Solo 401(k) Contribution Deadline

The solo 401(k) contribution deadline for self-employed individuals splits into two distinct dates: elective salary deferrals—the employee portion—must be made by April 15 (or earlier if your business doesn’t file on extension), while employer profit-sharing contributions can be deferred until the business’s tax-filing deadline, including extensions. Missing either deadline forfeits that year’s contribution, so understanding both dates is essential.

The two contribution buckets

A solo 401(k) combines two roles: employee and employer. This dual nature creates two separate contribution deadlines.

Elective deferrals are the employee portion. As an employee, you direct the solo 401(k) plan to withhold a portion of your net self-employment income and put it into the plan. This is salary reduction—you’re deferring your own pay. The maximum elective deferral for 2024 is $23,500 (or $31,000 if you’re 50 or older and make the catch-up contribution). This contribution reduces your taxable income dollar-for-dollar.

Employer profit-sharing contributions are discretionary payments your business makes to the plan on behalf of yourself (the employee). Unlike elective deferrals, these are not from your paycheck but from your business’s profits. You choose the amount—up to 25% of your net self-employment income (after the self-employment tax adjustment)—and you decide each year whether to contribute at all. Many business owners contribute $0 in weak years and larger amounts in strong years.

The critical distinction: your elective deferral is mandatory if you set up the election; your employer contribution is optional, made only if your business has profits and you decide to contribute.

Elective deferral deadline: April 15 (or before)

Elective deferrals must be deposited into your solo 401(k) plan by April 15 of the following year. If you file a tax extension (Form 4868), you get an automatic extension to October 15 for your tax return, but your elective deferral deadline remains April 15. The IRS does not extend the deferral deadline.

Why the earlier date? The IRS treats elective deferrals as if they occur during the tax year to which they relate. For the 2024 tax year, your deferrals must be in the plan by April 15, 2025. This is the same deadline for contributions to traditional and Roth IRA plans, making it a unified deadline across retirement accounts.

The April 15 deadline is strict. There is no “reasonable cause” exception; you cannot make up a missed deferral later. If you intended to defer $20,000 in 2024 but only made $15,000 in contributions by April 15, 2025, the shortfall is gone forever. You can contribute that missed $5,000 to a separate traditional-ira or Roth IRA if eligible, but not back into the solo 401(k) for that year.

Employer contribution deadline: October 15 (with extension)

Employer profit-sharing contributions have a much later deadline. You can contribute up to your business’s tax-filing deadline, including extensions.

For a self-employed individual filing as a sole proprietor or S-corp:

• No extension: Deadline is typically April 15, 2025 (for 2024 tax year)
• With extension: Deadline extends to October 15, 2025

For an LLC taxed as a S-corp or C-corp, the deadline may differ based on your entity’s fiscal year, but the principle is the same: the deadline is your business tax-filing deadline.

This flexibility is one of the solo 401(k)’s greatest advantages. You don’t have to decide on your employer contribution until you finish your tax return and know your final profits. If your business had a strong year, you can contribute a large employer portion. If it was weak, you can contribute less or nothing.

How a tax extension affects the dates

If you file a tax extension, your elective deferral deadline does not move—it stays April 15. Your employer contribution deadline does extend to October 15.

This creates a situation where you might make an elective deferral by April 15 but delay finalizing your employer contribution (and thus your business’s tax liability) until October 15 when your extension is due. This is legal and common.

Practical example: You’re a self-employed consultant with a calendar-year business. You file Form 4868 on April 14, 2025, extending your 2024 tax return to October 15. You must still make your 2024 elective deferrals by April 15. But you can wait until September to finalize your books and decide on your employer contribution, then contribute by October 15 when you file your return (the extended deadline).

Coordination with payroll and estimated taxes

If you have a solo 401(k) with elective deferrals, you must ensure your plan’s setup allows for timely contributions. If you contribute through automated payroll, set it up so that contributions post to the plan before April 15. If you contribute via a lump-sum deposit, make the deposit at least a few days before April 15 to account for processing time.

Elective deferrals also reduce your estimated tax payments (because they’re tax-deductible), so if you make a large deferral, recalculate your estimated tax for the current year and avoid underpayment penalties.

Employer contributions, by contrast, don’t affect current-year estimated taxes—they’re deducted on your final tax return filed by October 15. If your employer contribution is large, make sure your tax withholding (from W-2 income, if you have any) or your estimated tax payments are sufficient to cover your final tax bill after deducting the contribution.

What happens if you miss the deadline

Missed contributions are generally not recoverable. The IRS has provided limited relief in rare cases (natural disasters, serious illness), but routine missed deadlines cannot be fixed.

If you miss the April 15 deferral deadline, you lose that year’s deferral opportunity. You cannot “catch up” by contributing extra next year. Similarly, if you miss the October 15 employer contribution deadline (with extension), you cannot make that contribution for the past year.

However, you can contribute to the next year’s plan. If you missed the 2024 deadline, you can still make 2025 contributions by the 2025 deadline.

Solo 401(k) vs. SEP-IRA: different rules

It’s worth noting that a SEP-IRA (Simplified Employee Pension IRA) has the same October 15 (with extension) deadline for all contributions. There’s no April 15 deferral cutoff for a SEP. But a SEP-IRA allows only employer contributions, not elective deferrals, so the structure is different. Some self-employed individuals use a SEP precisely because they don’t want to manage two deadlines.

See also