How FICA Applies to Solo 401(k) Contributions in an S-Corp

If your self-employed business is taxed as an S-corporation and sponsors a Solo 401(k), one of the most common (and costly) points of confusion is FICA taxes.

What Is FICA Tax?

FICA stands for the Federal Insurance Contributions Act.
It’s a U.S. payroll tax that funds two major social insurance programs:

Watch: Learn when FICA Applies to Solo 401k contributions

• Social Security (6.2% employee + 6.2% employer, up to the wage base)
• Medicare (1.45% employee + 1.45% employer, plus Additional Medicare Tax where applicable)

For S-corp owners, FICA applies only to W-2 wages, not to distributions.

Many S-corp owners assume that all Solo 401(k) contributions avoid payroll taxes—but that’s not how the IRS rules work. Unlike sole proprietors and single-member LLCs taxed as sole proprietors, S-corporation owners are treated as W-2 employees of their business. Let’s break it down clearly.

That distinction is critical because:

FICA applies to W-2 wages — not to employer-paid benefits. As a result, how a Solo 401(k) contribution is made determines whether it is subject to:

• Social Security tax
• Medicare tax

Employee Salary Deferrals (Pre-Tax or Roth)

FICA treatment: Yes, subject to FICA

• These are deducted from W-2 wages

• Social Security & Medicare taxes apply

• Reduce taxable income, but not payroll taxes

• This is true for:

  • Pre-tax employee Solo 401(k) contributions

  • Pre-tax Roth Solo 401(k) contributions

Why?
The IRS treats S-corp owners as employees, not self-employed individuals, for payroll purposes.

Employer Profit-Sharing Contributions

FICA treatment: NOT subject to FICA

• Made by the S-corp, not withheld from wages

• Calculated as up to 25% of W-2 compensation

• Deductible to the corporation

• Not subject to:

  • Social Security tax

  • Medicare tax

 Voluntary After-Tax Contributions (Mega Backdoor Roth)

FICA treatment: Yes, subject to FICA

• These contributions:

  • Come from W-2 wages

  • Are made after income tax

  • Still pass through payroll

• Therefore:

  • FICA applies

  • Income tax already paid

Important:
Even though after-tax contributions are later rolled into a Roth IRA or Roth Solo 401(k), the initial contribution is still FICA-taxed.

Summary Table

Key Takeaway for S-Corp Owners

Only employer profit-sharing contributions escape FICA.
Every Solo 401(k) contribution that comes from W-2 wages—including Roth and Mega Backdoor Roth contributions—is subject to Social Security and Medicare taxes

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