Solo 401(k) contribution deadlines split along two lines: the type of contribution (employee deferral or employer profit-sharing) and your business entity. Sole proprietors and single-member LLCs generally have until the tax filing deadline, April 15, with a six-month extension available to October 15. S-Corporation owners have a stricter rule: the employee deferral must be elected by December 31, and the corporate return is due March 15 (September 15 with an extension). The employer profit-sharing side always tracks the business return deadline, including extensions.
Sole Proprietors and Single-Member LLCs
If you file a Schedule C, both halves of the Solo 401(k) contribution follow the same clock. You can elect and deposit your employee deferral, and fund the employer profit-sharing portion, right up to your tax filing deadline. That’s April 15 of the year after the tax year. File Form 4868 on time and you push both deadlines to October 15.
The reason the deferral gets this flexibility is practical. Self-employment income isn’t final until the return is done, so the IRS lets you decide the deferral amount at that point rather than during the tax year itself. Many sole proprietors file an extension routinely, even when the return is otherwise ready, just to keep the funding window open.
S-Corporation Owners
S-Corp owners work under a tighter rule that trips up a lot of people. The election to defer compensation must happen by December 31 of the tax year, because the deferral reduces W-2 wages and has to show up in that year’s payroll records. Deciding in February to defer prior-year salary doesn’t work. The election window has closed.
The actual deposit of the deferred money has some flexibility and can follow the corporate return deadline, but the paper trail — the election, the payroll adjustment, the W-2 treatment — must be in place by year-end.
The employer profit-sharing contribution for an S-Corp is due by the corporate return deadline, March 15, or September 15 with a timely extension.
Employer Profit-Sharing Deadlines
The employer profit-sharing contribution rule is the same regardless of entity type: it’s due by the business tax return filing deadline, including extensions.
- Sole proprietors and single-member LLCs: April 15, or October 15 with a timely extension.
- S-Corporations and partnerships: March 15, or September 15 with a timely extension.
Filing a tax extension automatically extends this contribution deadline. There’s no separate request needed for extra time on the contribution itself.
Deadline to Open a Solo 401(k) for the Prior Year
Before SECURE 2.0, you had to formally adopt a Solo 401(k) by December 31 of the tax year to claim contributions for it. Miss that date and the plan couldn’t reduce that year’s taxes at all. SECURE 2.0 changed this. For plan years beginning after December 29, 2022, a self-employed individual can adopt a Solo 401(k) after year-end and treat the plan as if it existed on the last day of the prior tax year, as long as adoption happens before the tax filing deadline (with extensions).
Both employer profit-sharing contributions and employee elective deferrals can now be made retroactively for that prior year under a plan set up after December 31. The establishment deadline follows your return date: April 15 (or October 15 with an extension) for Schedule C filers; March 15 (or September 15 with an extension) for S-Corps and partnerships.
2026 Contribution Limits
The deadlines matter more when you know what you’re funding. For 2026, the total annual additions limit — the combined ceiling for employee deferrals plus employer profit-sharing — is $72,000 before catch-up amounts.1Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
The base employee elective deferral limit is $24,500. If you’re 50 or older, you can add an $8,000 catch-up, taking the deferral ceiling to $32,500. Participants aged 60 through 63 get a higher SECURE 2.0 catch-up of $11,250 instead of $8,000, pushing the maximum deferral for those ages to $35,750.2Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 At 64, you drop back to the standard $8,000.
Deferral limits are aggregate across every 401(k) you participate in during the year. If you also have a W-2 job with its own 401(k), your combined deferrals cannot exceed the annual cap. Employer profit-sharing is calculated separately for each plan, but the deferral limit belongs to you as a person.
The maximum compensation the IRS lets you use in the employer profit-sharing calculation is $360,000 for 2026.1Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions S-Corp and C-Corp owners can contribute up to 25% of W-2 compensation on the employer side.3Internal Revenue Service. One-Participant 401(k) Plans Sole proprietors work through a circular calculation in Publication 560 that lands the effective employer rate at roughly 20% of net self-employment earnings after subtracting half of self-employment tax.4Internal Revenue Service. Self-Employed Individuals – Calculating Your Own Retirement Plan Contribution and Deduction
If You Contribute Too Much
The deadline to correct an excess employee deferral is April 15 of the year following the over-contribution, and this one does not move with a tax extension.5Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan The corrective distribution must include earnings the excess generated during the calendar year of the over-contribution.
Miss April 15 and you’re taxed twice: the excess is included in your taxable income for the year you contributed it, and taxed again when you eventually withdraw it from the plan.5Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan A plan that fails to distribute excess deferrals also risks disqualification.
This problem shows up most often when someone runs a Solo 401(k) alongside a W-2 employer’s 401(k) in the same year and doesn’t coordinate the two. If you change jobs mid-year, or hold overlapping self-employment and W-2 income, track your combined deferrals against the $24,500 limit as the year goes rather than after.
Roth Timing to Know About
If your plan document allows it, you can designate some or all of your employee deferrals as Roth. There’s no immediate deduction, but qualified withdrawals in retirement are tax-free. Starting in 2027, if your FICA wages from the prior year were $150,000 or more, any catch-up contributions you make must go into a Roth account.6Internal Revenue Service. Treasury, IRS Issue Final Regulations on New Roth Catch-Up Rule, Other SECURE 2.0 Act Provisions For 2026, that mandatory Roth rule is not yet in effect, so catch-up contributions can still be pre-tax if you prefer.