A Solo 401(k) almost always lets you contribute more than a SEP IRA at the same income, because it stacks an employee deferral on top of the employer contribution. A SEP IRA only gives you the employer side. Below roughly $140,000 of net self-employment income, that difference can mean thousands of extra dollars sheltered from tax every single year. Above that, the gap narrows but never fully closes.
Both are retirement accounts built for people who work for themselves. Both get you a tax deduction now and tax-deferred growth later. That is where the similarity ends. One account rewards you for understanding the rules. The other rewards you for not wanting to think about it. Picking wrong does not ruin your retirement, but it does leave money on the table every year you make the wrong call.
A SEP IRA contribution is employer-only. You, as your own employer, can put in up to 25% of your net self-employment compensation, capped at $69,000 for 2024 and $70,000 for 2025. That sounds generous until you run the actual formula for a sole proprietor, which nets out closer to 20% of net profit after you account for the self-employment tax adjustment and the contribution itself reducing the base.
A Solo 401(k) splits into two pieces. You get an employee deferral, up to $23,000 in 2024 ($23,500 in 2025), plus a catch-up of $7,500 if you are 50 or older. Then you add an employer contribution of up to 20-25% of compensation, depending on your business structure. The two pieces stack, up to the same overall cap as the SEP, $69,000 or $70,000, with catch-up pushing it higher.
Here is why that stacking matters so much at lower income. Say you net $60,000 in self-employment income. A SEP IRA caps you around $11,000. A Solo 401(k) lets you put in roughly $23,000 as employee deferral plus another $11,000 employer piece, landing near $34,000. That is not a rounding error. That is three times the contribution on the same income, just because the account lets you wear two hats instead of one.
Numbers settle arguments faster than opinions do. Here is the rough contribution ceiling at a few income levels for a sole proprietor under 50, using 2024 limits.
| Net Self-Employment Income | SEP IRA Max | Solo 401(k) Max |
|---|---|---|
| $40,000 | ~$7,400 | ~$23,000 (deferral covers most of it) |
| $80,000 | ~$14,800 | ~$37,900 |
| $150,000 | ~$27,800 | ~$50,800 |
| $300,000+ | ~$69,000 (capped) | ~$69,000 (capped) |
Notice where the lines converge. At very high income, both accounts hit the same federal cap, so the advantage of the Solo 401(k) shrinks. Below six figures, the Solo 401(k) is not a marginal improvement. It is a completely different savings plan wearing the same tax label.
Pick the Solo 401(k) if you have no full-time employees besides yourself and maybe a spouse, and you want to maximize contributions on moderate income. Freelancers, consultants, single-member LLC owners, and small S-corp owners paying themselves a modest salary all benefit the most here, because the employee deferral lets you front-load savings before the employer formula even kicks in.
The Solo 401(k) also allows Roth contributions on the employee deferral portion. A SEP IRA, traditionally, does not offer a Roth option at all, though newer SEP Roth provisions exist and remain rare in practice because almost no custodian has built the infrastructure for it yet. If you want Roth flexibility without opening a second account type, the Solo 401(k) wins outright.
It also allows a loan provision, up to 50% of the balance or $50,000, whichever is lower. A SEP IRA never allows loans. If you want the account to double as emergency liquidity in a pinch, that feature matters more than people admit until they actually need it.
Pick the SEP IRA if you hate paperwork and your income is irregular or high enough that the Solo 401(k) gap stops mattering. SEP IRAs have almost no ongoing administration. No annual Form 5500 filing until the account crosses $250,000, no plan document maintenance, and most brokerages let you open one in ten minutes with no setup fee.
A Solo 401(k) requires a formal plan document, and once your account balance exceeds $250,000, you owe an annual Form 5500-EZ filing with the IRS. Miss it and the penalties are not trivial. If your business is a side venture that might not exist in two years, or you genuinely do not want another compliance obligation, the SEP IRA removes that friction entirely.
SEP IRAs also work better if you have a few part-time or seasonal employees you are not ready to cover under a 401(k) plan. SEP rules require you to contribute the same percentage for eligible employees as you do for yourself, which gets expensive fast with full staff, but for a solo operator with the occasional contractor, it stays manageable.
If you also do backdoor Roth IRA conversions personally, a SEP IRA balance counts against you under the pro-rata rule, the same way a traditional IRA does. That turns a clean backdoor conversion into a partially taxable mess. A Solo 401(k) balance does not count toward that pro-rata calculation at all, because it lives outside the IRA universe entirely.
This single fact pushes a lot of high earners toward the Solo 401(k) even when the SEP IRA would be simpler on paper. If backdoor Roth conversions are part of your long-term tax strategy, carrying a large SEP balance quietly sabotages it every single year until you roll that balance into a 401(k) plan to get it out of the IRA math.
SEP IRA contributions can be made as late as your tax filing deadline, including extensions, for the prior year. That flexibility helps if you do not know your net profit until your accountant finishes your return in September. Solo 401(k) employee deferrals, by contrast, generally need to be elected by December 31 of the tax year, though the employer contribution can still follow the extended filing deadline.
You can switch from a SEP IRA to a Solo 401(k) in a later year, and many self-employed people do exactly that once income climbs past the point where the stacking advantage justifies the extra paperwork. Opening the Solo 401(k) needs to happen by December 31 of the year you want contributions to count, so this is not a decision to leave until tax season. Decide in Q4, not April.
My take: for anyone netting under $150,000 from self-employment with no full-time staff, the Solo 401(k) is the better default almost every time. The paperwork is a minor cost relative to the extra five-figure contribution room, and once your custodian sets up the plan document, the annual maintenance is closer to a formality than a burden. Reach for the SEP IRA only when simplicity genuinely outweighs the math, or when you are covering part-time employees and do not want the complexity of a staffed 401(k) plan.
Yes, but the total contributions across both accounts still count toward the same combined annual limit, so there is no advantage to running both simultaneously for contribution purposes. Some people keep an old SEP IRA balance from prior years while contributing new money to a Solo 401(k), which is fine as long as the deadlines and formulas are tracked separately.
Yes, a spouse who earns income from the same business can also participate in the Solo 401(k), effectively doubling the household contribution room. This only works if the spouse is on payroll or receives documented self-employment income from the business, not just informal help.
SEP IRAs typically have lower or no setup fees since they are just a specialized IRA with no plan document required. Many major brokerages now offer free Solo 401(k) plans too, but smaller providers sometimes charge annual administration fees once the account requires a Form 5500-EZ filing.
Once you hire a full-time employee who works more than 1,000 hours a year, you generally must stop treating the plan as a Solo 401(k) and convert it to a regular 401(k) plan that covers that employee. This adds compliance costs and nondiscrimination testing requirements that a true solo plan avoids.
A SEP IRA tends to suit highly variable income better because contributions are flexible and can be decided as late as the extended tax filing deadline with no prior election required. A Solo 401(k) still offers flexibility on the employer portion, but the employee deferral generally needs to be elected before year-end, which is harder to plan around with unpredictable cash flow.