Solo 401(k) vs. SEP IRA: Which One Actually Gives You More Room

A version of this question shows up constantly on Bogleheads: someone has self-employment income, maybe a full side business, maybe consulting work on top of a W-2 job, and they're deciding between a Solo 401(k) and a SEP IRA. Both are IRS-approved retirement accounts built for people without an employer-sponsored plan covering that income. Both let you deduct the contribution now and defer tax on the growth. Past that, they're not close to equivalent, and for most self-employed people, the SEP IRA leaves a meaningful amount of contribution room on the table.

Here's how each one actually works, the exact 2026 numbers, and a worked comparison so you can see the gap for yourself.

The structural difference

A SEP IRA only allows one kind of contribution: an employer contribution, calculated as a percentage of compensation. If you're self-employed, you're both the employer and the employee, but the SEP only recognizes the employer side. There's no separate employee deferral.

A Solo 401(k) allows two kinds of contributions, stacked on top of each other. You can make an employee elective deferral, the same kind of contribution a W-2 employee makes to a company 401(k), up to a flat dollar limit regardless of income level. On top of that, you can also make the same employer contribution a SEP would allow, calculated the same way.

That employee deferral is the entire reason a Solo 401(k) usually wins. It's a fixed amount you get to contribute before the percentage-of-income calculation even starts.

The 2026 numbers

For 2026, the employee elective deferral limit for a 401(k), including a Solo 401(k), is $24,500. If you're 50 or older, an additional $8,000 catch-up contribution brings your deferral limit to $32,500. A newer catch-up tier from SECURE 2.0 applies to people who are 60, 61, 62, or 63 specifically: their catch-up is $11,250 instead of $8,000, bringing their deferral limit to $35,750 (IRS Notice 2025-67, the annual cost-of-living adjustment notice covering 2026 retirement plan limits).

The employer contribution, whether inside a Solo 401(k) or a standalone SEP IRA, is nominally capped at 25% of compensation. For a self-employed person filing a Schedule C, that 25% figure is misleading. The actual calculation is circular: your net self-employment earnings are reduced by half of your self-employment tax deduction, and then reduced again by the contribution itself, before the 25% is applied. IRS Publication 560 walks through the math, and the shortcut that comes out the other end is that self-employed people should use an effective rate of about 20% of net self-employment earnings (net profit minus the deduction for one-half of self-employment tax), not the full 25%.

Both the employee deferral and the employer contribution, combined, are capped by the overall annual additions limit under Internal Revenue Code Section 415(c), which is $72,000 for 2026 (before catch-up contributions, which stack on top). Compensation used in any of these calculations is itself capped at $360,000 for 2026 under Section 401(a)(17).

A worked comparison

Take someone with $80,000 in net self-employment earnings for the year, after the deduction for one-half of self-employment tax, under age 50.

SEP IRA. The only contribution available is the employer contribution: 20% effective rate × $80,000 = $16,000.

Solo 401(k). The employee deferral is capped at the lesser of $24,500 or their net self-employment earnings; $80,000 comfortably supports the full $24,500. The employer contribution is calculated the same way as the SEP: 20% × $80,000 = $16,000. Total: $24,500 + $16,000 = $40,500.

Same $80,000 of income. A $24,500 difference in how much of it can go into a tax-advantaged account this year, purely because of which account type holds it. At a 24% marginal federal tax bracket, that gap alone is worth roughly $5,880 in current-year tax deferral, before counting decades of tax-deferred growth on the difference.

The gap narrows for very low self-employment income, since the employee deferral is capped at your actual earnings, and it narrows again for very high earners once the $72,000 combined limit is reached by both account types, at which point they converge. For a broad middle range of self-employment income, roughly $25,000 to $250,000 a year, the Solo 401(k) gives meaningfully more room in nearly every case.

Where a SEP IRA still makes sense

The Solo 401(k)'s bigger number doesn't make it the automatic right answer for everyone.

A SEP IRA is simpler to open and maintain. Most brokerages let you open one online in minutes with a short form, and there's no annual government filing requirement regardless of the account's size. A Solo 401(k), once its assets exceed $250,000, requires filing Form 5500-EZ with the IRS every year, with real penalties for missing it.

A SEP IRA works better if you have common-law employees, not just yourself. SEP contributions must be made at the same percentage rate for every eligible employee, including you. If you're likely to hire W-2 employees soon, a SEP's simplicity can be worth more than the extra Solo 401(k) contribution room, though a Solo 401(k) generally has to be converted or closed once you have non-owner, non-spouse employees anyway, since "solo" is doing real work in that name.

A SEP IRA can be opened later in the year, up to your tax filing deadline including extensions. A Solo 401(k) has to be established by December 31 of the tax year to make an employee deferral for that year, though the employer contribution itself can still be made up until the filing deadline. If you're setting this up in March for the prior tax year, only the SEP (or a Solo 401(k) opened in the prior year) is still an option for the full contribution.

The backdoor Roth complication

If you also do backdoor Roth IRA conversions in the standard non-employer way, a SEP IRA balance creates a problem a Solo 401(k) doesn't. The IRS pro-rata rule treats all of your traditional IRA balances, including SEP and SIMPLE IRAs, as one combined pool when you convert any of it to a Roth IRA. A large SEP balance sitting alongside a small nondeductible traditional IRA contribution means most of your "backdoor" conversion ends up taxable, not tax-free.

A Solo 401(k) balance isn't an IRA at all, so it doesn't enter that calculation. Someone who wants to keep doing clean backdoor Roth conversions every year, on top of maxing out self-employment retirement savings, generally needs to avoid a SEP IRA balance for exactly this reason, or roll an existing SEP balance into a Solo 401(k) if their provider allows incoming rollovers.

Roth contributions

SECURE 2.0 technically allows Roth contributions inside a SEP IRA as of 2023, but actual provider support has been slow to roll out, and plenty of major brokerages still don't offer it. Roth Solo 401(k)s are far more broadly supported: most Solo 401(k) providers let you designate part or all of your employee deferral as Roth, and SECURE 2.0 extended that option to the employer contribution as well. If a Roth option matters to your strategy, that's another point in the Solo 401(k)'s favor today, though it's worth checking current provider offerings directly since this is an area that's still catching up to the law.

Choosing between them

For most people with steady self-employment or side-business income who don't have employees, the Solo 401(k) is the better default: more contribution room across nearly the entire realistic income range, broader Roth support, and no pro-rata complication for backdoor Roth conversions. The tradeoffs, mainly the Form 5500-EZ filing above $250,000 in assets and the December 31 deadline to establish the account, are manageable for most people who plan a few months ahead rather than scrambling in March.

Whichever account you choose, it's worth tracking business equity and retirement balances together as part of your full net worth, not as a separate mental bucket. A Solo 401(k) or SEP balance is real, growing net worth, even though you won't touch it for years.

Frequently asked questions

Can I have both a Solo 401(k) and a SEP IRA at the same time?

You can technically hold both accounts open, but for the same self-employment business, the combined contributions across both are still bound by the same $72,000 annual additions limit for 2026, so there's no extra room gained by splitting contributions between the two. Most people who choose a Solo 401(k) for its higher limit simply stop contributing to an old SEP IRA rather than funding both.

What happens to my Solo 401(k) if I hire an employee?

A Solo 401(k) is only valid when the business has no employees other than the owner and the owner's spouse. Hiring a non-spouse employee generally requires converting to a standard 401(k) plan that covers eligible employees, or exploring a SEP IRA instead, since SEP IRAs are built to cover multiple employees from the start.

Does the Solo 401(k) contribution limit include my day job's 401(k)?

The $24,500 employee elective deferral limit for 2026 is a per-person limit across all 401(k) plans you contribute to, including a W-2 employer's plan and your own Solo 401(k). If you already deferred $15,000 into your day job's 401(k), you have $9,500 of employee deferral room left for your Solo 401(k) that year. The employer-side contribution, based on 20% of your self-employment earnings, is separate and isn't reduced by your day job's plan.

Is a SEP IRA or Solo 401(k) better for a small side income under $20,000 a year?

At lower income levels the dollar gap between the two narrows, since the employee deferral in a Solo 401(k) is capped at your actual net earnings, not the full $24,500. A SEP IRA's simpler setup and lack of an annual filing requirement can make it the more practical choice for a modest, steady side income where the extra contribution room wouldn't be fully usable anyway.