What Is a Solo 401(k)?
A Solo 401(k) — officially called a one-participant 401(k) plan by the IRS — is a standard qualified 401(k) structured for self-employed workers who have no full-time employees other than a spouse. The IRS governs it under the same rules as any employer 401(k), but because only the owner (and potentially a spouse) participates, it skips the nondiscrimination testing and top-heavy rules that apply to multi-employee plans.
The defining feature: a Solo 401(k) lets you wear two hats in the same plan — employee making elective deferrals, and employer making profit-sharing contributions. That dual structure is what allows moderate-income self-employed workers to outpace what a SEP-IRA permits at the same income level.
Who Can Open a Solo 401(k)?
Eligibility has two requirements:
- Self-employment income. You need net earnings from self-employment — a sole proprietorship (Schedule C), single-member LLC, partnership, S-corp, or C-corp all qualify.
- No full-time employees other than a spouse. "Full-time" in this context generally means employees who have worked more than 1,000 hours in a year across three consecutive years. Part-time workers, independent contractors, and seasonal workers who stay below that threshold typically don't break eligibility.
A spouse who earns compensation from your business can participate in the same plan, with their own independent contribution ceiling.
2026 Solo 401(k) Contribution Limits
The IRS announced the 2026 retirement plan limits in IR-2025-229, and the exact figures are confirmed in IRS Notice 2025-67. For a one-participant 401(k) plan in 2026:
- Employee elective deferral: up to $24,500, or 100% of compensation — whichever is less. This can be traditional (pre-tax) or Roth (after-tax) if your plan document allows it.
- Standard catch-up (age 50–59 and 64+): An additional $8,000 in elective deferrals under the IRS catch-up rules, raising the personal deferral ceiling to $32,500.
- SECURE 2.0 super catch-up (age 60–63): an enhanced catch-up of $11,250 instead of the standard $8,000, raising the personal deferral ceiling to $35,750.
- Employer profit-sharing: up to 25% of W-2 compensation (for S-corp owners) or approximately 18.6% of net Schedule C profit (for sole proprietors — see calculation below).
- Total annual additions limit (Section 415(c)): $72,000 per participant, not counting catch-up contributions. Catch-up contributions sit on top.
| Participant Age | Employee Deferral Ceiling | Total Maximum |
|---|---|---|
| Under 50 | $24,500 | $72,000 |
| 50–59 and 64+ | $32,500 | $80,000 |
| 60–63 (SECURE 2.0) | $35,750 | $83,250 |
How Sole Proprietors Calculate the Employer Contribution
For sole proprietors and single-member LLC owners, the profit-sharing contribution uses net self-employment earnings as the base — not gross revenue. That base is reduced by: (1) the deduction for one-half of self-employment (SE) tax, and (2) the retirement contribution itself — a circular dependency.
IRS Publication 560 contains the worksheet to resolve that circularity. The practical result: the employer profit-sharing rate works out to approximately 18.587% of net Schedule C profit — the same effective rate that applies to a SEP-IRA's employer contribution.
Example — $100,000 net Schedule C profit:
- Employee deferral: $24,500
- Employer profit-sharing: ~$18,587 (18.587% × $100,000)
- Total contribution: ~$43,087
A SEP-IRA at the same income would cap at ~$18,587. The employee deferral — unavailable in a SEP — is the entire difference.
When Solo 401(k) Beats a SEP-IRA
The SEP-IRA guide on this site covers the SEP's strongest advantage: simplicity. No plan document, no annual filing until plan assets exceed a high threshold, and contributions are due by the tax filing deadline (including extensions) — which means you can open and fund a SEP the same day you file in October.
The Solo 401(k) gives up some of that convenience in exchange for higher contributions at moderate income levels. The crossover point is roughly $233,000 in net self-employment income — below that level, the Solo 401(k)'s employee deferral stacks enough on top of the profit-sharing contribution to exceed what a SEP-IRA permits.
Other Solo 401(k) advantages over a SEP:
- Roth option. SEP-IRA contributions are traditional (pre-tax) only. Solo 401(k) deferrals can be Roth — creating tax-free growth for any portion of your savings.
- Loan provision. You can borrow up to 50% of your vested balance or $50,000, whichever is less — an option unavailable in a SEP-IRA.
- Creditor protection. 401(k) assets receive strong federal ERISA protections in bankruptcy. SEP-IRA protection varies by state law.
The 2026 SECURE 2.0 Roth Catch-Up Mandate
Starting in 2026, SECURE 2.0 requires that catch-up contributions — the portion above the $24,500 base deferral — be designated Roth for participants whose prior-year wages from the same employer exceeded $145,000 (indexed for inflation). For an S-corp owner-employee, this means: if your 2025 W-2 wages from your own S-corp exceeded $145,000, your entire 2026 catch-up must go to a Roth account.
The practical effect: the immediate deduction disappears for the catch-up amount, but the contribution grows tax-free. Your plan document must explicitly allow Roth contributions for this to work — if the plan does not support Roth, you lose catch-up eligibility entirely under this rule.
Sole proprietors without W-2 wages have less exposure to this rule, but should confirm the IRS treatment for self-employment income with their plan administrator. For full context on how SECURE 2.0 changes retirement rules more broadly, see the SECURE 2.0 Act overview on this site.
Setup, Deadlines, and Administration
Plan establishment deadline: December 31 of the tax year in which you want contributions to count. You can fund the plan — both deferrals and profit-sharing — up to the tax filing deadline including extensions (typically October 15 for sole proprietors on extension), but the plan document must exist by year-end. This is the critical difference from a SEP-IRA, which can be established retroactively.
Where to open one: Most major brokerages offer free Solo 401(k) prototype plan documents — Fidelity, Vanguard, Schwab, and E*TRADE are common providers. Roth support and loan provisions depend on the specific plan document; confirm before choosing.
Form 5500-EZ: Once year-end plan assets exceed $250,000, you must file Form 5500-EZ with the IRS annually. Missing the deadline triggers a penalty of $250 per day, up to $150,000. This filing is not required below the $250,000 threshold.
How a Solo 401(k) Interacts With Other Self-Employed Deductions
Both the employee deferral and the employer profit-sharing contribution reduce your federal adjusted gross income — a Schedule 1 above-the-line deduction. That means the deduction applies whether you itemize or take the standard deduction, and it layers on top of other self-employed deductions.
If you claim the QBI deduction under Section 199A, note that your Solo 401(k) contributions reduce qualified business income — slightly lowering the QBI deduction base. At typical contribution levels this is a minor interaction, not a reason to avoid the plan.
The self-employed health insurance deduction stacks cleanly: both reduce adjusted gross income independently, with no shared floor or combined ceiling.
Neither deduction affects self-employment tax, which is calculated on net earnings before the retirement and health insurance deductions are applied.
This article is for educational purposes only. ClearValue Lending is a small business funding platform, not a tax advisor or financial planner. Consult a qualified tax professional before establishing or contributing to a retirement plan.
