Self-employed? Your solo 401(k) has split deadlines. Employee deferrals are due December 31. Employer profit-sharing contributions follow your tax filing deadline. Miss either one, and that portion of your contribution is gone for the year.
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The combined total of both contribution types cannot exceed $70,000 in 2026 ($77,500 if you're 50 or older). For the full limit breakdown, see the 2026 contribution limits guide.
This is the deadline that catches first-time solo 401(k) owners. Unlike a SEP IRA, which can be established up to your tax filing deadline, a solo 401(k) plan must be adopted (paperwork signed with your plan provider) by December 31 of the tax year you want to contribute for.
If you're considering a solo 401(k) for the current tax year, don't wait until December to start the paperwork. Most providers (Fidelity, Schwab, Vanguard, eTrade) take 1-3 weeks to process applications. Starting in November gives you a buffer.
No plan by December 31 = no solo 401(k) contributions for that tax year. No exceptions.
IRS Publication 560
Filing a tax extension extends only the employer contribution deadline. It does not extend the employee deferral deadline.
| Business type | Standard deadline | With extension |
|---|---|---|
| Sole proprietor (Schedule C) | April 15 | October 15 |
| Single-member LLC | April 15 | October 15 |
| S-Corp (Form 1120-S) | March 15 | September 15 |
| Partnership (Form 1065) | March 15 | September 15 |
IRS filing deadlines, 2026
The extension gives you extra months for the employer profit-sharing portion, which is often the larger piece for high earners. But you must file the extension before the original deadline. A 401(k) deadline reminder set for both December 31 and your tax filing date covers both halves.
It depends on the contribution type. Employee salary deferrals (elective deferrals) are due by December 31. Employer profit-sharing contributions are due by your business tax filing deadline, including extensions. For sole proprietors, that means April 15 (or October 15 with an extension).
Only the employer contribution portion. Filing a tax extension (Form 4868 for sole proprietors, Form 7004 for S-corps) extends the employer contribution deadline to October 15. Employee deferrals are always due December 31, regardless of any extension.
The plan must be established (adoption agreement signed) by December 31 of the year you want to make contributions for. You cannot retroactively open a solo 401(k) after year-end, unlike a SEP IRA which can be established up to the tax filing deadline.
Up to $24,500 as employee deferrals, plus up to 25% of net self-employment income (20% for sole proprietors after the self-employment tax deduction) as employer profit-sharing. The combined total cannot exceed $70,000 ($77,500 if 50 or older).
A solo 401(k) allows both employee deferrals and employer contributions, while a SEP IRA only allows employer contributions. The solo 401(k) typically lets you contribute more at lower income levels because of the employee deferral component. The tradeoff: the plan must be established by December 31, not the tax filing deadline.
Those employee deferrals are permanently lost for that tax year. You cannot make them up later. You can still make employer profit-sharing contributions until your tax filing deadline (with extensions), but the employee deferral portion is gone.
Set a free reminder for December 31 so your employee deferrals don't slip. Then set another for your tax filing deadline to handle employer contributions.
Set My Solo 401(k) ReminderLast modified: