A Solo 401(k) and a SEP IRA can both help self-employed workers save for retirement, but they work very differently. A Solo 401(k) often allows a larger contribution at lower and moderate income because the owner can contribute in two roles — as employee and employer — while a SEP is built around employer contributions and is usually simpler to administer.
Neither plan is automatically better. Income, age, employees, another workplace retirement plan, Roth preferences, and administrative complexity can all change the answer.
Quick Answer
A Solo 401(k), also called a one-participant 401(k), is generally designed for a business owner with no common-law employees other than a spouse. The owner can make an elective deferral as an employee and can also make an employer nonelective contribution.
A SEP, or Simplified Employee Pension, allows an employer — including a self-employed person — to make contributions to SEP-IRAs for eligible participants. A modern SEP does not offer normal employee salary deferrals. However, a participant may be able to make a separate regular IRA contribution to the same SEP-IRA if the account permits it, subject to the ordinary IRA rules. Grandfathered SARSEPs operate under different rules.
For 2026, the standard 401(k) elective-deferral limit is $24,500, while the overall defined-contribution limit is generally $72,000before catch-up contributions. A SEP contribution is generally limited to the lesser of 25% of an employee’s compensation or $72,000, with a special calculation required for self-employed owners.
What Is a Solo 401(k)?
The IRS calls a Solo 401(k) a one-participant 401(k) plan. It is not a separate category of retirement plan; it is a regular 401(k) designed for a business with no common-law employees other than the owner, or the owner and spouse.
The business owner wears two hats:
- Employee: the owner can make elective deferrals, subject to the annual personal deferral limit and available earned income.
- Employer: the business can make an additional employer nonelective contribution, subject to the plan rules and annual limits.
A spouse who genuinely works in the business and has compensation or earned income from it can generally participate as well. The spouse does not automatically receive a second full contribution limit simply by being married to the owner; the spouse must have qualifying compensation or earned income and satisfy the plan’s terms.
Solo 401(k) plans can also offer features such as designated Roth contributions and participant loans, but only if the plan document provides for them.
What Is a SEP IRA?
A SEP plan is an employer retirement arrangement under which contributions are made to IRAs established for eligible participants. Any employer, including a self-employed individual, can establish a SEP.
Modern SEPs are funded through employer SEP contributions. They do not allow normal employee elective salary deferrals. The important exception is a grandfathered SARSEP established before 1997, which is outside the scope of this comparison.
If the SEP-IRA permits it, an individual can separately make a regular traditional IRA contribution to the same account. That is an IRA contribution, not a SEP employer contribution, and it follows the normal IRA contribution and deduction rules.
SEP Employee Eligibility
Under the standard IRS SEP framework, an employer generally must include an employee who:
- Has reached age 21
- Worked for the employer in at least 3 of the preceding 5 years
- Received at least $800 in compensation from the employer in 2026
An employer can use less restrictive eligibility rules. Certain employees, such as some union employees and certain nonresident aliens, may be excluded under the applicable rules.
Most SEPs, including the IRS model Form 5305-SEP, require contributions to be allocated proportionally so eligible participants receive the same stated percentage of compensation. A self-employed owner’s own contribution is calculated differently because the tax law requires a reduced contribution-rate calculation for self-employment income.
Solo 401(k) vs SEP IRA: Side-by-Side Comparison
| Feature | Solo 401(k) | SEP IRA |
|---|---|---|
| Typical user | Business owner with no common-law employees other than spouse | Businesses of any size, including self-employed owners |
| Employee elective deferrals | Yes | No in a modern SEP; grandfathered SARSEPs differ |
| Employer contributions | Yes | Yes |
| 2026 elective-deferral limit | $24,500, subject to earned income and shared-deferral rules | Not applicable to SEP employer contributions |
| 2026 overall defined-contribution limit | $72,000 before catch-up contributions | $72,000 maximum SEP contribution, subject to compensation rules |
| Catch-up contribution | Yes, if eligible and the plan permits it | No SEP-plan catch-up; separate IRA contribution rules may apply |
| Roth option | May permit Roth elective deferrals and certain Roth employer contributions if the plan supports them | Roth SEP contributions are legally permitted, but plan/custodian support varies |
| Participant loans | May be permitted by the plan | Not permitted |
| Employees | Eligible common-law employees end the one-participant treatment and must be handled under regular 401(k) rules | Eligible employees generally must receive SEP contributions under the plan’s allocation formula |
| Annual government filing | Form 5500-EZ may become required | Generally no Form 5500-series filing for a standard SEP arrangement |
| Administration | Higher | Generally lower |
2026 Contribution Limits
Solo 401(k) Employee Elective Deferral
The standard elective-deferral limit for 401(k) plans in 2026 is $24,500.
This is a personal limit, not a separate limit for every 401(k) account. If someone participates in more than one 401(k) plan during the year, elective deferrals generally must be coordinated across those plans. Elective deferrals to certain other plan types can also count toward the same individual limit. Governmental 457(b) plans generally have a separate deferral limit.
Solo 401(k) Employer Contribution
The employer contribution can generally be up to 25% of compensation for a common-law employee. For a self-employed owner, however, the IRS requires a special calculation because the owner’s contribution itself affects the definition of earned income.
When a plan uses a 25% employer contribution rate, the corresponding reduced rate for a self-employed owner is generally 20% of adjusted net earnings used in the retirement-plan calculation — not 25% of raw Schedule C profit.
For more context on how Schedule C profit relates to self-employment calculations, see Schedule C Explained: How Freelancers Report Business Income and Expenses.
Overall 2026 Limit
The general Section 415(c) limit for annual additions to a defined-contribution plan is $72,000 in 2026, or 100% of compensation if lower. Catch-up contributions are not counted inside that $72,000 limit.
Catch-Up Contributions
For 2026:
- Participants age 50 or older who are not in the special age-60-to-63 group can generally make up to an additional $8,000 catch-up contribution.
- Participants who turn age 60, 61, 62, or 63 during 2026 can generally use the higher $11,250 catch-up limit instead of the $8,000 amount.
That means the potential total can reach $80,000 for someone using the standard $8,000 catch-up or $83,250 for someone eligible for the $11,250 catch-up, assuming sufficient compensation and all other requirements are met.
Mandatory Roth Catch-Up Rule
Beginning in 2026, the IRS says participants whose prior-year wages from the plan sponsor exceed $150,000 are generally subject to the SECURE 2.0 Roth catch-up requirement when making catch-up contributions under an applicable plan with the required Roth feature.
The rule looks to prior-year wages from the sponsoring employer. Pure sole-proprietor arrangements can present different facts because self-employment earnings are not the same as W-2 wages. A self-employed owner affected by this rule should confirm the treatment with the plan provider or a retirement-plan professional rather than assuming the rule does or does not apply.
SEP IRA Contribution Limit
For 2026, employer contributions to an employee’s SEP-IRA generally cannot exceed the lesser of:
- 25% of compensation, or
- $72,000
For a self-employed owner, the special reduced-rate calculation applies. The IRS’s rate table shows that a 25% plan contribution rate becomes a 20% reduced contribution rate for the self-employed calculation.
2026 Compensation Limit
The maximum compensation that can generally be taken into account for 401(k) employer contributions and SEP contributions is $360,000 in 2026.
Why a Solo 401(k) Can Allow More at Lower Income
The key difference is the employee elective deferral.
A SEP contribution for a sole proprietor is percentage-based. A Solo 401(k), by contrast, can combine that employer contribution with an employee elective deferral of up to $24,500 in 2026, assuming enough earned income and no competing elective deferrals that use up the personal limit.
At lower and moderate income levels, that extra contribution layer can create a large gap between the plans.
Example: $80,000 of Schedule C Net Profit
Assume a sole proprietor has:
- $80,000 of Schedule C net profit
- No employees
- No other retirement plan
- Is under age 50
- A plan using the standard maximum 25% employer contribution formula
For simplicity, assume the regular self-employment tax calculation applies and there are no other wages affecting the Social Security portion.
| Step | Approximate Amount |
|---|---|
| Schedule C net profit | $80,000 |
| Self-employment tax | $11,303.64 |
| Deductible one-half of SE tax | $5,651.82 |
| Adjusted net earnings used for this simplified contribution calculation | $74,348.18 |
| Maximum employer contribution using 20% reduced rate | About $14,869.64 |
SEP IRA
Under these assumptions, the maximum SEP employer contribution would be approximately:
$14,870
Solo 401(k)
The Solo 401(k) could potentially combine:
- $24,500 employee elective deferral
- About $14,870 employer contribution
That produces a potential total of approximately:
$39,370
In this simplified example, the Solo 401(k) offers about $24,500 more contribution room because the employer-contribution calculation is essentially the same while the Solo 401(k) adds an employee elective deferral.
This is an educational example, not a universal maximum. Actual contribution calculations can change when a person has other wages, other retirement plans, different business entities, employees, or other tax adjustments.
What If You Also Have a W-2 Job?
A self-employed person can generally maintain a Solo 401(k) while also participating in a separate employer’s 401(k).
The important distinction is between the personal elective-deferral limit and the annual-additions limit.
If you defer $15,000 into an unrelated employer’s 401(k) in 2026, you would generally have only $9,500 of the standard $24,500 elective-deferral limit remaining for a Solo 401(k).
Employer contributions can work differently. The Section 415(c) annual-additions limit can apply separately to plans maintained by unrelated employers. The IRS provides examples in which a worker who maxes out elective deferrals at a W-2 employer can still make a substantial employer nonelective contribution to a Solo 401(k) maintained by an unrelated self-employed business.
Related businesses are more complicated. Controlled-group and affiliated-service-group rules can cause businesses to be treated as a single employer for retirement-plan purposes. Someone who owns part of the W-2 employer or operates related businesses should not assume separate limits apply.
Solo 401(k) Roth vs Traditional Contributions
A Solo 401(k) can permit traditional pre-tax elective deferrals and designated Roth elective deferrals if the plan document supports those options.
- Traditional elective deferrals generally reduce current federal taxable income, subject to the applicable tax rules.
- Roth elective deferrals are included in current taxable income and do not provide the same current-year income-tax reduction. Qualified Roth distributions can be tax-free if the requirements are met.
SECURE 2.0 also permits plans to allow certain employer matching and nonelective contributions to be designated as Roth, subject to the statutory requirements, vesting rules, plan design, and provider implementation.
Can a SEP Have Roth Contributions?
Yes. SECURE 2.0 changed the law so a SEP arrangement can allow contributions to a Roth SEP IRA.
That does not mean every financial institution currently supports the feature. Plan documents, custodians, and operational systems can differ, so a self-employed person interested in a Roth SEP should verify that the chosen provider actually supports it.
The current-year tax treatment of Roth SEP contributions is different from a traditional SEP contribution, so do not assume that designating a SEP contribution as Roth produces the same current-year deduction outcome for the participant.
What Happens If You Hire Employees?
Solo 401(k)
The one-participant advantage exists because the plan does not cover common-law employees other than the owner and spouse.
If the business hires employees who satisfy the plan’s eligibility requirements, they generally must be included. The plan then becomes subject to the rules that apply to a regular 401(k), including nondiscrimination testing unless an applicable exception such as a safe-harbor design applies.
That does not necessarily mean the legal plan must be completely replaced, but the simple owner-only treatment disappears and the plan document/provider must be able to operate a compliant employee plan.
SEP IRA
A SEP can continue when a business hires employees, but eligible employees generally must be covered under the plan’s contribution formula.
For a standard proportional SEP, eligible employees generally receive the same stated contribution percentage of compensation. The self-employed owner’s own contribution may look like a lower percentage of Schedule C-derived earnings because the IRS requires the special self-employed calculation.
For example, a plan with a stated 25% contribution rate can produce a 20% reduced rate for the owner’s self-employed calculation while common-law employees receive contributions based on the plan’s stated 25% compensation formula. This distinction is important when estimating the cost of adding employees.
Administration and Paperwork
Solo 401(k)
A Solo 401(k) generally requires:
- A written plan document
- Contribution and election records
- Ongoing compliance with 401(k) rules
- Potential Form 5500-EZ filing
For a one-participant plan, Form 5500-EZ is generally not required when the combined assets of all one-participant plans maintained by the employer are $250,000 or less at the end of the plan year. Once combined assets exceed $250,000, the filing requirement generally applies. A final return is required for the final plan year regardless of the asset amount.
For a calendar-year plan, the normal filing deadline is the last day of the seventh month after year-end — generally July 31.
Form 5500-EZ can be filed electronically through EFAST2. Some filers are required to file electronically under IRS e-filing rules; otherwise a paper filing may still be available under the applicable rules.
SEP IRA
A SEP is generally simpler. An employer can establish one using Form 5305-SEP when eligible to use that model document, an IRS-approved prototype, or an individually designed SEP document.
Standard SEP arrangements generally do not have an annual Form 5500-series filing requirement, which is one reason they appeal to self-employed workers who prioritize administrative simplicity.
Contribution and Setup Deadlines
SEP IRA
A SEP can generally be established for a year as late as the due date of the business’s income-tax return for that year, including extensions. Employer SEP contributions are generally also due by the employer’s tax-return due date, including extensions, for the contribution to be treated for that year under the applicable rules.
Solo 401(k)
401(k) timing is more complicated because plan adoption, elective-deferral elections, and employer contributions have separate rules.
Under the SECURE Act, a qualified plan can generally be adopted retroactively by the employer’s tax-return due date, including extensions, for certain employer-contribution purposes.
Elective deferrals are different. SECURE 2.0 created a special first-year rule for an individual who owns an entire unincorporated business and is the only employee. That sole proprietor can adopt a new 401(k) after year-end and make a first-year elective-deferral election for the prior year by the individual’s tax-return due date without extensions.
This exception is narrower than the general plan-adoption rule and should not be assumed to apply to an S corporation, C corporation, or a business with employees.
Can You Borrow From the Plan?
Solo 401(k)
A 401(k) plan may permit participant loans if the plan document allows them.
The general federal maximum is based on the lesser of:
- $50,000, subject to reductions for certain prior outstanding loans, or
- 50% of the participant’s vested account balance
There is also a rule that can allow a plan to permit up to $10,000 in certain cases when 50% of the vested balance is less than $10,000. Plans are not required to offer this exception.
Plan loans also have repayment and documentation requirements.
SEP IRA
SEP-IRAs cannot offer participant loans. Borrowing from an IRA can create a prohibited transaction and serious tax consequences.
Which Plan Is Easier to Manage?
The SEP IRA is generally easier to administer. There is no employee elective-deferral tracking, and standard SEP arrangements generally avoid annual Form 5500-series filings.
The Solo 401(k) generally requires more administration but offers features a SEP may not provide, including employee elective deferrals, potential catch-up contributions, and possible participant loans.
Which Plan Is Better for High-Income Self-Employed Workers?
For someone under age 50 with enough eligible income, both plans can eventually reach the general $72,000 2026 contribution ceiling.
However, a catch-up-eligible Solo 401(k) participant can potentially exceed that $72,000 amount because catch-up contributions sit on top of the Section 415(c) limit. A SEP does not provide a SEP-plan catch-up contribution.
Other considerations at higher income include:
- Roth contribution availability
- Whether another workplace 401(k) is already using the personal deferral limit
- Employees and required coverage
- Loan features
- Administrative burden
Which Plan Is Better for Freelancers Just Starting Out?
A Solo 401(k) can be especially powerful at lower and moderate income because the employee elective deferral is not limited to the same percentage-based formula as the employer contribution.
A SEP can still be attractive for someone who values simplicity, wants flexible employer contributions, and does not need the Solo 401(k)’s additional features.
The best comparison is not «which plan has the bigger published maximum?» It is «how much can I actually contribute under my income, employee, and other-plan situation?»
Common Solo 401(k) vs SEP IRA Mistakes
- Assuming everyone can contribute $72,000. Actual limits depend on compensation or earned income.
- Applying 25% directly to Schedule C profit. Self-employed owners require a special reduced-rate calculation.
- Forgetting the personal elective-deferral limit. A second 401(k) does not create a second full $24,500 deferral limit.
- Assuming the annual-additions limit always combines across unrelated employers. The rules distinguish personal deferrals from employer-by-employer plan limits.
- Keeping owner-only treatment after eligible employees are hired.
- Forgetting Form 5500-EZ when the filing requirement applies.
- Assuming every Solo 401(k) offers Roth contributions or loans. Those features depend on the plan.
- Assuming every SEP custodian supports Roth SEP contributions.
- Confusing Roth contributions with current-year deductible contributions.
- Assuming a SEP gives employees normal salary-deferral rights.
- Using the wrong setup or contribution deadline.
Decision Checklist
- Do you have employees other than a spouse?
- Do you also participate in another employer’s 401(k) or similar plan?
- How much can you realistically contribute based on your earned income?
- Are you age 50 or older and eligible for catch-up contributions?
- Do you want a Roth contribution option?
- Would a participant-loan feature matter to you?
- How much administration are you willing to handle?
- Could you hire employees soon?
Frequently Asked Questions
Is a Solo 401(k) better than a SEP IRA?
Not universally. A Solo 401(k) often offers more contribution flexibility and plan features, while a SEP is generally simpler to establish and administer. Employees, income, age, other retirement plans, and Roth preferences can change the answer.
Can I have both a Solo 401(k) and a SEP IRA?
Potentially, but maintaining both does not automatically double the contribution limit. If the same self-employed business maintains multiple defined-contribution plans, annual additions generally must be coordinated under the applicable limits. Using both plans can also prevent use of the simplest Form 5305-SEP arrangement in some cases.
Can I contribute to a Solo 401(k) if I have a W-2 job?
Yes. But your personal elective-deferral limit is generally shared across your 401(k) plans, so contributions to the workplace plan reduce the amount of employee deferral available for the Solo 401(k).
Can a SEP IRA have Roth contributions?
Yes, current federal law permits Roth SEP contributions, but the SEP arrangement and financial institution must support the feature.
Can I borrow from a Solo 401(k)?
Only if the plan document permits participant loans and the loan satisfies the federal plan-loan requirements.
Can I borrow from a SEP IRA?
No. SEP-IRAs are IRA-based arrangements and cannot offer participant loans.
What happens if I hire an employee?
A Solo 401(k) loses its one-participant simplicity once eligible common-law employees must be covered. A SEP can continue, but eligible employees generally must receive contributions under the SEP’s allocation formula.
Which plan lets me contribute more?
At many lower and moderate income levels, a Solo 401(k) can allow more because it combines an elective deferral with an employer contribution. At sufficiently high income, a person under age 50 can potentially reach the same $72,000 general 2026 ceiling under either plan. Catch-up-eligible Solo 401(k) participants may be able to contribute above that amount.
Are Solo 401(k) contributions tax deductible?
Traditional elective deferrals and qualifying employer contributions generally receive current federal tax advantages, but the exact deduction and reporting treatment depends on the contribution type and business structure. Roth contributions are generally included in current taxable income rather than receiving the same current-year income-tax exclusion.
Do I need Form 5500-EZ?
A one-participant plan is generally exempt from Form 5500-EZ while the combined assets of the employer’s one-participant plans are $250,000 or less at year-end. Once those assets exceed $250,000, filing generally becomes required. A final return is also required in the plan’s final year regardless of asset size.
Bottom Line
A Solo 401(k) generally provides more contribution flexibility: an employee elective deferral, an employer contribution, catch-up contributions for eligible participants, and potentially Roth and loan features.
A SEP IRA generally offers a simpler administrative path and can work well for self-employed people who do not need the Solo 401(k)’s additional features.
The published maximums alone do not decide the winner. Earned income, employees, age, another workplace plan, business ownership relationships, contribution deadlines, and provider features can materially change what is actually available.
This article is for general educational purposes and is not individualized tax, legal, retirement-plan, or investment advice. Retirement-plan rules are detailed and can change. Consider checking your calculation and plan terms with a CPA, enrolled agent, or qualified retirement-plan professional before making a maximum contribution or relying on a deadline.
Official Sources
- IRS — One-Participant 401(k) Plans
- IRS — 2026 Retirement Plan Contribution Limits
- IRS — 401(k) and Profit-Sharing Plan Contribution Limits
- IRS — Catch-Up Contributions
- IRS — SEP Contribution Limits
- IRS — SEP FAQs
- IRS — Simplified Employee Pension Plan
- IRS — Publication 560, Retirement Plans for Small Business
- IRS — Notice 2024-2, SECURE 2.0 Guidance
- IRS — Timing and Retroactive Adoption of 401(k) Plans
- IRS — Retirement Plan Loans
- IRS — Form 5500-EZ