P3 · Covering the business
Solo 401(k) vs. SEP IRA: Which Actually Lets You Save More
A Solo 401(k) and a SEP IRA can both reach the same total contribution cap, but the SEP IRA's employer-only structure makes it far harder to reach at moderate income.
Both a Solo 401(k) and a SEP IRA can reach the same overall contribution ceiling in a given year, but the path to that ceiling is completely different. A SEP IRA is funded entirely through an employer-style contribution capped as a percentage of compensation; a Solo 401(k) lets you contribute as both employee and employer, which matters enormously at moderate income levels.
Why the same total limit produces very different real contributions
A SEP IRA contribution is capped at roughly 20 percent of net self-employment earnings for a sole proprietor (the math works out lower than the stated 25 percent employer-contribution rule once self-employment tax adjustments are applied). A Solo 401(k) lets you make an employee deferral up to a fixed dollar limit regardless of a percentage-of-income cap, plus an employer contribution on top — which is why a moderate-income solo operator can often contribute meaningfully more into a Solo 401(k) than a SEP IRA at the same income level.
Solo 401(k) vs. SEP IRA decision points
| Contribution structure | SEP IRA: employer-only, capped near 20% of net self-employment earnings. Solo 401(k): employee deferral up to a fixed dollar limit, plus an employer contribution on top. |
|---|---|
| Catch-up contributions | Available only in a Solo 401(k) for savers 50 and older, with a larger window for a specific age band closer to retirement. SEP IRA has none. |
| Employee eligibility limit | Solo 401(k) requires no common-law employees besides a spouse. SEP IRA has no such restriction but requires proportional contributions for all eligible employees. |
| Annual filing | Solo 401(k) requires Form 5500-EZ once plan assets cross the filing threshold. SEP IRA has no annual filing requirement. |
| Best fit | Solo 401(k) generally allows higher contributions at moderate income and adds catch-up room; SEP IRA fits a business that may add employees or wants the simplest possible paperwork. |
The catch-up contribution only exists in one of these plans
Only the Solo 401(k) allows catch-up contributions for savers age 50 and older, with an even larger catch-up window available to savers in a specific age band close to retirement. A SEP IRA has no catch-up provision at any age — if you're over 50 and trying to maximize retirement savings quickly, this difference alone can be decisive.
Employee eligibility is where SEP IRA keeps its advantage
A Solo 401(k) is only available to a business with no common-law employees other than a spouse — the moment you hire even a part-time non-spouse employee who meets eligibility rules, the Solo 401(k) structure no longer fits and you'd need a different plan design. A SEP IRA has no such restriction; a business of any size can use one, provided contributions are made proportionally for all eligible employees, not just the owner.
Administrative burden differs even though both are relatively simple
A Solo 401(k) requires an annual Form 5500-EZ filing once plan assets cross a certain threshold, plus a formal plan document you adopt when opening the account. A SEP IRA has no annual filing requirement and a simpler one-page adoption agreement, which is part of why some solo operators choose it even when the contribution math slightly favors the Solo 401(k) — the paperwork tradeoff is real, even if smaller than a company-sponsored 401(k) would require.
Roth options exist in one plan type, not the other by default
Many Solo 401(k) providers offer a Roth option for the employee-deferral portion, letting you choose between pre-tax and after-tax treatment on that piece of your contribution. A traditional SEP IRA does not offer a Roth version through the standard SEP structure, though a separate SEP-Roth option has become available through some providers more recently — confirm directly with your specific provider which version they actually offer before assuming Roth treatment is available under either plan by default.
The same $100,000 in profit, two different contribution ceilings
A solo consultant nets $100,000 in self-employment profit after expenses. Under a SEP IRA, the contribution is capped near 20 percent of that net figure after the self-employment tax adjustment, landing in the high-teens-thousands range. Under a Solo 401(k), the same consultant can make an employee deferral up to the fixed annual dollar limit, then add an employer contribution calculated on the same net-earnings basis as the SEP — the combination reaches a meaningfully higher total than the SEP IRA alone at this income level, sometimes more than double. The gap narrows at much higher income levels, where both plans approach the same overall dollar ceiling, but at moderate income the two plans are not close substitutes for maximizing contributions.
Where solo operators pick the wrong plan for their situation
The most common mistake is choosing a SEP IRA purely because it's simpler to set up, without running the actual contribution-ceiling comparison at your specific income level first. At moderate self-employment income, the SEP IRA's employer-only structure can leave real contribution room on the table compared to what a Solo 401(k) would allow — the paperwork savings can be smaller than the retirement-savings gap it creates over several years.
The second common mistake runs the other direction: opening a Solo 401(k) and then hiring a part-time employee without checking whether that hire breaks Solo 401(k) eligibility. Once a business has a non-spouse common-law employee who meets the plan's eligibility rules, the Solo 401(k) generally has to be converted to a different plan design or wound down — which is a much bigger administrative event than simply choosing a SEP IRA would have been from the start, if hiring was already part of the plan.
Before you open either account
- Run your actual contribution ceiling under both plans at your real net self-employment income, not a rounded estimate.
- Confirm whether you plan to hire any non-spouse employees in the next few years.
- Check whether you're 50 or older and would benefit from the Solo 401(k)'s catch-up provision.
- Ask your plan provider directly about the Form 5500-EZ filing threshold and whether your expected balance will cross it.
- Confirm the account can be opened and funded before your tax-filing deadline, including extensions, for the year you want the contribution to count.
Questions to answer before choosing between the two
Can I have both a Solo 401(k) and a SEP IRA at the same time?
Generally you can maintain both, but total contributions across all your own retirement plans in a given year are still subject to the overall combined annual limit, so having both doesn't let you exceed that ceiling — it mainly adds administrative complexity without adding extra room in most cases.
What happens to my Solo 401(k) if I hire my first non-spouse employee?
Once a non-spouse common-law employee meets the plan's eligibility requirements, the Solo 401(k) structure generally no longer fits your business, and you'd typically need to convert to a different employer plan design or wind the Solo 401(k) down, since it's specifically built for owner-only or owner-plus-spouse businesses.
Is the SEP IRA contribution really only around 20%, not 25%, for a sole proprietor?
Yes for a sole proprietor or single-member LLC taxed as a sole proprietor — the stated 25% employer-contribution rule applies to compensation as defined for a corporation, but for self-employment income, a required adjustment for self-employment tax brings the effective contribution rate down to roughly 20% of net earnings in practice.
Do I need a financial advisor to open a Solo 401(k)?
Not necessarily — many brokerages offer a Solo 401(k) plan document and setup process directly, without requiring a paid advisor, though a tax professional's input is worth getting if you're unsure how the contribution math applies to your specific net self-employment income.