Retirement planning is crucial for self-employed individuals and small business owners who don’t have access to traditional employer-sponsored plans. Without the benefit of company-matched 401(k)s, entrepreneurs must take charge of their retirement savings strategy. Two of the most popular and effective options are the SEP IRA and Solo 401(k). While both plans offer substantial tax advantages and high contribution limits, they function differently and serve distinct business situations.
Understanding these differences is essential for maximizing your retirement savings potential while staying compliant with IRS regulations. The choice between these plans can significantly impact your annual contribution capacity, tax strategy, and administrative burden.
What Is a SEP IRA?
A Simplified Employee Pension (SEP) IRA is a retirement plan specifically designed for self-employed individuals and small business owners. It operates as an individual retirement account where the employer makes contributions on behalf of eligible employees, including themselves if they’re self-employed. The SEP IRA allows business owners to contribute up to 25% of compensation or $70,000 for the 2025 tax year, whichever is less.
Key Features
Employer-Only Contributions: One of the defining characteristics of a SEP IRA is that only employers can make contributions. Employees cannot contribute their own money to the plan, which simplifies the structure but also limits flexibility.
High Contribution Limits: SEP IRAs allow annual contributions of up to $70,000 in 2025, representing 25% of eligible employee compensation. For self-employed individuals, the calculation is slightly different, typically limiting contributions to about 20% of net self-employment income after deducting half of self-employment taxes.
Simple Setup and Maintenance: SEP IRAs are among the easiest retirement plans to establish and maintain. They require minimal paperwork, no annual filing requirements, and can be set up quickly through most financial institutions.
What Is a Solo 401(k)?
A Solo 401(k), also known as an Individual 401(k) or One-Participant 401(k), is designed specifically for self-employed individuals and business owners with no employees other than a spouse. This plan functions as a traditional 401(k) but is tailored for single-participant businesses.
Key Features
Dual Contribution Structure: The Solo 401(k)’s biggest advantage is its ability to accept contributions from both employee and employer perspectives. As the business owner, you can contribute as both the employee (through salary deferrals) and the employer (through profit-sharing contributions).
Higher Potential Contribution Limits: For 2025, individuals can contribute up to $23,500 in salary deferrals to their 401(k) plans, plus additional employer contributions up to 25% of compensation, potentially reaching the same $70,000 maximum as SEP IRAs but through a different structure.
Additional Options: Solo 401(k)s often provide features unavailable in SEP IRAs, including Roth contribution options, loan capabilities (allowing you to borrow up to $50,000 or 50% of the account balance), and catch-up contributions for those over 50.
Side-by-Side Comparison: SEP IRA vs. Solo 401(k)
Contribution Limits and Business Structure Impact
Both SEP IRAs and Solo 401(k) plans share the same annual contribution ceiling of $70,000 for 2025, but the path to reaching this limit varies dramatically based on your business structure. The key difference lies in how each plan defines “compensation” and structures contributions. For corporate shareholders, contribution limits are tied strictly to W-2 wages, while for partnerships and sole proprietors (SMLLCs), limits are tied to net self-employment income.
SEP IRA Contribution Structure
SEP IRAs follow a straightforward formula: up to 25% of compensation with a maximum of $70,000 for 2025. However, the definition of eligible compensation creates significant planning considerations depending on your business entity:
- S-Corp and C-Corp Shareholders: Your contributions are strictly limited to W-2 wages only. Distributions, dividends, or other forms of business income don’t count toward your contribution base. This creates a common planning dilemma since many corporate owners intentionally keep W-2 wages low to minimize payroll taxes, inadvertently restricting their retirement contribution capacity.
- Partnerships and Single-Member LLCs: Your eligible compensation equals net earnings from self-employment, calculated through a circular formula: Net profit − ½ self-employment tax − SEP contribution itself. This complex calculation effectively reduces your maximum contribution rate to approximately 20% of adjusted net earnings rather than the stated 25%. Despite the mathematical complexity, this structure often allows higher contributions than corporate owners with artificially suppressed wages.
Solo 401(k) Contribution Advantage
Solo 401(k) plans offer a more flexible dual contribution structure that can help business owners maximize retirement savings regardless of their entity type:
- Employee Deferral Component: Contribute up to $23,500 (plus $7,500 catch-up if age 50+) from your earnings, similar to traditional employer 401(k) plans.
- Employer Contribution Component: Add up to 25% of compensation using the same entity-specific rules as SEP IRAs.
The strategic advantage becomes clear when examining the employee deferral portion. This $23,500 contribution can be made independently of the employer contribution formula, making it significantly easier for business owners with lower W-2 wages or modest self-employment income to achieve substantial retirement contributions. While SEP IRA participants might struggle to contribute meaningful amounts due to compensation limitations, Solo 401(k) participants can often reach the combined maximum of $70,000 (or $77,500 with catch-up) more readily.
Eligibility Requirements
SEP IRAs are available to self-employed individuals, sole proprietors, partnerships, and corporations, making them one of the most flexible retirement plan options for a wide range of business structures. The trade-off is that SEP IRAs have mandatory employee inclusion rules. Any employee who is at least 21 years old, has worked for the business in at least three of the last five years, and earned at least $750 in compensation during the current year (2024 threshold) must be included in the plan.
Employers must contribute the same percentage of compensation for every eligible employee. For example, if you contribute 15% of your own compensation, you must also contribute 15% of each eligible employee’s pay. This can be costly for growing businesses with multiple employees.
Family members are treated just like other employees under these rules. Children under 21 are excluded until they meet the minimum age requirement, which gives business owners flexibility to employ minor children without triggering additional SEP contributions. Spouses, however, must be included if they meet eligibility requirements. While this doubles the contribution obligation, it also doubles the family’s tax deduction and retirement savings.
Solo 401(k) plans are more restrictive in terms of eligibility. They are designed specifically for business owners with no employees, although a spouse who works in the business can also participate. By rule, a Solo 401(k) cannot cover full-time employees working 1,000 hours or more annually. This makes it an excellent fit for self-employed individuals and owner-only businesses, but less practical for companies planning to hire staff.
Part-time employees working under 1,000 hours are generally excluded, though starting in 2025, the SECURE Act 2.0 will require most 401(k) plans to include employees who work at least 500 hours in two consecutive years.
For family planning, Solo 401(k) rules mirror many of the SEP provisions: children under 21 are typically excluded, which allows owners to employ their minor children without affecting plan eligibility. Once a child turns 21 and meets service requirements, the business would need to either convert to a traditional 401(k) that includes them or amend the plan to bring them in as participants, ending the plan’s “solo” status.
Contribution Timing Considerations
Both SEP IRA and Solo 401(k) contributions can be made up until your tax filing deadline, including extensions, for the previous tax year. However, there’s one critical requirement that many business owners overlook: the retirement account must be established by December 31st of the tax year for which you want to make contributions. This December 31st deadline is absolute and cannot be extended.
You cannot retroactively open an account after the year ends and still make contributions for that tax year, regardless of when you file your taxes. For example, to make 2025 contributions, your account must be opened by December 31, 2025, even though you have until April 15, 2026 to actually contribute. Missing this deadline means losing an entire year’s contribution opportunity, potentially tens of thousands in retirement savings.
Administrative Requirements
SEP IRA: Extremely simple administration with no annual reporting requirements to the IRS. Setup involves a basic agreement and individual IRA accounts for each participant.
Solo 401(k): More complex administration, especially as assets grow. Once plan assets exceed $250,000, annual Form 5500 filing becomes required, adding to the administrative burden and potential costs.
Payroll Reporting and W-2 Treatment
The payroll and tax reporting requirements differ substantially between SEP IRAs and Solo 401(k) plans, creating important compliance considerations for business owners and their payroll systems.
SEP IRA Payroll Simplicity
SEP IRAs offer the ultimate in payroll simplicity because all SEP contributions are employer contributions only—there are no employee deferrals, no pre-tax withholding, and no complex payroll calculations.
The payroll treatment is straightforward:
- No Payroll Processing: SEP contributions are not listed on an employee’s individual W-2 form since they don’t run through payroll at all. Contributions are made directly to the custodian outside of payroll processing.
- W-2 Reporting: SEP contribution amounts do not appear in any wage boxes (1, 3, or 5) and receive no Box 12 coding. The only W-2 requirement is checking Box 13 “Retirement Plan” for any employee who received a SEP contribution during the year.
- Tax Deduction: SEP contributions are employer contributions. They are deducted on the business tax return—Form 1120-S for S corporations, Form 1065 for partnerships, or Schedule 1, Part II, line 16 for sole proprietors.
Solo 401(k) Payroll Complexity
Solo 401(k) plans require more sophisticated payroll handling due to their dual contribution structure, with different treatment for employee deferrals versus employer contributions.
Employee Deferral Component (runs through payroll):
- Pre-tax Deferrals: Reduce Box 1 (federal income tax wages) but remain included in Boxes 3 and 5 (Social Security and Medicare wages). Report the total in Box 12, code “D” and check Box 13 “Retirement Plan.”
- Roth Deferrals: Included in all wage boxes (1, 3, and 5) since they’re made with after-tax dollars. Report the amount contributed in Box 12 with code “AA” and check Box 13 “Retirement Plan.”
- After-tax (Non-Roth) Contributions (if allowed by your plan): Included in all wage boxes but receive no Box 12 coding. These are tracked within the plan’s recordkeeping system and still require checking Box 13 “Retirement Plan.”
- Catch-up Contributions: The additional $7,500 catch-up limit (for individuals age 50 or older) applies only to employee deferrals, not employer contributions.
- Tip: If your plan allows after-tax (non-Roth) contributions, many advisors recommend converting or distributing them frequently to minimize small amounts of pre-conversion earnings.
Employer Contribution Component (off-payroll):
- No Payroll Processing: Employer contributions don’t run through payroll taxes and aren’t reported in any W-2 wage boxes or Box 12.
- Tax Deduction: Deducted on the business tax return—Form 1120-S for S corporations, Form 1065 for partnerships, or Schedule 1, Part II, line 16 for sole proprietors.
Owners by Entity Type
- S-Corp shareholder-employees: Use W-2. Deferrals are made via payroll. Employer contributions are made off-payroll.
- Sole proprietors / LLCs taxed as sole proprietors: No W-2. You must elect an “employee” deferral in writing by 12/31 and fund it from net self-employment earnings (not through payroll). Report the deduction on Schedule 1 (and Keogh/SEP worksheets as applicable).
- Partnerships / LLCs taxed as partnerships: Partners aren’t W-2 employees. Similar to sole proprietor treatment; no W-2.
Critical W-2 Compliance
The “Retirement Plan” indicator in Box 13 shows whether an employee is an active participant in your company’s plan and affects their ability to deduct traditional IRA contributions. This box must be checked for any employee who participated in either a SEP IRA or Solo 401(k) during the tax year, regardless of contribution amounts.
Common payroll mistakes include:
- Using incorrect codes in Box 12 of Form W-2 (e.g., code D for 401(k) elective deferrals incorrectly used for 403(b) or 457 plans).
- Forgetting to check Box 13.
- Accidentally running employer contributions through payroll processing.
Payroll Reporting Requirements
SEP IRA has minimal reporting requirements, which is one of its key administrative advantages. The annual reporting required for qualified plans (Form 5500 series) is normally not required for SEPs. Generally, you do not have to file any documents with the government. The financial institution that holds the SEP-IRA accounts handles most of the compliance reporting, making this an attractive option for business owners who prefer minimal administrative burden.
Solo 401(k) plans have conditional reporting requirements based on plan assets. A one-participant 401(k) plan is generally required to file an annual report on Form 5500-EZ if it has $250,000 or more in assets at the end of the year. Solo 401k plans with assets totaling less than $250,000 have no filing requirement at all. When required, the Form 5500-EZ is due every July 31st of the next plan year. This is an important consideration for high-contribution participants who may reach the $250,000 threshold relatively quickly.
Implementing Payroll Setup in Gusto
For businesses using Gusto payroll software, the setup process varies significantly between these two retirement plan types.
Setting Up Solo 401(k) in Gusto
Navigate to Benefits & Perks → Add benefits → Retirement and select 401(k).
Configure the deferral types your plan allows:
- Pre-tax 401(k): Gusto automatically reduces federal income tax wages while maintaining full FICA taxation.
- Roth 401(k): Set as post-tax with full taxation on all wage types.
- After-tax (Non-Roth): If permitted by your plan and not available natively in Gusto, create a Custom post-tax deduction.
Set employee elections as flat dollar amounts or percentage of gross pay. Add company contribution rules for matching or profit-sharing, coordinating with your plan recordkeeper for actual funding.
During payroll processing, Gusto automatically places code “D” for pre-tax and code “AA” for Roth deferrals in W-2 Box 12. Enable annual contribution limit tracking to prevent employees from exceeding the $23,500 employee deferral limit for 2025 (plus $7,500 if age 50+). Remember: this limit applies across all 401(k) plans an individual participates in, not per plan.
Year-end W-2 checks: Confirm that Box 12 “D” = total pre-tax deferrals, Box 12 “AA” = total Roth deferrals, and Box 13 is checked. No employer or after-tax contributions should appear in Box 12.
Owner without W-2 (sole prop/partner):
Don’t use payroll software for elective deferrals. Keep a written deferral election by 12/31, then fund it from net self-employment income. Report on Schedule 1.
SEP IRA Setup in Gusto
Create a company benefit under Benefits & Perks → Add benefits → Retirement → SEP-IRA and mark as employer-paid only with no employee deduction component. SEP contributions are funded directly with custodians, not through payroll.
Verify that Gusto doesn’t place SEP amounts in W-2 wage boxes or assign Box 12 codes. Only Box 13 “Retirement Plan” should be checked for covered employees.
Important Note: Payroll software interfaces (like Gusto) update periodically, so always verify the specific navigation steps within your live system when setting up retirement plan benefits.
Key Setup Reminders
- After-tax contributions must be coded correctly: non-Roth after-tax ≠ Roth.
- Always check the Box 13 retirement plan indicator.
- Never run employer contributions for a SEP IRA or 401(k) through payroll. Fund them directly with custodians and take the deduction on the business tax return.
Investment and Tax Options
SEP IRA: Limited to traditional pre-tax contributions only. All contributions are tax-deductible in the year made, and withdrawals in retirement are taxed as ordinary income.
Solo 401(k): Offers greater flexibility with both traditional pre-tax and Roth after-tax contribution options. Some plans also allow loans, providing access to funds before retirement age without penalties (though with interest requirements and repayment terms).
Pros and Cons of Each Plan
SEP IRA
Pros:
- Extremely simple setup and maintenance with minimal ongoing administration
- High contribution limits suitable for high-income earners
- Works well for businesses with employees, as long as equal contribution percentages are acceptable
- No annual filing requirements regardless of account size
- Immediate vesting of all contributions
Cons:
- No employee salary deferral options, limiting contribution flexibility
- Must contribute equally for all eligible employees, which can be costly
- No loan options or Roth contributions available
- No catch-up contributions for participants over age 50
Solo 401(k)
Pros:
- Highest potential contribution limits through dual employee/employer structure
- Flexible contribution options including Roth deferrals
- Loan capabilities provide access to funds if needed
- Catch-up contributions available for those over 50 (additional $7,500 in 2025)
- Greater control over investment options
- Mega backdoor Roth conversion capabilities for additional tax advantages in retirement planning
Cons:
- More complex administration and potential annual filing requirements
- Only suitable for businesses with no employees (other than spouse)
- Higher setup and maintenance costs compared to SEP IRA
- Loan feature, while beneficial, can reduce retirement savings if misused
Which Plan Fits Different Business Scenarios?
Best for Self-Employed with No Employees: Solo 401(k)
If you’re a freelancer, consultant, or solo entrepreneur with no plans to hire employees, the Solo 401(k) typically offers superior benefits. The higher contribution potential, Roth options, and loan capabilities provide greater flexibility for retirement planning and financial management.
Best for Small Business Owners with Employees: SEP IRA
Business owners with employees will find the SEP IRA more practical due to its simplicity and the ability to include staff members. While you must contribute equally for all eligible employees, the administrative ease often outweighs this cost, especially for businesses with stable, long-term employees.
Future Growth Considerations
Consider your business’s trajectory when choosing a plan. If you currently have no employees but plan to hire in the future, starting with a Solo 401(k) and later transitioning to a SEP IRA might be optimal. However, plan transitions can involve complexity and costs, so long-term planning is essential.
Conclusion
Both SEP IRAs and Solo 401(k)s serve as excellent retirement savings vehicles for self-employed individuals and small business owners. Both plans allow small business owners and the self-employed to save substantial amounts annually for retirement while generating significant tax breaks.
The right choice depends on three primary factors: your business structure, current and future employee situation, and contribution goals. Solo 401(k)s excel for solo entrepreneurs seeking maximum contribution flexibility and additional features like loans and Roth options. SEP IRAs shine for their simplicity and suitability for businesses with employees.
Evaluate your specific situation based on simplicity versus features, current contribution capacity, and long-term business plans. Consider consulting with a tax professional or financial advisor to ensure your choice aligns with your overall retirement and tax strategy, as the implications of these decisions extend well beyond just the annual contribution amounts.
