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

At $80,000 of net self-employment income, a Solo 401(k) shelters roughly $37,000 while a SEP IRA caps out around $14,800. The two plans share the same headline limit, which is why owners treat them as interchangeable, and the gap is widest in exactly the years when sheltering income matters most.

The Contribution Math Is Not Close

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. At $80,000 of net SE income, it 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 at 50-plus, with the enhanced catch-up for ages 60-63 reaching $34,750. As the employer, you contribute up to 25% of compensation on top. Same $80,000, and the Solo 401(k) gets you close to $37,000. You do not reach parity until net self-employment income approaches $300,000.

Roth Access and Tax Location

The Solo 401(k) is the only vehicle here that can offer a Roth option. Many plan documents now allow Roth deferrals, so the employee-deferral portion goes in after tax and grows tax-free. For an owner who expects higher income in retirement, that matters.

Some plans also allow after-tax non-Roth contributions that can then be converted, the mega-backdoor Roth. Whether it is worth structuring depends on your income trajectory and existing tax picture. In 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 the SEP 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. A Solo 401(k) must be established by December 31 of the plan year, though contributions can follow later. If you want a plan you can set up in April alongside your return, the SEP wins on logistics.

The SEP also wins the moment you hire. A Solo 401(k) is available only to owner-employees and their spouses. Add a W-2 employee and it becomes a traditional 401(k), with the testing, complexity, and cost that follow.

The Takeaway

The fact that decides this is not your current income. It is your headcount plan, because a structure that fits a solo practice today can become an expensive rebuild two hires from now.

The owners who get hurt here chose carefully. They compare the limits, correctly pick the Solo 401(k), fund it for three years, then hire their first employee and find that the plan they chose for flexibility now carries testing obligations and employer contribution costs they never priced in. Entity structure moves the math too, since S-corp rules change what counts as compensation on both plans.

If your income or headcount is likely to look different in three years, that is worth a conversation before you commit to a plan document.

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.