Most articles on Solo 401(k)s and SEP-IRAs describe both plans and leave you to guess. There's actually one question that decides which one wins for most small business owners, and it's not about how much you earn. It's about whether you have, or ever plan to hire, employees.
The Question That Actually Decides This
A SEP-IRA sounds simple: the business contributes up to 25% of compensation, done. The catch most people don't hear about until it's expensive: if you have eligible employees, you must contribute that same percentage for every one of them, not just yourself. Contribute 20% of your own income and you owe 20% of every eligible employee's pay too, whether they've been with you three months or three years.
A SEP-IRA is the simple choice — 25% of compensation, done.
Contribute 20% of your own income and you owe 20% of every eligible employee's pay too.
A Solo 401(k) sidesteps that problem in the cleanest way possible: it legally cannot have other employees. It's built for exactly one worker, the owner, plus a spouse who's also on payroll. The moment you hire someone who isn't your spouse and who meets basic eligibility, the Solo 401(k) has to convert to a regular 401(k) plan, real administration, real cost.
Why a Solo 401(k) Usually Lets You Save More
A SEP-IRA is employer contributions only. A Solo 401(k) has two pieces: an employee deferral, money you contribute as if you were your own employee, and an employer profit-sharing contribution on top. For 2026, the employee deferral alone is $24,500, or $32,500 if you're 50 to 59 or 64 and older, or $35,750 if you're 60 to 63, thanks to a higher SECURE 2.0 catch-up that applies only in that age window.
On top of that, you can add an employer contribution of up to 25% of compensation, the same percentage a SEP allows, capped so total contributions from both pieces don't exceed $72,000 for 2026 (higher with catch-up contributions added on). Compensation used in the calculation is capped at $360,000 either way.
| Solo 401(k) | SEP-IRA | |
|---|---|---|
| Who can contribute | Employee deferral + employer piece | Employer only |
| 2026 employee deferral | Up to $24,500 ($35,750 if 60–63) | Not allowed |
| 2026 employer contribution | Up to 25% of compensation | Up to 25% of compensation |
| 2026 combined maximum | $72,000 (higher with catch-up) | $72,000 |
| Can you have employees? | No (other than a spouse) | Yes, but must contribute the same % for all eligible staff |
| Annual filing | Form 5500-EZ once assets exceed $250,000 | None required |
| Loan option | Often allowed against the balance | Not allowed |
The Real Numbers, at $100,000 in Net Self-Employment Income
Here's where the difference stops being theoretical. Take a self-employed business owner netting roughly $100,000. A SEP-IRA, employer contribution only, generally works out to somewhere in the neighborhood of $18,000 to $19,000, once the self-employment adjustment is applied.
A Solo 401(k) at the same income starts with the same employer piece, then adds the full $24,500 employee deferral on top. That's a total contribution roughly double what the SEP alone allows, sheltering meaningfully more income from tax in the same year, out of the same paycheck.
Where a SEP-IRA Still Wins
The Solo 401(k) isn't better in every situation. A SEP-IRA is genuinely simpler to set up and maintain, no annual filing requirement even as the account grows, no deadline pressure around plan documents, and it's easy to open mid-year with almost no paperwork. If you're not sure whether you'll bring on employees in the next year or two, or you want the lowest-maintenance option available, a SEP is a completely reasonable choice, you're trading some contribution room for simplicity.
It's also worth knowing a SEP can be opened and funded right up until your tax filing deadline, including extensions, which makes it a useful last-minute move if you're realizing in March that last year's return could use a bigger deduction.
If You're an S-Corp Owner, Read This Part Twice
For S-Corp owners, both plans calculate the employer contribution off your W-2 wages, not your total business profit. That means the reasonable-salary number you land on has a direct effect on how much you can contribute to either plan, set the salary too low purely to save on payroll tax, and you've also quietly capped your own retirement contribution room. We cover exactly how that number gets decided, and the case that shows what happens when it's set too low, in our guide to setting a defensible S-Corp salary, and retirement contributions are exactly the kind of tradeoff worth running alongside that decision, not after it.
What to Do Before Year-End
If you want a Solo 401(k) for this tax year, open it before December 31
Unlike a SEP, the plan itself has to exist by year-end, even if you fund it later, up until your filing deadline.
Run the actual numbers for your income and entity type
rather than assuming one plan is universally better. The gap between the two grows with income, and shrinks or disappears if you're near the combined contribution ceiling either way.
If you're planning to hire in the next year or two,
factor that into the decision now. A SEP that looks affordable at one employee can get expensive fast once you're contributing the same percentage across a growing team.
Don't set this up and forget it
Contribution limits change every year, and what made sense at last year's income may not be the right move this year, especially after a strong year or a shift in how the business is structured.
Bring us your numbers. We'll run both scenarios for your actual income and entity structure and tell you honestly which plan puts more money in your pocket, and how to get it set up before the deadline that matters for you. Call (301) 662-6992.
Frequently asked questions
Technically yes, but it rarely makes sense. Both plans draw from the same overall contribution ceiling, so stacking them doesn't let you save more than a single Solo 401(k) would on its own, it just adds complexity for no real benefit. The exception is if you're transitioning from one plan to the other mid-year, which is a timing question worth walking through with a CPA rather than doing on your own.
