Supplemental Wage Withholding Gap Calculator 2026 β RSU, Bonus, Stock
Calculate the tax gap between 22% flat supplemental withholding and your actual marginal rate on bonuses, RSU vests, and stock options. See exactly how much you'll owe.
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Bonus, RSU vest, commission, or other supplemental wages %
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Withholding Gap (You'll Owe)
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Your Marginal Rate on Supp. Income
$0
Withheld at 22% Flat Rate
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Actual Tax at Marginal Rate
Withholding Gap Analysis
How Supplemental Wage Withholding Works in 2026
When your employer pays supplemental wages (bonuses, RSU vests, commissions) separately from your regular paycheck, they are required to withhold at the IRS flat supplemental rate of 22%. This is a simplified approach β but it creates a problem for higher earners whose marginal rate is above 22%.
The Withholding Gap Formula
Step 1: Calculate your marginal rate on supplemental income
(salary + supplemental income β marginal bracket)
Step 2: Actual tax on supplemental income
= supplemental income Γ marginal federal rate
Step 3: Withheld by employer
= supplemental income Γ 22% (flat rate)
Step 4: Gap = Actual Tax - Amount Withheld
(positive = you owe more; negative = over-withheld)
Note: State supplemental withholding varies.
CA: 10.23% | NY: flat rate | Others: vary
Safe Harbor to Avoid Underpayment Penalty
To avoid the IRS underpayment penalty, total withholding + estimated payments must cover either:
Option A: 90% of your current year (2026) federal tax liability
Option B: 100% of your prior year (2025) federal tax liability
Option B (high income): 110% of prior year tax if 2025 AGI exceeded $150,000
Quarterly estimated taxes are due: April 15 | June 16 | Sept 15 | Jan 15, 2027
Option A: 90% of your current year (2026) federal tax liability
Option B: 100% of your prior year (2025) federal tax liability
Option B (high income): 110% of prior year tax if 2025 AGI exceeded $150,000
Quarterly estimated taxes are due: April 15 | June 16 | Sept 15 | Jan 15, 2027
Extended
RSU Vest Schedule Gap Tracker
Enter up to 4 quarterly RSU vest amounts and see the cumulative withholding gap through the year.
Enter your quarterly RSU vest amounts to see the cumulative withholding gap throughout the year and the recommended Q4 estimated tax payment.
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Quarterly Vest Gap Tracker
| Quarter | Vest Amount | Withheld (22%) | Actual Tax | Quarterly Gap | Cumulative Gap |
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Frequently Asked Questions
What is the supplemental wage withholding rate in 2026?
For 2026, employers must withhold a flat 22% on supplemental wages (bonuses, RSU vests, commission payments) when these are paid separately from regular wages and the aggregate supplemental wages do not exceed $1,000,000 for the year. If supplemental wages exceed $1,000,000 in a year, the rate jumps to 37% on the excess over $1,000,000. Note: some employers may use the aggregate method instead, withholding at the effective rate on combined wages.
Why do I owe extra tax on my bonus or RSU vest?
The flat 22% supplemental withholding rate is designed for the "average" taxpayer. If your total income places you in the 24%, 32%, 35%, or 37% bracket, your bonus is actually taxed at that higher marginal rate β but only 22% was withheld. The "withholding gap" (the difference) becomes a balance owed when you file your return. Many high-income earners are surprised by a large tax bill after a good bonus year.
How do RSU vests work for taxes?
When RSUs (Restricted Stock Units) vest, the fair market value of the shares on the vest date is treated as ordinary income and reported on your W-2. Your employer withholds at the 22% supplemental rate (or allows you to cover taxes by selling shares β called "sell to cover"). If your marginal rate is above 22%, you will owe the difference. You can address this by increasing your W-4 withholding, paying quarterly estimates, or submitting a one-time additional withholding request to your employer.
How do I fix the supplemental withholding gap?
You have several options: (1) Submit a new W-4 with an additional withholding amount in Step 4(c) β this adds a fixed extra dollar amount to each regular paycheck. (2) Pay IRS quarterly estimated taxes using Form 1040-ES β due April 15, June 16, September 15, and January 15. (3) Ask your employer to withhold at a higher rate on your bonus if they use the aggregate method. (4) Make sure you have paid enough to avoid penalties β the safe harbor is 90% of current year tax or 100% of prior year tax (110% if prior AGI exceeded $150,000).
What is the IRS underpayment penalty?
If you underpay your federal tax liability by more than $1,000 and do not meet the safe harbor requirements, the IRS charges an underpayment penalty. The 2026 rate is the federal short-term rate plus 3 percentage points (approximately 7-8%). The penalty is calculated on the underpayment amount for each quarter. To avoid it, ensure total withholding + estimated payments cover at least 90% of your 2026 tax liability, or 100% of your 2025 tax liability (110% if 2025 AGI exceeded $150,000).