and Payroll Net Value Are Different

The calculator provides two different views of the vest.

Estimated Net Value After Modeled Tax subtracts the calculator's estimated tax attributable to the vest.

It is an economic after-tax estimate.

Value After Modeled Employer Withholding subtracts what payroll is expected to withhold.

That is closer to the immediate vest-event settlement.

Those numbers can differ when withholding does not match modeled tax.

Keeping both visible helps explain why an employee can receive a certain number or value of shares at vest and still discover later that withholding was too low or too high relative to the final tax calculation.

What Happens When You Sell the RSU Shares Later?

This calculator stops at the vest event.

A later sale is a separate tax event.

Under the standard scenario, the value already included as compensation becomes important to the tax basis of the shares.

If the shares are later sold for more than that basis, a capital gain can arise.

If they are sold for less, a capital loss can arise.

The holding period also matters when determining whether a later capital gain or loss is short-term or long-term.

This tool does not calculate those post-vest gains or losses.

That avoids a common mistake: taxing the full future sale proceeds as though none of the stock value had already been included as compensation at vest. (Note: ISOs use a different tax model, which you can estimate in our ISO AMT Calculator).

Do NOT:
tax the entire future sale proceeds as gain,
use grant-date price as basis,
assume a holding period,
calculate long-term vs short-term gains.

State Tax Is Deliberately Optional

State taxation of equity compensation is not uniform.

Some employees may also have allocation questions because they worked in more than one state during an RSU earning period.

A single national calculator cannot safely solve those situations with one built-in state percentage.

For that reason, the core RSU tax calculator is federal-focused.

Advanced Mode allows a user to enter a state marginal tax rate and a separate state withholding rate as optional assumptions.

Those figures are simply added to the scenario the user wants to test.

They are not a state-specific tax determination.

Common RSU Tax-Calculator Mistakes

One mistake is using only the shares delivered after withholding to calculate taxable compensation.

Tax withholding does not normally erase the compensation value of the shares used to fund it.

Another mistake is treating the employer's federal withholding percentage as the employee's final federal tax rate.

The two can differ substantially.

Social Security is also easy to overstate if the annual wage base is ignored.

Additional Medicare Tax creates the opposite problem: the employer's withholding threshold can differ from the employee's final filing-status threshold.

Finally, grant-date value should not be casually substituted for the taxable vest-event value in a standard RSU calculation.

The tax event and the employer's plan terms matter.

What This RSU Tax Calculator Does Not Determine

This calculator is a planning tool for a conventional 2026 U.S. RSU vest.

It does not determine:
your final Form 1040 liability
whether a particular award qualifies for special tax deferral
Section 83(i) treatment
deferred-settlement taxation
state-specific sourcing rules
local income tax
post-vest capital gains or losses
tax credits
Alternative Minimum Tax
tax on self-employment or RRTA income
every employer payroll method
the exact price obtained in a sell-to-cover transaction
whether you will owe money with your return
whether a withholding surplus will become a refund

The calculator is most useful when you understand those boundaries.

Its purpose is to separate three numbers that are often blurred together:
what vested, what the vest may actually add to tax, and what the employer withheld.

That makes it easier to identify a potential withholding gap before assuming that the number shown on a pay statement is the final tax cost of the RSU.