Tax simulator

RSU withholding gap estimator

When RSUs vest, your employer usually withholds federal tax at a flat supplemental rate that sits below the true marginal rate a high earner actually pays, so you can owe far more at filing than was withheld, often tens of thousands of dollars on income you already received as shares. This estimates the size of that gap from a few inputs and shows where it comes from: federal, state, and Additional Medicare.

Background reading: Equity compensation

RSU income (vesting this year)

Fair-market value of shares vesting this calendar year, taxed as supplemental wages.Other ordinary income

Approximate base salary plus cash bonus and other ordinary income, excluding the RSU income above.Filing statusDeductionsHow taxable income is estimated from your income.StateSkip it and the state piece is left out rather than counted as zero.

You may owe about this much more at filing than was withheld on your RSUs

$40,800

True marginal rate 49.2% vs 22.0% flat supplemental withholding

Where the gap comes from

Federal income tax$19,500

Additional Medicare (0.9%)$1,350

State income tax (estimated)$19,950

Estimated gap$40,800

Withheld rate vs true rate

Flat supplemental withholding applied22.0%

Effective federal rate withheld22.0%

Estimated true marginal rate49.2%

Where this goes next

This is a current-year estimate, not advice. For how to handle this gap before filing, talk to a CFP.

Assumptions and simplifications

The marginal rate is estimated from your income band midpoint, not an exact return.
The state rate is approximate (from your state or a coarse bucket) and is omitted when state is skipped.
This covers the ordinary-income withholding gap only; it excludes NIIT, AMT, and phaseout-cliff effects.
Social Security and regular Medicare are assumed correctly withheld and are out of scope.
State supplemental withholding is not modeled; the state gap is computed against zero state withholding.