Do RSU vests count for Social Security tax?

Generally yes—RSU wage income is subject to FICA like salary until the Social Security wage base is reached for the year.

Does Medicare stop at a wage cap?

Medicare Hospital Insurance continues without a wage cap; Additional Medicare may apply above statutory thresholds.

Are NSO exercises subject to FICA?

The spread on NSO exercise is commonly treated as wages for payroll tax purposes—confirm on your W-2.

Why did payroll taxes change late in the year?

Large vests can cross Social Security wage base thresholds or Additional Medicare thresholds—changing marginal payroll tax on incremental dollars.

Where can I model NSO payroll taxes?

Use the NSO exercise tax estimator for a simplified federal view.

FICA

Social Security

Medicare

Additional Medicare

RSU

NSO

ISO

W-2

Wage Base

FICA & Medicare on Equity Compensation: What Hits Your Paycheck (RSU, NSO, ISO)

Q: Are ISO exercises subject to FICA?

ISO exercises generally are not taxed as wages for income tax at exercise; payroll tax treatment differs from NSOs—verify forms and employer reporting.

How Social Security, Medicare, and Additional Medicare apply to RSU vesting, NSO exercises, and ISO exercises. Wage base limits, withholding gaps, and why equity can trigger surprise payroll taxes.

Updated April 2, 2026

2 min read

TL;DR•Full read: 2 min read

RSU and NSO spread income is generally subject to FICA like other wages—up to the Social Security wage base, then Medicare continues with Additional Medicare tax above thresholds. ISO exercises may not trigger FICA on the ISO spread itself in many cases, but RSU/NSO equity compensation almost always flows through payroll taxes when taxed as wages. Annual wage caps mean late-year vests can change marginal payroll tax bite.

Key Takeaways

Social Security tax stops after the annual wage base—verify each tax year’s number
Medicare continues without a wage cap; Additional Medicare applies above $200K single / $250K MFJ (withholding rules vary)
Supplemental wage withholding for income tax is separate from FICA mechanics
ISO exercises are treated differently than NSO exercises for payroll tax—confirm plan reporting
Multi-employer years can affect how much SS tax you pay—two jobs don’t double the base

Executive Summary

Quick Answer

Do I pay Social Security tax on RSU vesting?

Source: Employment tax rules on wages

Quick Answer

Why did Medicare tax spike on my big vest?

Source: IRC Section 3101(b)(2)

Quick Answer

Are ISO exercises subject to FICA?

Source: ISO vs NSO regimes

Employees often anchor on income tax brackets and forget payroll taxes on equity that flows through payroll. This guide frames how payroll taxes attach—not employer-specific payroll policies.

What Is “Wages” for FICA?

Typical equity event	Often treated as wages for FICA?
RSU vest / settlement	Yes (ordinary wages)
NSO exercise spread	Yes (compensation)
ISO exercise	Not usually ordinary wages at exercise for income tax—payroll tax treatment distinct

Each year, Social Security tax applies only up to the wage base. Equity that pushes wages above the base:

Still pays Medicare,
Stops Social Security on the incremental dollar (once at/over base).

Additional Medicare Tax

Employees may owe Additional Medicare on wages above thresholds; withholding can lag actual liability—true up at filing.

Planning Notes

Time vests across tax years if your plan allows—sometimes reduces concentration of payroll taxes and income tax.
Two employers in one year: one Social Security wage base per person—see job change guide.

Disclaimer

Educational only. Employer payroll systems vary; verify pay stubs and Forms W-2.

Primary sources

Source	URL
IRC FICA provisions	https://www.law.cornell.edu/uscode/text/26/subtitle-C
IRS Pub 15	https://www.irs.gov/publications/p15

Executive Summary

Do I pay Social Security tax on RSU vesting?

Why did Medicare tax spike on my big vest?

Are ISO exercises subject to FICA?

What Is “Wages” for FICA?

Social Security Wage Base (Conceptual)

Additional Medicare Tax

Planning Notes

Disclaimer

Primary sources