There is no universal winner between a one-participant 401(k), often called a Solo 401(k), and a SEP IRA. The better fit depends first on eligible employees, business compensation, other workplace plans, desired contribution types, plan-document features, deadlines, and administration. A Solo 401(k) can combine employee deferrals with employer contributions, while a SEP generally uses employer contributions only. The headline $72,000 limit does not mean either plan automatically accepts a $72,000 contribution.
How do Solo 401(k) and SEP IRA plans differ?
| Decision point | One-participant 401(k) | SEP IRA |
|---|---|---|
| Who can use it? | A business owner with no common-law employees, other than a spouse, who would be eligible under the plan | An employer of any size, subject to SEP eligibility and contribution rules for employees |
| Contribution sources | Employee elective deferrals plus employer nonelective or profit-sharing contributions | Employer contributions; ordinary employee salary deferrals are not part of a current SEP |
| 2026 employee deferral | Up to $24,500 across the person's applicable 401(k), 403(b), and similar deferrals, subject to compensation and plan terms | None under an ordinary SEP |
| 2026 annual additions | Generally limited to the lesser of $72,000 or 100% of applicable compensation before permitted catch-up contributions | Generally limited to the lesser of 25% of employee compensation or $72,000; self-employed owners use a special calculation |
| Age catch-up | Potentially $8,000 at age 50 or older, or $11,250 at ages 60 through 63, when the plan and participant qualify | No SEP catch-up contribution merely because the owner is age 50 or older |
| Employees | Eligible common-law employees change the plan from one-participant status and introduce coverage, testing, notice, and other plan duties | Eligible employees generally receive the same contribution percentage as the owner, based on their compensation |
| Loans and Roth features | May be available only when the plan document and provider support them | Loans are prohibited from IRAs; Roth SEP availability and administration depend on the plan and provider |
| Annual filing | Form 5500-EZ or another permitted filing can be required when assets cross the threshold and for the final plan year | The employer generally does not file an annual Form 5500 for the SEP itself |
| Administration | Requires a qualified plan document, operation under its terms, contribution tracking, and possible annual filing | Usually simpler, but employee eligibility, notices, allocation, and IRA setup still require administration |
The comparison starts with plan eligibility and compensation, not with a generic online calculator. The 2026 retirement contribution limit guide explains the federal ceilings that both plans interact with.
What is a one-participant 401(k)?
A one-participant 401(k) is a regular qualified 401(k) plan covering a business owner with no common-law employees and potentially the owner's spouse. The tax law does not create a separate Solo 401(k) account type with relaxed contribution limits. It applies ordinary 401(k) rules, with some nondiscrimination testing becoming unnecessary while no non-owner employees participate.
The owner can act in two contribution roles. As an employee, the owner can make elective deferrals up to the person's remaining annual limit. As the employer, the business can make a nonelective or profit-sharing contribution under the plan formula. The two amounts then interact with the annual-additions limit, compensation limit, deduction rules, and plan terms.
A spouse does not receive a second owner contribution merely because of marriage. The spouse must perform services for the business, receive eligible compensation, satisfy the plan terms, and be included as a participant. Each participant then has separate compensation and contribution calculations.
What is a SEP IRA?
A SEP is an employer retirement arrangement that funds traditional IRAs established for eligible participants. A current SEP generally receives employer contributions rather than employee elective salary deferrals. The employer decides whether to contribute for a year, but when it contributes, the same contribution percentage generally applies to each eligible participant's compensation under the plan.
That employee rule can dominate the decision. A self-employed owner with several eligible employees cannot contribute a high percentage only for the owner while giving employees zero under a standard SEP allocation. Before opening the plan, build the employee census and review related businesses, leased workers, service history, compensation, and the document's eligibility provisions.
A SEP's simpler setup does not mean there are no records. The employer must use an approved document, provide required information, establish participant IRAs, calculate allocations correctly, deposit contributions, and retain proof. A financial institution's product menu can be narrower than the federal rules.
How do the 2026 contribution limits work?
For 2026, the employee elective-deferral limit is $24,500. The standard age-50 catch-up is $8,000, while an eligible participant age 60 through 63 at year-end can have a larger $11,250 catch-up instead. The defined-contribution annual-additions ceiling is $72,000 before permitted catch-up contributions, and the qualified-plan compensation limit is $360,000.
Those numbers describe different layers. Employee deferrals, employer contributions, after-tax employee contributions when permitted, and catch-up contributions cannot be combined as though each were a separate independent allowance. A plan can also be more restrictive than the federal ceiling.
For a SEP employee, the employer contribution is generally limited to 25% of compensation and the annual dollar ceiling. For a sole proprietor or partner treated as self-employed, the owner contribution is not calculated by multiplying gross revenue or Schedule C profit by 25%. The IRS worksheet adjusts net earnings for the deductible portion of self-employment tax and for the contribution itself, producing a reduced contribution rate.
What if the owner also has a workplace 401(k)?
The employee elective-deferral limit belongs to the individual. Deferring at a W-2 job generally reduces the employee-deferral amount remaining for a one-participant 401(k). Opening another plan does not create another full $24,500 employee allowance.
Employer annual-additions limits can sometimes apply separately to plans of unrelated employers, but ownership, controlled-group, affiliated-service-group, common-control, and plan aggregation rules can change the result. Do not assume two businesses are unrelated merely because they have different names or tax identifiers. Disclose all ownership and plan participation to the administrator or tax professional performing the calculation.
A SEP contribution does not consume the personal $7,500 traditional-and-Roth IRA contribution limit. It can, however, change other tax calculations and must still satisfy its own employer contribution, compensation, deduction, and annual-additions limits.
What happens when the business hires employees?
A one-participant 401(k) loses its testing exemption when eligible common-law employees begin participating. The plan may need amendments, employee notices, payroll deferrals, employer contributions, nondiscrimination testing, bonding, and a different annual filing. Waiting until year-end to discover an eligible employee can create an operational failure.
A SEP can cover employees more simply, but it can become expensive when the owner wants a large contribution and the same percentage must be allocated to every eligible employee. Neither outcome makes one plan universally better. Model the actual census for the coming two or three years, not only today's owner-only business.
Related companies matter here too. Employees of businesses under common control or within an affiliated service group may need to be considered even when they are paid by another entity. This is a plan qualification question, not a branding or payroll-platform choice.
When must the plan be established and funded?
A SEP can generally be established for a tax year as late as the employer's federal income-tax return due date, including extensions, and the employer contribution generally follows the deductible contribution deadline. That flexibility can be useful after business results are known.
A one-participant 401(k) has separate plan-adoption, employee deferral-election, deposit, and employer-contribution deadlines. The answer varies by entity, compensation timing, plan year, and whether special first-year sole-proprietor rules apply. A contribution deposited by the tax-return deadline can still fail if the plan or election was not timely established.
Provider processing deadlines can precede statutory deadlines. Obtain a written calendar before year-end and keep the signed plan document, adoption agreement, election, payroll or earned-income calculation, deposit confirmations, and tax filing together.
What ongoing administration should be compared?
A one-participant plan must operate according to its written terms. When total plan assets exceed $250,000 at year-end, an annual Form 5500-EZ or permitted alternative is generally required; a filing is also generally required for the final plan year regardless of asset size. Late-filing relief has its own requirements and should not be treated as the operating plan.
Compare provider support for amendments, restatements, beneficiary records, required minimum distributions, loans, Roth accounting, rollovers, valuation, corrections, and final termination. A low opening fee does not reveal the complete administrative workload.
A SEP generally avoids Form 5500 filing, but each participant owns an IRA and receives IRA tax reporting. The employer still needs correct eligibility, allocation, notices, deposits, and plan records. SEP simplicity is real only when those duties fit the business.
How do these plans interact with a backdoor Roth IRA?
A traditional SEP IRA is included in the individual's Form 8606 traditional IRA aggregation calculation. A pretax SEP balance can therefore change the taxable portion of a Roth conversion even when the nondeductible contribution and conversion occur at another custodian.
A one-participant 401(k) is not a traditional IRA in that Form 8606 aggregation pool. Some plan documents accept eligible incoming IRA rollovers, but federal permission does not require the plan or provider to accept one. Fees, investments, protection, access, eligible amounts, and processing must be reviewed before using a qualified plan to change an IRA balance. See the 2026 backdoor Roth and pro-rata checklist.
Use this plan-selection checklist
- List every owner, spouse, employee, related business, and existing retirement plan.
- Classify the business entity and calculate plan compensation from the applicable tax records.
- Separate employee deferrals, catch-ups, employer contributions, and personal IRA contributions.
- Subtract employee deferrals already made through other applicable workplace plans.
- Calculate the Solo 401(k) and SEP contribution under current IRS worksheets rather than a gross-revenue percentage.
- Project employee eligibility and contribution cost for at least the next two years.
- Compare written plan features, provider support, investments, fees, and annual filing duties.
- Put adoption, election, deposit, return, extension, and Form 5500-EZ dates on one calendar.
- Reconcile the selected contribution with the business and individual tax returns before filing.
What can RetireStride calculate?
The RetireStride retirement projection can compare long-term annual saving assumptions, but it does not determine self-employment plan compensation, employee eligibility, controlled-group status, deduction limits, plan deadlines, or an allowable Solo 401(k) or SEP contribution. Review the methodology before using a contribution assumption.
Use the projection only after an allowable contribution range has been calculated from the business records and plan document. The traditional-versus-Roth IRA guide separately covers personal IRA eligibility and deductions.