Solo 401(k) vs SEP IRA: Which Retirement Plan Fits Your Business?

Most self-employed business owners assume these two plans are roughly equivalent and just pick whichever sounds simpler. They are not equivalent. At moderate income levels, the wrong choice can cost you $15,000 or more in annual tax-sheltered contributions, and the gap only widens once you factor in Roth access and long-term tax location.

The Contribution Math Is Not Even Close at Lower Income Levels

Both plans share the same annual additions limit in 2026: $70,000, or $77,500 if you are 50 or older. That number is not the whole story.

A SEP IRA limits you to 25% of net self-employment compensation, full stop. No employee deferral. So if your net SE income is $80,000, your SEP contribution maxes out around $14,800.

A Solo 401(k) lets you wear two hats. As the employee, you can defer up to $23,500 in 2026 (or $31,000 if 50-plus, with the enhanced catch-up for ages 60-63 reaching $34,750). Then, as the employer, you contribute up to 25% of compensation on top of that. Using the same $80,000 net SE income, a solo 401(k) gets you close to $37,000, more than double the SEP. You do not reach contribution parity until net self-employment income approaches $300,000.

This is the employee-deferral edge, and it matters most in the years when income is still climbing.

The Roth and Tax-Location Advantages

The Solo 401(k) is the only vehicle in this comparison that can offer a Roth option. Many plan documents now allow Roth deferrals, which means the employee-deferral portion can go in after-tax and grow permanently tax-free. For a business owner who expects higher income in retirement, that matters.

Some plans also allow after-tax (non-Roth) contributions that can then be converted, the mechanism commonly called the mega-backdoor Roth. Whether this is worth structuring depends entirely on your specific income trajectory and existing tax picture. In the language of our planning framework, this is Soil-layer work: getting the right assets into the right wrappers before the opportunity closes.

A SEP IRA is always pre-tax, always traditional. Simple, but inflexible.

When SEP Simplicity Is the Right Answer

Simplicity is a real advantage, not a consolation prize. A SEP can be opened and funded up to your tax-filing deadline including extensions, with almost no administrative overhead. Solo 401(k)s must be established by December 31 of the plan year (though contributions can follow later). If you missed the calendar-year deadline or want a plan you can set up in April alongside your tax return, the SEP wins on logistics.

The SEP also wins clearly the moment you hire employees. A Solo 401(k) is available only to owner-employees and their spouses. Add a W-2 employee and the solo 401(k) becomes a traditional 401(k) with all the complexity, testing requirements, and cost that entails. If you are scaling headcount, building your plan around a structure that holds beyond the solo phase is worth doing early.

What to Do Before Year-End

If you are self-employed and not yet maximizing a tax-advantaged plan, run your numbers at two income levels: current and projected. Check whether your existing plan document allows Roth deferrals. Confirm your entity structure, because S-corp compensation rules change the math.

If you would like a second opinion on which structure fits your trajectory, I am happy to have that conversation.

The information provided is for educational purposes only and does not constitute investment, legal, or tax advice. Consult with qualified professionals for guidance specific to your situation.