Both of these accounts let a self-employed person shelter a serious chunk of income from taxes, and both can reach the same high ceiling. The difference is how they climb to it, and that difference decides which one is actually right for you.

The Gist: if you have no employees other than a spouse, a Solo 401(k) is usually the stronger choice, because it lets you contribute as both employee and employer, so you can save far more at a modest income and add Roth, catch-up, and loan options a SEP cannot. A SEP IRA wins when you value simplicity, want the ability to open and fund it as late as your tax deadline, or you have eligible employees, since a Solo 401(k) is off the table the moment you have full-time staff beyond yourself. Same maximum in 2026, very different paths to it.

This article is general educational information, not financial, tax, or investment advice. Retirement account rules, contribution limits, and tax treatment are complex, depend on your specific income and business structure, and change over time. The figures here are for 2026 and may not apply to your situation. Confirm current limits with the IRS and consult a qualified tax or financial professional before opening or funding any retirement account.

The one question that usually decides it: do you have employees?

Before comparing a single dollar figure, answer this: does your business have full-time employees other than you and your spouse? If yes, a Solo 401(k) is simply not available to you, because it is designed for owner-only businesses, a point the IRS makes clear in its overview of one-participant 401(k) plans. A SEP IRA can cover employees, but it comes with a catch that surprises people: you generally must contribute the same percentage of pay for every eligible employee that you contribute for yourself. Give yourself 20% and you owe roughly 20% to each qualifying worker too, which gets expensive fast. So if you have staff, the realistic choice is usually a SEP IRA or a full traditional 401(k), not a Solo 401(k). If it is just you, both are open, and the rest of this guide helps you pick.

The 2026 numbers, side by side

Here is where the two plans line up and where they part ways for the 2026 tax year.

Feature	SEP IRA	Solo 401(k)
Who contributes	Employer only	You as employee and employer
Employee deferral (2026)	None	Up to $24,500
Employer share	Up to ~25% of compensation	Up to ~25% of compensation
Total cap (2026)	$72,000	$72,000, plus catch-up
Catch-up (age 50+)	Not allowed	$8,000, or $11,250 at 60 to 63
Roth option	Rarely offered	Commonly available
Loans	No	Often permitted
Setup and admin	Very simple, no annual filing	More admin, filing over $250k

The 2026 employee deferral limit of $24,500 and the $72,000 overall cap come from the IRS’s announcement of 2026 retirement plan limits. Note that for a self-employed person, the “25% of compensation” is calculated on net self-employment income after part of your self-employment tax, which works out closer to 20% of your net profit in practice.

Why a Solo 401(k) lets you save more at the same income

This is the part that changes people’s decision. A SEP IRA gives you only the employer contribution, so at, say, $80,000 of net self-employment income you might shelter roughly $15,000 to $16,000. A Solo 401(k) lets you add the full employee deferral on top of that same employer contribution, so at the identical income you could put away that employer amount plus up to $24,500, a dramatically larger total.

At the same modest income, a SEP IRA gives you only the employer contribution, while a Solo 401(k) stacks a flat employee deferral on top of it, so the total you can shelter is much larger.

The two plans only converge at higher incomes, where the 25% employer contribution alone is large enough to approach the $72,000 cap on its own. Below that level, the Solo 401(k)’s employee deferral is a genuine advantage, which is why high savers on moderate self-employment income so often prefer it. If your goal is to maximize tax-advantaged saving rather than simply have an account, this stacking is usually the deciding factor.

The tie-breakers most people miss

Beyond the headline contribution math, a few features quietly tip the decision. A Solo 401(k) commonly offers a Roth option, letting you contribute after-tax dollars that grow tax-free, which a SEP IRA typically does not. It allows catch-up contributions once you turn 50, worth an extra $8,000 in 2026 and more between ages 60 and 63, while a SEP offers none. It can permit loans from your balance in a pinch. And it plays nicely with a backdoor Roth strategy, because SEP IRA balances count in the pro-rata rule that can make backdoor Roth conversions partly taxable, whereas Solo 401(k) balances do not. Deciding between pre-tax and Roth in the first place is its own question, and our Roth versus traditional checklist is a useful companion here.

Where the SEP IRA still wins: simplicity and timing

None of that means the SEP is the lesser plan. Its advantages are real and, for many people, decisive. A SEP IRA is remarkably simple to open at a brokerage, requires essentially no ongoing administration, and has no annual government filing, whereas a Solo 401(k) generally requires filing a short Form 5500-EZ once its assets pass $250,000. The SEP also has the friendlier deadline: you can open and fund it as late as your tax-filing deadline, including extensions, which makes it the natural choice if you are looking to make a prior-year contribution after the year has already ended. A Solo 401(k) is best established by December 31 to capture your employee deferrals for that year, even though employer contributions can follow later. If your priority is low effort and last-minute flexibility rather than squeezing out the maximum, the SEP is hard to beat, and it pairs well with the broader money hygiene in our guide to keeping business and personal expenses separate.

How to choose, in plain terms

If you have employees beyond a spouse, use a SEP IRA or a full 401(k) and skip the Solo. If it is just you and you want to shelter as much as possible at a moderate income, or you want Roth, catch-up, or loan features, choose the Solo 401(k). If it is just you but you value simplicity, hate paperwork, or you are making a prior-year contribution after December 31, choose the SEP IRA. Many people start with a SEP for its ease and move to a Solo 401(k) as their income and savings ambitions grow, which is a perfectly reasonable path. Whichever you choose, the biggest win is starting, since both crush a plain taxable account for retirement saving, a theme our rundown of common retirement investing mistakes returns to repeatedly.

What to watch out for

• Do not open a Solo 401(k) if you have eligible employees. It is for owner-only businesses, and hiring full-time staff can force you to change plans.
• Do not forget the SEP employee-equity rule. With staff, you generally must fund the same percentage of pay for every eligible worker as for yourself.
• Do not assume 25% means 25% of gross. For the self-employed, the calculation is on net income after part of self-employment tax, closer to 20% of profit.
• Do not miss the Solo 401(k) year-end setup. Establishing it by December 31 protects your ability to make employee deferrals for that year.

Frequently Asked Questions Can I contribute more to a Solo 401(k) or a SEP IRA?

At the same modest income, a Solo 401(k) usually allows more, because you contribute as both employee and employer. It adds a flat employee deferral, up to $24,500 in 2026, on top of the same roughly 25% employer contribution a SEP offers. The two only converge at higher incomes where the employer contribution alone approaches the $72,000 cap.

Can I have a SEP IRA or Solo 401(k) if I have employees?

A Solo 401(k) is only for owner-only businesses, so full-time employees other than a spouse rule it out. A SEP IRA can include employees, but you generally must contribute the same percentage of pay for every eligible worker as you do for yourself, which can be costly. Businesses with staff often use a SEP IRA or a full traditional 401(k) instead.

Does a SEP IRA allow catch-up or Roth contributions?

Generally no. A SEP IRA has no catch-up contributions for those 50 and older, and Roth SEP contributions are rarely offered in practice. A Solo 401(k) commonly offers both a Roth option and catch-up contributions, worth an extra $8,000 in 2026, or $11,250 between ages 60 and 63 if the plan allows it. That is a meaningful edge for older or higher savers.

What is the deadline to set up each plan?

A SEP IRA can be opened and funded as late as your tax-filing deadline, including extensions, which makes it ideal for prior-year contributions after the year ends. A Solo 401(k) is best established by December 31 to make employee deferrals for that year, though employer contributions can generally be made later. Confirm current deadlines with the IRS or your provider.

Why does a SEP IRA affect a backdoor Roth?

Because SEP IRA balances count toward the pro-rata rule that determines how much of a backdoor Roth conversion is taxable. A large SEP balance can make those conversions mostly taxable. Solo 401(k) balances are not counted the same way, so people who want to keep using the backdoor Roth strategy often prefer a Solo 401(k). Ask a tax professional about your specific situation.

Key Takeaways

• The deciding question is usually whether you have employees: staff rule out a Solo 401(k), leaving a SEP IRA or full 401(k).
• For an owner-only business, a Solo 401(k) lets you save more at a modest income by stacking a $24,500 employee deferral on the employer contribution.
• Both cap at $72,000 in 2026, but the Solo 401(k) adds catch-up, Roth, and loan options a SEP lacks.
• A SEP IRA wins on simplicity and timing, with no annual filing and funding allowed up to your tax deadline.
• Self-employed contributions are based on net income after part of self-employment tax, closer to 20% of profit than 25%.

Practical Takeaway

Start with the employee question, because it often makes the decision for you. If you are a one-person business, ask whether you care more about maximizing contributions or minimizing hassle: the Solo 401(k) rewards the maximizer with its stacked deferral, Roth access, and catch-up room, while the SEP IRA rewards the simplifier with one-click setup and a generous funding deadline. Neither is a wrong answer, and you can move from one to the other as your business grows. The genuine mistake would be leaving the money in a taxable account while you decide. Run your real numbers, or have a tax professional run them, and open the account that fits the business you actually have. For more on building long-term wealth as a founder or freelancer, browse Venture-Lab’s Investment section.