What this calculator does
This calculator is designed for tech workers and corporate employees receiving Restricted Stock Units (RSUs) who want to avoid surprise tax bills. When your RSUs vest, the IRS requires employers to withhold taxes. However, they typically apply a flat statutory rate that is often significantly lower than your actual marginal tax rate. This tool calculates the shortfall between what your employer automatically withholds and your true tax liability, helping you estimate potential IRS Form 2210 underpayment penalties so you can take proactive measures like adjusting your W-4 or making estimated payments. If you also hold ISOs, see our ISO vs NSO Stock Options AMT Calculator to model alternative minimum tax scenarios.
How the math works
The computation follows federal guidelines for supplemental wages, as outlined in IRS Publication 15 (Circular E). By default, employers are required to withhold a flat 22% for federal income tax on supplemental wages up to $1 million.
The calculator determines your true federal marginal tax bracket based on your combined base salary and annual vesting RSU value for the 2026 tax year (up to the maximum 37% bracket). It then adds your selected state tax rate. The tax shortfall is the difference between your true tax liability on the RSUs and the default withheld amount. If this shortfall exceeds $1,000, the tool applies an estimated 8% interest rate to calculate the potential IRS Form 2210 underpayment penalty.
Worked example
Let's walk through a concrete scenario for a single filer in California during the 2026 tax year:
- Inputs: A base salary of $180,000 and an annual vesting RSU value of $120,000. Filing status is Single, and the state tax rate is 9.3% (California).
- True Marginal Rate: The combined income is $300,000, placing the filer in the 35% federal marginal tax bracket. Adding the 9.3% state tax results in a true marginal tax rate of 44.3%.
- Actual Withheld: The employer withholds the standard 22% federal rate plus the 9.3% state rate, totaling 31.3%.
- Tax Shortfall: The true tax on the $120,000 RSUs is $53,160 (44.3%), while only $37,560 (31.3%) is withheld. This creates a shortfall of $15,600.
- Penalty: Since the shortfall is over $1,000, the estimated 8% IRS underpayment penalty is $1,248.
Frequently Asked Questions
Why do employers only withhold 22% on RSUs?
Under IRS rules, RSUs are considered supplemental wages. Employers are legally required to withhold a flat 22% on supplemental wages under $1 million, regardless of your actual overall income or tax bracket. They do not calculate your exact marginal rate when shares vest.
How can I avoid the Form 2210 underpayment penalty?
You can avoid the penalty by meeting the IRS Safe Harbor rules. You must pay either 90% of your current year's tax or 100% of your previous year's tax (110% if your adjusted gross income was over $150,000). You can achieve this by filing a new Form W-4 to increase payroll withholding or by making quarterly estimated tax payments.
Can I ask my employer to withhold taxes at my actual rate?
Most employers do not allow custom withholding rates for RSUs specifically because their payroll systems automate the statutory 22% default. However, you can offset this by adjusting your standard W-4 withholdings from your regular base salary.
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Disclaimer: This tool is for educational and scenario-analysis purposes only. It does not constitute tax, legal, or investment advice. Always consult a licensed CPA or financial advisor. Figures reflect 2026 statutory guidelines and may change.
Built and verified by The Core-AI Engineering Desk — last reviewed August 2026. Calibrated strictly to 2026 statutory figures.