Solo 401k vs SEP IRA: Which Saves Small Owners More

Only 13% of self-employed Americans have any retirement savings at all. That number stopped me cold the first time I saw it, because I work with small business owners every day and I still wasn’t prepared for how stark it was. Most of them are pouring every dollar back into the business, telling themselves they’ll “figure out retirement later.” Later has a way of never arriving.

If you’re a sole proprietor or a single-member LLC with no employees (other than maybe a spouse), you’re sitting on access to two of the most powerful retirement vehicles the IRS offers: the Solo 401(k) and the SEP-IRA. The question isn’t which one sounds better. The question is which one actually lets you keep more of your money given your specific income, your business structure, and honestly, how much paperwork you’re willing to deal with.

I’ll be honest: I used a SEP-IRA myself for the first four years after going independent. Thought it was simpler, thought it was fine. What surprised me was how much contribution room I was leaving on the table compared to what a Solo 401(k) would have allowed at my income level. The math was embarrassing. Let me show you what I found.

The Real Difference in How These Plans Are Built

Both plans share the same IRS contribution ceiling for 2026: $70,000 (plus a $7,500 catch-up if you’re 50 or older, but only for the Solo 401(k) side). But the way you get there is completely different, and that difference is where most people get confused.

A SEP-IRA works on one lever: employer contributions only. The IRS caps those at 25% of net self-employment income (after the deduction for half of self-employment tax). So if your net SE income is $120,000, your maximum SEP contribution is roughly $27,500. Not bad. But nowhere near the $70,000 ceiling.

The Solo 401(k) has two levers. You’re wearing two hats: employee and employer. As the “employee,” you can defer up to $23,500 of your compensation in 2026, dollar for dollar, before the 25% employer calculation even starts. Then, as the “employer,” you can contribute an additional 25% of net SE income on top. That double-stacking is what makes the Solo 401(k) so powerful at moderate income levels, say, $80,000 to $200,000, where the SEP alone would leave you well under the cap.

(Note: chart shows Solo 401(k) maximums; SEP-IRA at these same income levels would be roughly 25% of each figure, before SE tax adjustment.)

Side-by-Side: What Actually Matters

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One thing that table doesn’t fully capture: the Roth Solo 401(k) option is genuinely underused and potentially very valuable if you expect to be in a higher tax bracket later, or if you’re in a lean year and the upfront deduction matters less. I’ve had clients in their early 40s with moderate income run the numbers on Roth vs. traditional and come out ahead by $60,000-plus in after-tax retirement value over 20 years. The research on tax diversification in retirement is actually pretty clear here, though predicting your future bracket is not a science.

Where the Solo 401(k) Wins, Clearly

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Here’s a worked example that I’ve replicated in various forms with actual clients:

Scenario: Freelance marketing consultant, net SE income of $95,000, age 44, no employees.

SEP-IRA route: After SE tax deduction, contributes roughly $16,700 (approximately 25% of adjusted net SE income after the self-employment tax deduction).

Solo 401(k) route: Employee deferral of $23,500 PLUS employer contribution of approximately $16,700. Total: around $40,200.

That’s an additional $23,500 in tax-deferred savings for the exact same year, exact same income. At a 24% marginal rate, that’s roughly $5,640 in federal taxes saved that year alone, not counting state taxes or the compounding effect over time.

The IRS lays out the self-employed contribution limits clearly in Publication 560, and I’d actually recommend reading it, or at least having your CPA walk through it with you. It’s more readable than most IRS documents, which I realize is a low bar.

Where the SEP-IRA Still Makes Sense

Don’t dismiss it. For some business owners, the SEP is exactly right.

If you’re making $300,000 or more in net SE income, both plans effectively top out at the same $70,000 ceiling, and the SEP gets you there without the plan document, the Form 5500-EZ once you cross $250K in assets, or the December 31 setup deadline pressure. You also have until your tax filing extension date (typically October 15 for sole proprietors) to open AND fund a SEP-IRA for the prior year. A Solo 401(k) has to be established by December 31 of the year you want to contribute for, even if you fund it later.

I’ve seen people lose a Solo 401(k) contribution year because they missed that December 31 setup window. One client, a graphic designer in Austin, came to me in February 2025 asking why her financial advisor hadn’t set up her plan yet. The advisor hadn’t warned her about the December deadline. She had to use a SEP-IRA for that tax year, which cost her about $18,000 in contribution room at her income level. That deadline is not flexible.

Worked example two: Solopreneur, net SE income of $280,000, age 52.

SEP-IRA contribution: approximately $69,000 (near max). Solo 401(k) contribution: $70,000 plus $7,500 catch-up = $77,500.

Here the Solo 401(k) still wins on raw contribution amount because of the catch-up, but the difference is much smaller, and if the business owner values simplicity and already has a good CPA handling the SEP math, the administrative edge of the SEP might genuinely outweigh the extra $8,500 room.

The Employee Problem Nobody Mentions

If you ever plan to hire W-2 employees (other than your spouse), the Solo 401(k) becomes unavailable. Full stop. The IRS defines a Solo 401(k) for owner-only businesses, and one regular W-2 employee other than a spouse disqualifies you. You’d need to either convert to a traditional 401(k) plan (which comes with significant administrative costs and potentially mandatory employer contributions to employees) or roll into a SEP-IRA.

This is worth thinking about before you get attached to the Solo 401(k). The U.S. Small Business Administration has resources on business growth planning, but they don’t always connect the dots to how hiring affects your retirement plan eligibility. That gap is where business owners get caught.

The SEP-IRA, by contrast, does allow employees, though you’d be required to contribute the same percentage of compensation for all eligible employees as you do for yourself. That can get expensive fast, which is why growing businesses often end up moving to a SIMPLE IRA or a full 401(k) plan. But that’s a different article.

How to Actually Open One

For the Solo 401(k), you’ll need an IRS-approved plan document (most custodians like Fidelity, Vanguard, or Charles Schwab provide these for free), an EIN even if you’re a sole proprietor, and you’ll need to establish the plan by December 31. Fidelity’s self-employed 401(k) is free to set up and has no annual fees, which is where I’d point most people starting out. Vanguard’s version requires a $1,000 initial investment.

SEP-IRA: Open it at any brokerage, fill out IRS Form 5305-SEP (one page, literally), done. You can do this the morning you file your taxes in October and still contribute for the prior year. It’s genuinely that simple.

One thing nobody tells you: when you call to open a Solo 401(k) at a large custodian, the phone rep will often ask you to confirm your business has no full-time employees. Make sure you understand the eligibility rules before that call, because answering incorrectly can create problems later.

Sources

• IRS Publication 560, Retirement Plans for Small Business (2026): Official IRS guidance on SEP, SIMPLE, and qualified plans for self-employed individuals, including current contribution limits.
• IRS Self-Employed Individuals Tax Center: IRS guidance hub for sole proprietors and single-member LLCs covering retirement deductions and plan types.
• U.S. Small Business Administration: Retirement Plans: SBA overview of retirement plan options for small business owners.
• EBRI (Employee Benefit Research Institute) 2025 Retirement Confidence Survey: Data on self-employed retirement savings rates and participation gaps, including the 13% statistic on self-employed workers with active retirement savings.
• IRS Form 5500-EZ Instructions (2026): Required annual filing for Solo 401(k) plans when plan assets exceed $250,000.

If you want to go deeper on the mechanics before talking to a CPA, Mike Piper’s Taxes Made Simple (Amazon, affiliate link) covers self-employment tax and retirement deductions in plain language better than anything else I’ve found at that price point. It’s $12 and worth every dollar.

But please: don’t make this decision based on an article alone, including this one. The numbers above are current as of July 2026, but your specific situation, your income, your business structure, your other accounts, deserves a real conversation with a CPA who knows your file. The plan type matters less than actually opening something and funding it consistently. That 13% figure doesn’t have to include you.

Photo: SHVETS production via Pexels

This article is for general informational purposes only and does not constitute financial, tax, or legal advice. Business finance and tax rules vary by entity type, state, and individual circumstances. Consult a qualified CPA, enrolled agent, or business attorney for advice specific to your situation.

Recommended Resources

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• Mastering QuickBooks 2025 (~$32), The most comprehensive QuickBooks 2025 guide, covers bookkeeping, payroll, invoicing, tax prep, and cash flow.
• Accounting for Small Business Owners (~$14), Beginner-friendly accounting guide covering basic bookkeeping, financial statements, and managing business taxes.