Solo 401(k) vs SEP IRA for 2026: max contribution math, mega-backdoor Roth, beneficiary rules

A solo 401(k) usually wins on maximum contribution flexibility for a self-employed person with moderate income because it combines an employee elective deferral with an employer nonelective or profit-sharing contribution. A SEP IRA is simpler and can still be powerful, but it is funded by employer contributions only. The 2026 IRS dollar limits make that difference obvious: the elective deferral limit for 401(k) plans is $24,500, the age-50 catch-up limit is $8,000, the special SECURE 2.0 age-60-to-63 catch-up limit is $11,250, and the defined contribution annual additions limit is $72,000, excluding catch-up contributions. SEP maximum contribution is also $72,000 for 2026, but SEP plans do not permit elective salary deferrals or catch-up contributions.

The IRS confirmed the 2026 numbers in IR-2025-111 and its COLA table. Publication 560, the IRS small-business retirement-plan publication, repeats the 2026 compensation limit of $360,000, elective deferral limit of $24,500, defined contribution limit of $72,000, and catch-up rules. Those are the numbers this page uses.

The decision is not only about the biggest deduction. A solo 401(k) can allow Roth salary deferrals, participant loans if the plan document permits, after-tax employee contributions for mega-backdoor Roth design if the provider supports them, and more detailed beneficiary planning. A SEP IRA is usually cheaper to establish, easier to maintain, and easier to fund after year-end. The best choice depends on income level, age, cash flow, Roth goals, whether common-law employees exist, and whether the owner wants simple administration or advanced features.

For a W-2 owner of an S corporation, employer retirement contributions are generally based on W-2 compensation, subject to plan terms and limits. For a Schedule C sole proprietor or partner, the math is less intuitive. The IRS one-participant 401(k) page says self-employed individuals must use a special computation, and that earned income means net earnings from self-employment after deducting one-half of self-employment tax and the contribution for the owner. Pub. 560 provides the rate table and worksheets.

The simplified planning shortcut is that a self-employed person's maximum employer contribution is roughly 20% of adjusted net earnings, not 25% of raw Schedule C profit. That happens because the contribution itself reduces the compensation base. A SEP IRA uses the same self-employed contribution logic. A solo 401(k) adds the employee deferral on top of that employer piece, which is why it often produces a much larger contribution at moderate income levels.

These examples are estimates to show the structure. A real return should use Pub. 560 worksheets, actual self-employment tax, other W-2 401(k) deferrals, controlled-group rules, and provider plan documents.

Assume Jordan has $80,000 of Schedule C net profit in 2026, no employees, no other 401(k) deferrals, and is under age 50. A rough self-employment-tax calculation produces about $11,303 of self-employment tax and a one-half SE tax deduction of about $5,652. Adjusted net earnings before the retirement contribution are about $74,348.

With a SEP IRA, the contribution is roughly 20% of $74,348, or about $14,870. With a solo 401(k), Jordan can first make a $24,500 employee elective deferral, then add the same rough employer contribution of $14,870, for a total near $39,370. The solo 401(k) contribution is more than double the SEP contribution because the employee deferral works even when business profit is not high enough to reach the $72,000 annual additions limit through employer contributions alone.

This is the classic solo 401(k) advantage. If the owner wants the biggest current-year retirement contribution on a moderate self-employment income, the solo 401(k) usually produces more room. The SEP IRA is still valid, but its simplicity comes with a lower ceiling at this income level.

Assume Priya has $200,000 of Schedule C net profit in 2026, no employees, no other workplace plan deferrals, and is under age 50. Because the 2026 Social Security wage base is $184,500, a rough self-employment-tax estimate is about $28,234, with a one-half SE tax deduction near $14,117. Adjusted net earnings before the retirement contribution are about $185,883.

A SEP IRA contribution is roughly 20% of $185,883, or about $37,177. A solo 401(k) can combine the $24,500 employee deferral with the same rough employer contribution, for a total near $61,677. That is still below the $72,000 annual additions limit, so the limiting factor is earned income, not the annual dollar cap.

If Priya is age 50 or older and the solo 401(k) allows catch-up, she may add the $8,000 catch-up for a total near $69,677. If she is 60, 61, 62, or 63 and the plan implements the SECURE 2.0 higher catch-up, the catch-up amount is $11,250 for 2026, bringing the total near $72,927. Catch-up contributions are not counted against the $72,000 annual additions cap, but they still require compensation and plan support.

The mega-backdoor Roth strategy generally requires after-tax employee contributions to a 401(k) plan, followed by an in-plan Roth conversion or an in-service distribution to a Roth IRA. The annual additions limit creates the outer cap. For 2026, that cap is $72,000 excluding catch-up contributions. If an owner contributes $24,500 of employee deferrals and $20,000 of employer contributions, there may be room for after-tax employee contributions up to the plan's annual additions limit, if the plan document permits them and the provider can administer the conversion.

A SEP IRA does not offer the same architecture. A regular SEP is an IRA-based employer contribution arrangement. It does not have employee elective deferrals, after-tax employee 401(k) contributions, in-plan Roth conversion mechanics, or participant loan features. SECURE 2.0 section 601 allows employers maintaining SEP or SIMPLE IRA arrangements to offer Roth treatment for certain contributions, and the IRS has issued W-2 reporting guidance for Roth SEP and Roth SIMPLE contributions. That is not the same as a mega-backdoor Roth pipeline. It is a Roth contribution feature inside an IRA-based arrangement, subject to provider availability and IRS reporting rules.

The practical warning is that not every solo 401(k) sold online supports the mega-backdoor Roth. Many low-cost prototype plans allow pre-tax and Roth employee deferrals but do not allow after-tax employee contributions or in-service conversions. If the mega-backdoor Roth is a primary reason for choosing a solo 401(k), read the adoption agreement before opening the account. The tax strategy exists only if the plan document and custodian can actually process it.

SECURE 2.0 affects this comparison in several ways. First, the higher catch-up limit for ages 60 through 63 is active: for 2026 it remains $11,250 for most 401(k)-type plans. That gives solo 401(k) users in that age band more room than SEP users, because SEP plans have no catch-up contributions. Second, designated Roth accounts in qualified plans are no longer counted for lifetime RMDs before the participant's death. Pub. 560 explains that lifetime required minimum distributions are no longer required from a designated Roth account in a qualified plan, but distributions are still required after death.

Third, SECURE 2.0 opened the door to Roth SEP and Roth SIMPLE contributions. The IRS SECURE 2.0 W-2 reporting guidance says section 601 allows an employer that maintains a SEP or SIMPLE IRA plan to offer participating employees the option to designate a Roth IRA as the IRA to which contributions are made. That makes the SEP IRA more flexible than it used to be, but provider availability remains the practical bottleneck.

Fourth, high-earner mandatory Roth catch-up rules continue to require plan-administration attention. The user-facing takeaway is simple: a solo 401(k) can be more powerful, but it is also more dependent on the plan document staying current with law changes. A SEP IRA may be less customizable, but fewer moving parts can be valuable for an owner who wants a deduction without plan design complexity.

Beneficiary planning is often ignored until it matters. A SEP IRA is an IRA, so beneficiary administration usually follows the IRA custodian's inherited IRA process. A spouse beneficiary often has rollover or own-IRA options. Many non-spouse designated beneficiaries are subject to the 10-year rule, with important distinctions for eligible designated beneficiaries such as surviving spouses, minor children of the participant, disabled or chronically ill beneficiaries, and beneficiaries not more than 10 years younger than the owner.

A solo 401(k) is a qualified plan. It has a plan document, beneficiary designation, possible Roth and pre-tax subaccounts, and Form 5500-EZ filing once assets exceed the threshold. The final required minimum distribution regulations published at 89 Fed. Reg. 58886 and reproduced in IRB 2024-33 implement SECURE Act and SECURE 2.0 changes for qualified plans and IRAs. Those rules can require annual beneficiary distributions during the 10-year period when the original owner died after the required beginning date, and they preserve special rules for eligible designated beneficiaries.

The practical difference is control. A solo 401(k) may allow more plan-level design, Roth subaccount separation, and custom beneficiary procedures, but it also puts more responsibility on the owner to keep beneficiary forms, plan documents, and successor trustee access current. A SEP IRA is usually easier for heirs and custodians to administer. Estate planners often care about this more than the contribution limit because a poorly documented plan can create administrative friction after death.

One court case often cited in inherited IRA planning is Clark v. Rameker, 573 U.S. 122 (2014), where the Supreme Court held that inherited IRA funds were not "retirement funds" for the federal bankruptcy exemption at issue. It is not a contribution-limit case, but it is a reminder that inherited retirement accounts can have different legal character from accounts held by the original saver.

Choose a solo 401(k) when the owner has no eligible common-law employees, wants to contribute more at moderate income, wants Roth deferrals, wants age-50 or age-60-to-63 catch-up contributions, wants possible loans, or wants to evaluate mega-backdoor Roth design. This is especially compelling for consultants, creators, physicians with side income, high-income freelancers, and owner-only S corporations that can handle plan administration.

Choose a SEP IRA when simplicity is the main goal, the owner wants employer-only contributions, the business may not contribute every year, setup needs to be fast, or the owner does not need employee deferrals or Roth 401(k) features. A SEP can also be useful when the owner already maxes out a W-2 401(k) at an unrelated employer and wants to make an employer contribution from side-business income, though the shared elective deferral limit and controlled-group rules must be checked.

Pause before using either plan if the business has employees or may hire soon. A solo 401(k) loses its owner-only simplicity when eligible employees enter the plan. A SEP IRA generally requires equal-percentage contributions for eligible employees. The wrong plan can become expensive if a business grows from a one-person operation into a small team without updating its retirement strategy.

• IRS 2026 limits: IR-2025-111 and IRS COLA table.
• Small-business retirement plan mechanics: Publication 560, IRS one-participant 401(k) plans, and IRS SEP contribution limits.
• SECURE 2.0 Roth guidance: IRS SECURE 2.0 W-2 reporting guidance.
• Beneficiary and RMD regulations: 89 Fed. Reg. 58886, IRB 2024-33, and Clark v. Rameker.