Solo 401(k) Rules
A Solo 401(k) — also called an Individual 401(k) or One-Participant 401(k) — is a retirement plan for self-employed individuals with no employees other than a spouse, and it offers the highest retirement contribution limits available to solo self-employed workers. Under 26 U.S.C. §§ 401 and 415, the solo 401(k) allows contributions in two capacities simultaneously: as an employee (up to $23,500 in elective deferrals in 2026, plus $7,500 catch-up for those 50+) and as the employer (profit-sharing contributions of up to 25% of net self-employment income). Combined, contributions can reach $70,000 ($77,500 with catch-up). This is the highest single-account retirement savings vehicle available to a solo operator — particularly powerful for lower-income earners, where the flat $23,500 employee deferral far exceeds what a SEP-IRA's 20%-of-income formula would allow. Solo 401(k) plans also permit Roth contributions, participant loans, and investment in alternative assets — features unavailable in a SEP-IRA. One deadline matters: the plan must be established by December 31 of the tax year (not just funded by the tax return due date, as with a SEP-IRA). Once plan assets exceed $250,000, a Form 5500-EZ must be filed annually with the IRS.
Current Law (2026)
A Solo 401(k) (Individual 401(k)) is a retirement plan for self-employed individuals with no employees other than a spouse, combining employee deferrals with employer profit-sharing contributions.
| Parameter | 2026 Value |
|---|
| Employee deferral | $23,500 | | Catch-up (50+) | $7,500 | | Enhanced catch-up (60-63) | $11,250 | | Employer profit-sharing | Up to 25% of compensation | | Total limit (under 50) | $70,000 | | Total limit (50+) | $77,500 | | Total limit (60-63) | $81,250 |
Legal Authority
- 26 U.S.C. § 401 — Qualified pension, profit-sharing, and stock bonus plans
- 26 U.S.C. § 415 — Limitations on benefits and contribution under qualified plans
- IRC Section 401(k) — Cash or deferred arrangements
- IRC Section 415(c) — Defined contribution limit
How It Works
The Solo 401(k)'s power comes from combining two contribution types simultaneously. As an employee, you can defer up to $23,500 in 2026 ($7,500 additional catch-up if 50+, or $11,250 if ages 60–63) — a flat dollar amount unrelated to income level. As the employer, you can make profit-sharing contributions of up to approximately 20% of net self-employment income (25% of W-2 wages for incorporated businesses), calculated after first subtracting the self-employment tax deduction. At $80,000 in net SE income, a sole proprietor can contribute roughly $23,500 (employee) + $14,870 (employer) = $38,370 — compared to only $14,870 in a SEP-IRA at the same income. The structural advantage is the flat employee deferral: it doesn't shrink as income falls, making the Solo 401(k) decisively better than a SEP-IRA for most earners below approximately $350,000.
Employee deferrals can be designated as Roth (after-tax, grows tax-free). Under SECURE 2.0, employer profit-sharing contributions can also be designated as Roth if the plan document allows — though not all Solo 401(k) providers have updated their documents to support this feature yet. Solo 401(k) plans can include a loan provision — borrowing up to the lesser of 50% of your vested balance or $50,000 — a feature SEP-IRAs structurally cannot offer. A spouse who works in the business can participate as a full plan member with their own employee deferral and employer profit-sharing allocation, potentially doubling household contribution capacity to more than $140,000/year. Once total plan assets exceed $250,000, you must file Form 5500-EZ annually by July 31 of the following year — failing to file triggers a $250/day penalty up to $150,000.
Solo 401(k) balances are not IRAs — a crucial distinction for anyone using the backdoor Roth strategy. SEP-IRAs are IRAs and trigger the pro-rata rule (IRC § 72) when converting non-deductible IRA contributions to Roth, forcing you to recognize a portion of pre-tax money as income. A Solo 401(k) is invisible to the pro-rata calculation, which means rolling an existing SEP-IRA balance into the Solo 401(k) zeroes out your IRA balance and enables clean backdoor Roth conversions every year thereafter. The plan remains available only as long as the business has no non-spouse W-2 employees; hiring even one non-spouse employee (including part-time employees who meet the SECURE 2.0 long-service threshold) typically requires converting to a standard 401(k) with nondiscrimination testing — adding administrative cost and potentially requiring matching contributions.
How It Affects You
If you're a self-employed freelancer or consultant: The Solo 401(k) almost always beats a SEP-IRA until you're earning above roughly $350,000. Here's why: at $80,000 in net self-employment income, a SEP-IRA allows approximately $14,900 in contributions (20% of net SE income). The Solo 401(k) lets you contribute that same $14,900 as employer profit-sharing PLUS $23,500 as an employee deferral — a total of roughly $38,400. The employee deferral is fixed regardless of your income level, which is the structural advantage for earners below the high end.
If you use the backdoor Roth strategy: The Solo 401(k) is the right plan structure for high-income freelancers who want clean backdoor Roth conversions. SEP-IRAs are IRAs — they trigger the pro-rata rule when you convert a non-deductible IRA contribution to Roth, forcing you to recognize a proportionate share of pre-tax money as income. Solo 401(k) plans are not IRAs, so they're invisible to the pro-rata calculation. The move: roll any existing SEP-IRA balance into your Solo 401(k) to zero out your IRA balance, then do clean backdoor Roth conversions every year. This is one of the most valuable planning moves for self-employed people earning over $150,000.
If your spouse works in the business: Each spouse who works in the business can make their own $23,500 employee deferral plus receive employer profit-sharing contributions from the business. For a couple where both spouses work in the same sole proprietorship or S-corp, the combined contribution capacity can exceed $140,000 per year — and up to $162,500 if either spouse is ages 60-63 and uses the enhanced catch-up. This makes the Solo 401(k) one of the most tax-efficient structures available to small family businesses.
If you're ages 60-63: The SECURE 2.0 enhanced catch-up provision — $11,250 instead of the standard $7,500 — applies to Solo 401(k) plans. That means your employee deferral alone can be $34,750 in 2026, plus employer profit-sharing on top, for a total that can approach $81,250. This four-year window (ages 60-63) is the highest annual contribution period available under any defined contribution plan structure. If you're self-employed and in this window, maximizing Solo 401(k) contributions should be near the top of your tax planning priorities.
If you're watching the non-spouse employee trigger: The Solo 401(k) is only available to businesses with no employees other than a spouse. If you hire even one non-spouse W-2 employee — part-time or full-time — you typically must convert to a standard 401(k) plan subject to nondiscrimination testing. SECURE 2.0 lowered the hours threshold for long-service part-time employees, so if you regularly use part-time help, review your status carefully before assuming you still qualify. The conversion requirement adds administrative cost and may require adding matching or safe harbor contributions to pass nondiscrimination tests.
State Variations
Federal rules; no state variation in plan structure or limits.
Implementing Regulations
- 26 CFR Part 1 — Income tax regulations (§§ 1.401(k)-1 through 1.401(k)-6 — cash or deferred arrangement requirements, ADP testing, one-participant plans; § 1.415-1 — contribution limits)
Pending Legislation
- Starter 401(k): SECURE 2.0 created a simplified "starter" 401(k) for small employers, which could expand access but differs from the Solo 401(k).
Recent Developments
- Enhanced catch-up (ages 60-63) now available in Solo 401(k): SECURE 2.0's enhanced catch-up provision — $11,250 for participants ages 60-63 vs. $7,500 for other age-50+ participants — applies to Solo 401(k) plans as well as employer-sponsored plans. For a self-employed person in the 60-63 window, this means $34,750 in employee deferrals alone in 2026, plus employer profit-sharing on top. Over the 4-year window, that's an extra $15,000 in potential contributions beyond the standard catch-up.
- Roth employer contributions now permitted (SECURE 2.0): SECURE 2.0 allowed employer contributions to 401(k) plans (including Solo 401(k)s) to be designated as Roth. For Solo 401(k) owners, this means the employer profit-sharing portion — which was previously always pre-tax — can now be contributed on an after-tax Roth basis if the plan document allows and the provider supports it. This is a significant planning option for self-employed owners in high-bracket years who want to maximize tax-free retirement assets. Not all Solo 401(k) providers have updated their plans to allow this.
- Backdoor Roth advantage preserved: Solo 401(k) plans are not IRAs and do not create pro-rata issues for the backdoor Roth conversion strategy. Self-employed individuals who had previously kept a SEP-IRA (which is an IRA and does create pro-rata issues) can roll the SEP-IRA balance into the Solo 401(k), resetting the IRA basis to zero and enabling a clean backdoor Roth. This is one of the top planning moves for high-income freelancers and consultants.
- Non-spouse employee hiring triggers conversion requirement: Many Solo 401(k) owners don't realize that hiring even one non-spouse W-2 employee — even part-time — can trigger the requirement to convert to a standard 401(k) with nondiscrimination testing. The threshold for "full-time" purposes in 401(k) plans was lowered by SECURE 2.0 (for long-service part-time employees), which means self-employed owners who regularly use part-time help should review their employee status more carefully. The conversion requirement can be administratively burdensome and may require adding plan features (safe harbor, matching) to pass nondiscrimination testing.