Solo 401(k) vs SEP-IRA: A Decision Guide

Self-employed taxpayers have access to two main retirement vehicles offering far higher contribution limits than typical employer 401(k)s: the Solo 401(k) and the SEP-IRA. The right choice depends on income level, whether you want Roth contributions, whether you might want to borrow against the account, and your tolerance for paperwork. This guide breaks down the math and the practical decision.

Contribution limits at a glance (2026)

• SEP-IRA: Employer contribution only — up to 25% of net SE income (after half-SE deduction adjustment), capped at $72,000.
• Solo 401(k): Employee deferral up to $24,500 ($32,500 if 50+, $36,500 if 60-63 per SECURE 2.0 enhanced catch-up) PLUS employer contribution up to 25% of net SE income, total capped at $72,000 (under 50), $80,000 (50+), or $84,000 (60-63).

Side-by-side comparison

At very high incomes ($300k+ net SE income) the two plans converge — both cap at the §415(c) annual addition limit ($72,000 in 2026). At moderate incomes ($100-200k), Solo 401(k) wins because of the employee deferral, which lets you reach the cap with much lower SE income than SEP-IRA's 25% formula requires.

The math at different income levels

Net SE income $50,000:

• SEP-IRA max: ~$9,300 (≈18.6% effective)
• Solo 401(k) max: $24,500 (employee) + $9,300 (employer) = $33,800
• Solo 401(k) wins by $24,500

Net SE income $100,000:

• SEP-IRA max: ~$18,600
• Solo 401(k) max: $24,500 + $18,600 = $43,100
• Solo 401(k) wins by $24,500

Net SE income $250,000:

• SEP-IRA max: ~$46,500
• Solo 401(k) max: $24,500 + $46,500 = $71,000 (capped at $72,000)
• Solo 401(k) wins by $25,000

Net SE income $400,000:

• SEP-IRA max: $72,000
• Solo 401(k) max: $72,000
• SEP-IRA wins by $500 (essentially a tie)

The takeaway: at moderate incomes ($50-300k), Solo 401(k) wins by ~$24,500/year because of the employee deferral. At very high incomes, the two are roughly equal.

Roth option

Solo 401(k) plans (most providers — Fidelity, Schwab, ETrade) offer a Roth option for the employee deferral portion. SEP-IRA does not — all contributions are pre-tax.

Why this matters: Roth contributions don't reduce current-year tax, but withdrawals in retirement (after 59.5) are tax-free. For high-income earners expecting equal or higher future tax brackets, Roth is valuable.

If you want Roth flexibility (mix Roth and traditional contributions across years), Solo 401(k) is the only path.

Loan provision

Solo 401(k) plans (with the right provider) allow loans up to 50% of the account balance, max $50,000. SEP-IRA does not allow loans.

For most self-employed people this isn't relevant, but if you might want emergency access to retirement funds without triggering early-withdrawal penalties, Solo 401(k) wins.

Setup complexity

SEP-IRA: Trivial. Open an account at any major brokerage in 15 minutes. No annual paperwork until you cross $250k in account value (then a Form 5500-EZ is required).

Solo 401(k): Slightly more setup. Open at Fidelity, Schwab, or E-Trade. Annual Form 5500-EZ filing required once account exceeds $250k. Choosing a "non-prototype" Solo 401(k) (like at MySolo401k or with a custom plan provider) gives you more features (Roth, loans, mega-backdoor Roth) but adds paperwork.

The "spousal contribution" advantage

Both account types allow contributions for a spouse who works in your business. Solo 401(k) wins here too — your spouse can contribute the full employee deferral ($24,500) on top of their salary, plus 25% employer match, doubling your household retirement contributions.

Mega-backdoor Roth (advanced)

Some Solo 401(k) plans (custom providers, not vanilla Fidelity) allow after-tax contributions beyond the employee deferral, then in-plan Roth conversion. This can push total Roth contributions to $70k+/year. Not available with SEP-IRA. Highly specialized — talk to a CPA.

When SEP-IRA might still be the right choice

1. You're earning very high income ($350k+ net) and want maximum simplicity.
2. Your income is wildly variable and you want zero paperwork in low-income years.
3. You're already maxing employer 401(k) at a day job and don't need the employee deferral here.
4. You hate paperwork.

When Solo 401(k) clearly wins

1. Net SE income $30k-$300k
2. You want Roth contribution flexibility
3. You might want loan access
4. You have a working spouse who'd contribute too
5. You're already past the Roth IRA income limits (~$179k single) and want continued Roth access

Setup deadlines

• SEP-IRA: Can be set up and funded as late as your tax filing deadline (April 15, or October 15 with extension).
• Solo 401(k): Must be ESTABLISHED by December 31 of the year you want to contribute for. Contributions can be made up to tax filing deadline, but the plan itself has to exist by year-end.

This catches many freelancers — they realize in March they want a Solo 401(k) for last year, only to learn it's too late. Open the account by December even if you won't fund it until April.

Common mistakes

• Defaulting to SEP-IRA because it's "simpler". The simplicity costs you ~$24,500/year in deferred contributions.
• Missing the December 31 Solo 401(k) deadline. Open the account before year-end even if not funding immediately.
• Forgetting to file Form 5500-EZ once Solo 401(k) hits $250k. Late filing penalties are $250/day.
• Not using the Roth option. If your tax bracket is already high and might stay high, Roth Solo 401(k) is valuable.
• Contributing to both at high income. You can have both, but employer-side contributions across both accounts are aggregated against the 25% net-SE limit.

Where to open each account — provider comparison

Both Solo 401(k) and SEP-IRA can be opened at any major brokerage. Key differences:

• SEP-IRA — open at Fidelity, Schwab, Vanguard, or any major broker in 15 minutes online. No paperwork, no plan document, no annual filing. Just an IRA with an "SEP" designation. Standard IRA fees apply (essentially $0 at major brokers).
• Solo 401(k) — basic — Fidelity, Schwab, and ETrade offer free Solo 401(k) accounts. Requires you to adopt a "plan document" (provided by the broker), assign yourself as plan administrator, and track contributions. The brokers' free plans usually DO NOT support: Roth, mega-backdoor Roth, or after-tax contributions.
• Solo 401(k) — advanced features — third-party providers (MySolo401k, Carry, Solo401k.com) charge $300-$1,000/year setup + $150-$500/year maintenance but enable Roth, mega-backdoor Roth, in-plan conversions, and self-directed alternative investments. Worth it for high earners ($100k+ net) who want maximum tax flexibility.
• Form 5500-EZ — Solo 401(k) requires annual filing once plan assets exceed $250,000. Free to file, takes 30 minutes. SEP-IRAs never require Form 5500.

For a freelancer starting out, Fidelity's free Solo 401(k) is fine. If/when you want Roth or mega-backdoor Roth, migrate to a third-party administrator.

Coordination with W-2 401(k) — the deferral cap is shared

If you have a day-job W-2 401(k) AND a Solo 401(k) for your side freelance income, the EMPLOYEE deferral limit ($24,500 in 2026) is shared across both plans. You can't defer $24,500 at your day job AND another $24,500 in your Solo 401(k) — the $24,500 cap applies to the sum across all your employee deferrals.

However, the EMPLOYER profit-sharing piece is plan-specific. Even if your day-job W-2 employer maxed your employee deferral, you can still make EMPLOYER contributions to your Solo 401(k) (up to 25% of net SE × 0.9235). This is how moderate-income side-hustlers still extract value from a Solo 401(k) when the deferral side is used elsewhere.

SEP-IRA contributions are NOT subject to the employee deferral cap — they're purely "employer" contributions. So if you've maxed your W-2 401(k) deferral, a SEP-IRA on your freelance income can be more flexible than a Solo 401(k) in some scenarios. Run the math both ways.

Frequently asked questions

I forgot to open my Solo 401(k) by December 31. Am I out of luck?
For 2026 contributions: yes for the employee deferral portion (must establish by 12/31/2026 to defer 2026 income). The EMPLOYER profit-sharing portion has an extended deadline — you can open and fund by your tax filing deadline (including extensions, so up to October 2027 for 2026 contributions). The SECURE 2.0 Act loosened the establishment deadline somewhat; check with your provider for specifics on retroactive plan adoption.

SEP-IRA has no deadline issue, right?
Correct — SEP-IRA can be established AND funded by your tax filing deadline (Oct 15 with extension). Procrastinator-friendly. This is one real advantage SEP keeps over Solo 401(k).

Can I contribute to both a Solo 401(k) AND a Traditional/Roth IRA?
Yes — they're separate limits. $24,500 Solo 401(k) deferral + $7,000 IRA contribution (or $8,000 if 50+) are completely independent. High-income earners may face Roth IRA income limits ($150k MFJ phase-out start in 2026), but Traditional IRA is always available (deductibility separate question).

What happens to my Solo 401(k) if I stop being self-employed?
The plan stays open. You can't make NEW contributions without earned SE income, but existing balances continue growing tax-deferred. When you eventually retire, normal 401(k) distribution rules apply. Or you can roll the balance to a Traditional IRA when convenient.

I'm an S-corp owner. Can I still use a Solo 401(k)?
Yes — S-corp owners use Solo 401(k) extensively. Your "employee deferral" comes from your W-2 wages (the salary you pay yourself from the S-corp). Your "employer profit-sharing" comes from the S-corp's payroll calculation (up to 25% of W-2 wages, not 25% of S-corp profit). This is one of several reasons S-corp election makes sense at higher income — better retirement leverage.

Bottom line

For most self-employed people earning $30k-$300k net, Solo 401(k) is the better choice — the employee deferral component lets you save ~$24,500/year more than SEP-IRA at the same income. Open it at Fidelity or Schwab (free) for basic use, or a third-party administrator if you want Roth and mega-backdoor Roth flexibility. Open before December 31 for current-year deferrals; fund by tax filing deadline. Use the Roth option for at least part of your contributions. See how retirement contributions affect your taxes in our calculator.

This article is for educational purposes only. It is not personalized tax, legal, or financial advice. Quarterly1099 is published by Vincent Roy and is not a CPA, EA, or licensed tax preparer. All content is sourced from IRS publications and current tax law. Fact-checked against IRS publications and 2026 Rev. Proc. 2025-32. For your specific situation, consult a licensed CPA or Enrolled Agent. See our full disclaimer.

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