RSU Tax Shortfall Calculator
Your employer withholds federal tax at a flat 22% on RSU vests (37% above $1M YTD). At higher marginal rates, that creates a tax shortfall you only discover at filing time. Estimate yours below.
Tax year 2026 · Last updated May 3, 2026
mathstub / rsu shortfall
How much more do you owe on your RSU vest?
Your employer withheld 22% federal (IRC § 3402(g)). If you're in a higher bracket, the IRS expects the rest by April 15 — or sooner via quarterly estimates.
Estimated shortfall
On $50,000 vest · CA · tax year 2026
$6,035
extra owed beyond what your employer withheld at vest
Federal shortfall
$5,000
IRC § 3402(g)
State shortfall
$1,035
CA DOR
Per paycheck (W-4 4c)
$431.07
Form W-4, line 4(c)
Show the mathCitations included
Next steps & resources
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- Every claim cites IRC § or IRS PubReviewed against IRS primary sources.
You still owe
$6,035
How it works
- Enter your vest details. Type in the gross dollar value of the RSU vest you received (or are about to receive).
- Add your year-to-date wages. Include both regular W-2 wages and any prior supplemental wages (other RSU vests, bonuses) paid earlier this year.
- Pick your filing status and state. These determine your federal marginal rate and whether state withholding applies.
- Read the shortfall and suggested action. If the shortfall exceeds $1,000, consider a quarterly estimated payment or extra W-4 withholding for the rest of the year.
Frequently asked questions
Why does my employer withhold only 22% on RSUs?
IRC §3402(g) and IRS Pub 15 (Employer's Tax Guide) treat RSU vests as "supplemental wages" with a fixed 22% federal withholding rate up to $1,000,000 in YTD supplemental wages, then 37% on the excess (Reg. §31.3402(g)-1). Your actual marginal rate may be much higher (32%, 35%, or 37%), creating a shortfall reconciled at filing time.
What is the $1,000,000 threshold?
Per Reg. §31.3402(g)-1(a)(2), when your year-to-date supplemental wages cross $1M with this employer, every dollar above the threshold is withheld at 37% (the highest individual rate). The calculator blends the rate when a single vest crosses the threshold.
Do I need to make estimated tax payments?
You may, especially if the shortfall is over $1,000 (the IRC §6654 safe-harbor threshold). The IRS safe-harbor rules in §6654(d) let you avoid an underpayment penalty by paying either 90% of this year's tax or 100% of last year's (110% if prior-year AGI > $150k). See IRS Publication 505 (Tax Withholding and Estimated Tax) for your situation.
What if I am over-withheld?
A negative shortfall means you’ll get a refund on the vest portion at filing time. The calculator labels this clearly. Some people deliberately accept overwithholding as a forced savings mechanism.
Does this work for double-trigger RSUs?
Yes — once the second trigger occurs (typically IPO), the shares vest for tax purposes and the same supplemental-withholding math applies.
Does it cover ISO/NSO/ESPP?
Not directly. Those have different tax mechanics (AMT for ISOs, ordinary income for NSO exercise, qualifying-disposition rules for ESPP). Adjacent calculators are planned.
How accurate is the state tax estimate?
v1 uses the top marginal rate per state as an approximation. For users above the highest bracket the estimate is exact; for lower-income users it overstates state tax by a few percentage points. Use the override field to enter your exact rate.
Is this tax advice?
No — it’s an estimate based on published rules and your inputs. It does not consider AMT, multi-state residency, or other facts a CPA would catch. For decisions involving real money, talk to a licensed tax professional.
Spotted a bug, edge case, or numbers that look off? Tell us — we read every report.
