Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total tax you still owe (or refund if negative)
$6,000
Federal shortfall
$6,000
State shortfall
$0
Federal tax actually withheld
$13,200
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How to use this
- 1Enter total rsu value vesting this year ($).
- 2Enter employer's supplemental withholding rate (%).
- 3Enter your actual federal marginal rate (%).
- 4Enter state tax rate on the vest (%).
- 5Enter state withholding rate applied (%).
- 6Read your total tax you still owe (or refund if negative) on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Most employers withhold federal tax on RSU vesting at the flat supplemental wage rate of 22% (37% above $1 million in supplemental wages in a year), plus Social Security, Medicare, and state tax. If your actual marginal tax bracket is higher than 22% — which is common once RSU income stacks on top of base salary — the withholding falls short, and you'll owe the difference at tax time, often with an unpleasant surprise and possibly an underpayment penalty. This calculator compares what was actually withheld against what you'll actually owe at your real marginal rate, so you can set aside the shortfall or adjust your W-4 and estimated payments before the bill arrives in April.
Worked example
Using the values the calculator loads with:
Inputs
- Total RSU value vesting this year: 60000 $
- Employer's supplemental withholding rate: 22 %
- Your actual federal marginal rate: 32 %
- State tax rate on the vest: 6 %
- State withholding rate applied: 6 %
Results
- Total tax you still owe (or refund if negative): $6,000
- Federal shortfall: $6,000
- State shortfall: $0
- Federal tax actually withheld: $13,200
What each field means
Inputs
- Total RSU value vesting this year ($)
- The total rsu value vesting this year used in the calculation, measured in $. Starts at 60000 $ so you have a working example on load.
- Employer's supplemental withholding rate (%)
- The employer's supplemental withholding rate used in the calculation, measured in %. Starts at 22 % so you have a working example on load. Accepted range: 0–40 %.
- Your actual federal marginal rate (%)
- The your actual federal marginal rate used in the calculation, measured in %. Starts at 32 % so you have a working example on load. Accepted range: 0–50 %.
- State tax rate on the vest (%)
- The state tax rate on the vest used in the calculation, measured in %. Starts at 6 % so you have a working example on load. Accepted range: 0–15 %.
- State withholding rate applied (%)
- The state withholding rate applied used in the calculation, measured in %. Starts at 6 % so you have a working example on load. Accepted range: 0–15 %.
Results
- Total tax you still owe (or refund if negative)
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Federal shortfall
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- State shortfall
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Federal tax actually withheld
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why is RSU withholding almost always too low for higher earners?
The IRS default supplemental wage withholding rate is a flat 22% up to $1 million of supplemental income in the year, regardless of your actual bracket. If your combined salary and RSU income puts you in the 32%, 35%, or 37% bracket, that flat 22% withholding on the RSU chunk leaves a real gap that compounds every vest date.
How do I avoid an underpayment penalty from this gap?
Either make estimated quarterly tax payments to cover the shortfall, or increase withholding elsewhere — many people add extra flat-dollar withholding on their regular paycheck via Form W-4 to cover the gap without having to manage separate quarterly payments.
Does the same withholding issue apply to bonuses?
Yes, bonuses are also 'supplemental wages' under IRS rules and get the same flat 22% federal withholding treatment (or 37% above the $1 million supplemental threshold in a year), so the same shortfall math applies.
Can I ask my employer to withhold more than 22% on RSUs?
Most payroll systems apply the flat supplemental rate automatically and don't offer an easy per-vest override, though some allow you to elect additional withholding on your regular paycheck. Check with payroll/HR — a few employers do allow supplemental withholding elections above the statutory minimum.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). RSU Vest Tax Withholding Gap Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/rsu-vest-withholding-gap
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/rsu-vest-withholding-gap" target="_blank" rel="noopener">RSU Vest Tax Withholding Gap Calculator — RevenueLab</a> (2026).</p>
Source: [RSU Vest Tax Withholding Gap Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/rsu-vest-withholding-gap) (2026).