HOW TO: Report Solo 401k Contributions on Tax Return (S-Corp 1120-S)

Watch: Detailed explanation of how to report Solo 401(k) contributions for S-Corporations

For small business owners operating as an S-corporation, understanding how to properly report Solo 401(k) contributions on your tax return is crucial for maximizing retirement benefits while staying compliant with IRS regulations. Solo 401(k) plans offer some of the highest contribution limits available to self-employed individuals, but the reporting requirements vary depending on the type of contributions you make.

Understanding Solo 401(k) Contribution Types

Before diving into tax reporting procedures, it’s important to understand the different types of contributions you can make to a Solo 401(k) plan:

Contribution Type	Subtypes	Tax Treatment	2025 Limits
Employee Contributions	Pre-tax (Traditional)	Reduces current taxable income	$23,500 ($31,000 if age 50+ or $34,750 if age 60-63)
	Roth	After-tax; tax-free growth	$23,500 ($31,000 if age 50+ or $34,750 if age 60-63)
	Voluntary After-tax	After-tax; used for mega backdoor Roth	Overall Limit: $70,000 ($77,500 if Age 50+ $81,250 if age 60-63)
Employer Contributions	Pre-tax	Deductible business expense	Up to 25% of w-2 wages for a business taxed as an S-corp (sum of employee, employer and after-tax can’t excess lesser of w-2 wages or overall limit)
Employer Contributions	Roth (SECURE Act 2.0)	Deductible for business; taxable to employee

Reporting Employee Contributions

Pre-Tax Employee Contributions

Pre-tax employee contributions reduce your taxable income for the year in which the contribution is made. For S-corporation owners, this means:

Your W-2 Box 1 (Wages, tips, other compensation) is reduced by the contribution amount
The contribution is reported in Box 12 of your W-2 using Code D
The “Retirement plan” box (Box 13) is checked

Example: If your S-corporation pays you $100,000 in wages and you make a $23,000 pre-tax employee contribution, Box 1 of your W-2 would show $77,000 ($100,000 – $23,000).

Roth Employee Contributions

Roth contributions are made with after-tax dollars, so they don’t reduce your current taxable income:

Your W-2 Box 1 remains unchanged (reports full wages)
The contribution is reported in Box 12 of your W-2 using Code AA
The “Retirement plan” box (Box 13) is checked

Example: If your S-corporation pays you $100,000 in wages and you make a $23,000 Roth employee contribution, Box 1 of your W-2 would still show the full $100,000.

Voluntary After-Tax Contributions

Voluntary after-tax contributions are often used for “Mega Backdoor Roth” strategies:

These contributions don’t reduce Box 1 income
Reporting on the W-2 is optional according to IRS guidelines
If reported, they appear in Box 14 of the W-2
When these funds are later converted to Roth accounts, the conversion is reported on Form 1099-R

Reporting Employer Contributions

Contribution Type	Where to Report	Tax Form	Additional Reporting
Pre-Tax Employer	Line 17	Form 1120-S	None
Roth Employer	Line 17	Form 1120-S	Form 1099-R in year of deposit

Pre-Tax Employer Contributions

Reported as a business expense on your S-corporation’s Form 1120-S, Line 17
These contributions are tax-deductible for the business
They don’t appear on your W-2

Roth Employer Contributions

Under SECURE Act 2.0, employer contributions can now be designated as Roth contributions. Per IRS Notice 2024-2:

Still reported as a deductible business expense on Form 1120-S, Line 17 for the year in which the contribution is made
Reported as taxable income to the employee for the year in which the contribution is deposited via Form 1099-R
This creates a unique situation where contributions made for tax year 2024 but deposited in 2025 are deductible on the 2024 business return but taxable to the employee in 2025

W-2 Reporting Quick Reference

Contribution Type	Box 1 Impact	Where to Report	Code/Field
Pre-tax Employee	Reduced by contribution amount	Box 12	Code D
Roth Employee	No reduction (full wages)	Box 12	Code AA
Voluntary After-tax	No reduction (full wages)	Box 14 (optional)	No specific code
All types	N/A	Box 13	Check “Retirement plan”

Important Deadlines for Solo 401(k) Contributions

Solo 401(k) contributions can be made up to the business tax return deadline, including extensions. This means:

For calendar year S-corporations, contributions for 2024 can be made until March 17, 2025 (or 9/15/2025 with timely filed extension)

Mega Backdoor Roth Reporting Considerations

The mega backdoor Roth strategy involves a two-step process:

Making voluntary after-tax contributions
Converting those funds to a Roth account (either Solo 401(k) Roth or a Roth IRA)

While the initial contribution reporting is optional, the conversion must be reported on Form 1099-R (note: My Solo 401k Financial offers as an optional free service for those that provide the information needed to prepare the 1099-R in a timely fashion).

Common Reporting Mistakes to Avoid

Failing to reduce W-2 Box 1 income for pre-tax employee contributions
Not using the correct codes in Box 12 (D for pre-tax, AA for Roth)
Forgetting to check the retirement plan box on your W-2
Misreporting employer contributions on the wrong line of Form 1120-S
Overlooking Form 1099-R requirements for Roth employer contributions or after-tax conversions

Conclusion

Properly reporting Solo 401(k) contributions on your S-corporation tax return is essential for claiming the tax benefits these retirement accounts offer. The reporting method varies based on whether you’re making employee or employer contributions and whether they’re pre-tax, Roth, or voluntary after-tax.

By understanding these reporting requirements, S-corporation owners can confidently build their retirement savings while properly documenting their contributions on their tax returns.

Disclaimer: This article is provided for educational purposes only and should not be construed as tax, legal, or investment advice. Please consult with your tax professional for guidance specific to your situation.