For most self-employed workers, a Solo 401(k) lets you save more than a SEP IRA, because it adds a $24,500 employee salary deferral on top of the same employer contribution a SEP IRA allows, all built on the same underlying tax rules. On $100,000 of net self-employment income, that gap works out to roughly $24,500 more you can shelter in the Solo 401(k) in 2026. The trade-off is timing and paperwork: a Solo 401(k) has to be opened by December 31 of the year you're contributing for, while a SEP IRA can be opened and funded as late as your tax filing deadline, extensions included.
What's the Actual Difference Between a SEP IRA and a Solo 401(k)?
A SEP IRA only allows one kind of contribution: an "employer" contribution, capped at roughly 20% of your net self-employment earnings after the self-employment tax adjustment. A Solo 401(k) allows that same employer contribution, plus a separate employee salary deferral of up to $24,500 in 2026, the same dollar limit a W-2 worker gets in a regular 401(k). Both accounts are funded entirely by you as a self-employed person wearing two hats, employer and employee, but only the Solo 401(k) lets you use the employee half of that equation.
How Much More Can a Solo 401(k) Actually Get You? A Real Example
Say you're a freelancer or sole proprietor with $100,000 in net profit for 2026. After the standard self-employment tax adjustment (net profit reduced by the 92.35% factor, then by half of the resulting self-employment tax), your net earnings for retirement plan purposes come out to roughly $92,935. Apply the 20% employer-contribution rate self-employed people actually use, and both a SEP IRA and a Solo 401(k) support about $18,587 in employer contributions. The SEP IRA stops there, at $18,587 total for the year. The Solo 401(k) adds the $24,500 employee deferral on top, bringing the total to roughly $43,087, more than double the SEP IRA's number, on the exact same income.
Does the Solo 401(k) Advantage Shrink at Higher Income?
Yes, and it eventually disappears entirely. The combined employee-plus-employer limit for both account types is capped at $72,000 for 2026 (before any catch-up contributions). A SEP IRA reaches that $72,000 cap on its own once your adjusted net self-employment earnings hit about $360,000, since 20% of $360,000 is $72,000. Past that income level, a SEP IRA and a Solo 401(k) let you contribute the exact same amount, because the SEP's employer-only math alone is enough to hit the ceiling. The Solo 401(k)'s real edge is concentrated in the low-to-mid six-figure range, where the flat $24,500 deferral is large relative to 20% of your net earnings. At $50,000 in net profit, for example, a SEP IRA supports about $9,293, while a Solo 401(k) supports about $33,793, again almost entirely the size of the employee deferral.
What Are the Actual 2026 Contribution Limits for Each?
- SEP IRA: the lesser of $72,000 or roughly 20% of your net self-employment earnings after the self-employment tax adjustment (25% of compensation for a W-2 employee at an incorporated business). No separate employee deferral exists.
- Solo 401(k): an employee deferral of up to $24,500, plus an employer contribution of roughly 20% of net self-employment earnings, combined total capped at $72,000. Catch-up contributions add $8,000 more if you're 50 to 59 or 64 and older, or $11,250 if you're 60 to 63, on top of the regular limits.
- Compensation cap: only the first $360,000 of compensation counts toward either calculation for 2026.
What Are the Setup Deadlines You Can't Miss?
A Solo 401(k) plan has to be legally established, meaning the plan documents are signed and in place, by December 31 of the tax year you want the deduction for. Employer contributions can still be made up until your tax filing deadline including extensions, but the plan itself has to exist by year-end, and custodians commonly stop accepting new Solo 401(k) paperwork in late November as their year-end queue fills up. A SEP IRA has no such deadline. You can open and fully fund a SEP IRA as late as your extended tax filing deadline, which means you can realize in September that you want to shelter more 2025 income and still open a SEP IRA for that prior year. If you're reading this after December 31 and want to contribute for the current tax year, the SEP IRA is often your only remaining option.
Does Either Option Work If You Have Employees?
Not equally. A SEP IRA requires you to contribute the same percentage of pay for every eligible employee, not just yourself, and "eligible" is a low bar: anyone 21 or older who has worked for the business in 3 of the last 5 years and earned at least $800 in 2026. Hire a part-time assistant and keep them on for a few years, and you may owe them the same 20% employer contribution you're giving yourself. A Solo 401(k) is built for a business with no full-time common-law employees besides the owner and a spouse; bring on a regular employee who meets standard eligibility rules, and you generally have to convert to a regular 401(k) plan built to cover them too, with the added administration that comes with it.
What Should You Actually Do?
- Choose the Solo 401(k) if you have no other employees and can hit the December 31 deadline. On most income levels below roughly $360,000, it lets you shelter meaningfully more, dollar for dollar, on the same income.
- Choose the SEP IRA if it's already late in the year, or you want maximum flexibility. Its ability to be opened and funded up to your extended filing deadline makes it the practical fallback once December has passed.
- Choose the SEP IRA if you have or plan to hire eligible employees you don't want a large ongoing contribution obligation toward, since a Solo 401(k) forces the issue the moment you bring on real staff.
- Run your own numbers before assuming either is "better." The dollar gap between the two shrinks the closer your net self-employment earnings get to about $360,000, so the Solo 401(k)'s advantage matters most for early-stage and mid-income self-employed workers, not necessarily high earners.
Either account is a real, IRS-recognized way to shelter self-employment income the same way a traditional 401(k) or IRA does. If you're weighing whether to put new contributions into a Roth or traditional version of either account, our breakdown of Roth IRA vs. traditional IRA covers how to decide. And if you're maxing out a Solo 401(k) and still have room to save more, the mechanics behind a mega backdoor Roth 401(k) are worth understanding, since some Solo 401(k) providers support a version of the same after-tax contribution strategy.
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Spicy Investing