Why They Have Different Deadlines—and Why It Matters
One of the most common areas of confusion for self-employed business owners is the difference between Solo 401(k) contribution deadlines and Roth Solo 401(k) conversion deadlines. While both strategies are powerful tools for building tax-advantaged retirement wealth, they follow very different IRS rules—and misunderstanding those rules can lead to missed opportunities or unintended tax consequences.
This article breaks down the key differences so you can confidently time your contributions and conversions the right way.
Watch: Don’t Confuse the solo 401k contribution with the conversion rules
Solo 401(k) Eligibility Refresher
Before diving into deadlines, it’s important to confirm eligibility.
A Solo 401(k) is designed for owner-only businesses. You are eligible if:
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You have no non-owner W-2 employees working 1,000+ hours per year
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You may exclude:
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Employees under age 21
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Independent contractors (1099 workers)
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Spouses who work in the business may also participate
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Businesses with multiple owners may qualify, as long as there are no qualifying non-owner employees
If these conditions are met, your business can open a Solo 401(k).
Solo 401(k) Contribution Deadlines
Contributions Follow Tax-Return Rules
Solo 401(k) contributions are tied to your business tax return—not the calendar year.
Contributions are based on your net self-employment income and can generally be made up to your business tax-return due date, plus extensions.
Contribution Deadlines by Business Type
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Sole Proprietorships & Single-Member LLCs
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Deadline: April 15
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With extension: October 15
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Partnerships & S-Corporations
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Deadline: March 15
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With extension: September 15
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C-Corporations
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Deadline: April 15
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With extension: October 15
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These deadlines apply to all contribution types, including:
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Employee deferrals (pre-tax or Roth)
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Employer profit-sharing contributions (pre-tax or Roth)
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Voluntary after-tax Solo 401(k) contributions
This means you can often make prior-year contributions in the current year, as long as they are made by the applicable tax-return deadline.
Roth Solo 401(k) Conversion Deadlines
Conversions Follow Calendar-Year Rules
Roth Solo 401(k) conversions operate under a completely different rule set.
All Roth conversions must be processed by December 31 to count for that calendar year.
This rule applies to:
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Pre-tax Solo 401(k) → Roth Solo 401(k) conversions
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Voluntary after-tax Solo 401(k) → Roth Solo 401(k) or Roth IRA conversions
Unlike contributions, conversions cannot be backdated. A conversion is always reported in the year it actually occurs.
Voluntary After-Tax Solo 401(k) Conversions
Deadline Is Less Critical
When converting voluntary after-tax Solo 401(k) contributions, timing is usually far less critical from a tax perspective.
Why?
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Voluntary after-tax contributions were already taxed
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Only any earnings are taxable at conversion
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Whether the conversion happens in the contribution year or the following year, the tax outcome is typically the same
For example:
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You may make a 2025 voluntary after-tax contribution in 2026 (by the tax-return deadline)
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You then convert those funds to Roth in 2026
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The conversion is reported in 2026—but generally results in little or no tax
Missing the December 31 deadline for after-tax conversions is usually not a tax problem, though converting sooner minimizes earnings.
Pre-Tax Solo 401(k) Conversions
Deadline Is Critical
Pre-tax Solo 401(k) conversions are very different.
Key rules:
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The entire converted amount is taxable as ordinary income
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Taxed at your current marginal tax rate
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Must be completed by December 31 to count for that year
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Cannot be applied retroactively
If a pre-tax conversion is delayed into the following year, it may:
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Push you into a higher tax bracket
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Increase Medicare surtaxes
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Affect other income-based thresholds
Because taxes are owed, conversions should be carefully coordinated with your broader tax plan.
Tax Payment & Reporting Rules
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Conversions are reported on IRS Form 1099-R
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Issued by January/February of the following year
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Filed with the IRS by March 31
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Contributions are not reported on Form 1099-R
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They are reported on your personal and/or business tax return
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Taxes due on pre-tax conversions must be paid with personal funds
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No mandatory 20% withholding applies
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You must plan ahead to have cash available in your personal bank account to pay the tax bill
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Key Takeaways
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Solo 401(k) contributions follow tax-return deadlines
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Roth conversions follow calendar-year (December 31) deadlines
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Voluntary after-tax conversions are flexible and usually minimally taxable
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Pre-tax conversions are fully taxable and deadline-sensitive
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Contributions and conversions are reported differently
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Taxes on pre-tax conversions are paid outside the plan
Final Thoughts
Understanding the difference between contribution rules and conversion rules is essential for maximizing the tax benefits of a Solo 401(k) while staying compliant with IRS regulations. When properly coordinated, these strategies can significantly accelerate retirement savings and reduce lifetime taxes.
If you’re unsure how to time your contributions or conversions, working with a Solo 401(k) specialist can help ensure everything is done correctly and strategically.
