SEP IRA vs Solo 401(k) for the Self-Employed

Self-employed individuals and small business owners without employees have two powerful retirement plan options: the SEP IRA and the Solo 401(k). Both offer tax-deductible contributions and tax-deferred growth, but they differ in contribution limits, flexibility, and administrative requirements. Choosing the right plan can save you thousands in taxes while building a substantial retirement nest egg.

SEP IRA: Simple and High Limits

A Simplified Employee Pension IRA allows contributions of up to 25 percent of net self-employment income, with a maximum of 70,000 dollars for 2025. Setup is simple with just a one-page form, and there are no annual filing requirements. Contributions are made entirely as employer contributions, meaning there is no employee deferral component. You can open and fund a SEP IRA as late as your tax filing deadline including extensions.

Solo 401(k): Maximum Flexibility

A Solo 401(k), also called an individual 401k, allows both employee deferrals and employer contributions. The employee deferral limit is 23,500 dollars for 2025, plus a 7,500 dollar catch-up if you are 50 or older. On top of that, you can make employer contributions of up to 25 percent of net self-employment income. The combined maximum is 70,000 dollars, the same as a SEP IRA, but the employee deferral component lets you reach higher contribution amounts at lower income levels.

Key Differences

• At income below 100,000: Solo 401k allows larger contributions due to the employee deferral
• At income above 280,000: both plans reach the same 70,000 dollar maximum
• Solo 401k offers a Roth option for after-tax contributions; SEP IRA does not
• Solo 401k allows loans from the plan; SEP IRA does not
• SEP IRA has simpler setup and no annual filing unless assets exceed 250,000
• Solo 401k requires Form 5500-EZ filing once assets exceed 250,000

Which Plan Is Right for You?

Choose a SEP IRA if you want simplicity, have high self-employment income, and do not need a Roth option or loan feature. Choose a Solo 401k if you want to maximize contributions at moderate income levels, want the Roth option for tax diversification, or may need to borrow from the plan. You can maintain both plans simultaneously, but total contributions across all plans cannot exceed the annual limits.

The Contribution Formula Differences That Matter Most

Both SEP-IRAs and Solo 401(k)s share a 2025 combined contribution limit of $70,000 (Section 415(c)), but the math to get there is fundamentally different. SEP-IRA contributions are employer-only, capped at 25% of compensation (20% of net self-employment earnings after the SE tax adjustment). Solo 401(k)s add a $23,500 employee elective deferral on top of the same employer 20%-of-net-SE contribution — meaning lower-income self-employed taxpayers hit the max faster with a Solo 401(k).

The Crossover Income Level

• At $50,000 net SE income: SEP contribution ≈ $9,293; Solo 401(k) ≈ $32,793 (employee $23,500 + employer $9,293)
• At $100,000 net SE income: SEP ≈ $18,587; Solo 401(k) ≈ $42,087
• At $200,000 net SE income: SEP ≈ $37,174; Solo 401(k) ≈ $60,674
• At $350,000 net SE income (both hit $70,000 combined cap): equal at $70,000
• Takeaway: Solo 401(k) wins at every income level below the combined cap

Practical Differences Beyond the Math

Solo 401(k)s permit Roth contributions (for the $23,500 employee portion), accept participant loans up to $50,000, and can be converted to Mega Backdoor Roth via in-plan conversion at top-tier providers. SEP-IRAs are pre-tax only, no loans, and trigger pro-rata rules that poison future Backdoor Roth IRA conversions under Section 408(d)(2). Solo 401(k)s require Form 5500-EZ once plan assets exceed $250,000; SEP-IRAs have no annual filing requirement. The Solo 401(k) wins on flexibility; the SEP-IRA wins on setup simplicity — often openable with one online form at Fidelity or Schwab in under 15 minutes.

References

• IRS: SEP Plan FAQs (irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps)
• IRS: One-Participant 401k Plans (irs.gov/retirement-plans/one-participant-401k-plans)

Key Takeaways

• SEP-IRA allows 25% of net self-employment earnings up to $70,000 for 2025 — all employer contribution.
• Solo 401(k) allows $23,500 employee + 25% employer up to the same $70,000 total (plus catch-ups).
• Solo 401(k) often wins at lower incomes because the employee $23,500 stacks even when 25% is small.
• SEP is simpler — no Form 5500 unless assets exceed $250k; Solo 401(k) requires 5500-EZ once over the threshold.
• Both allow Roth contributions now (SECURE 2.0), though Solo 401(k) rollout is more mature.

Common Mistakes to Avoid

• Picking SEP over Solo 401(k) without modeling — many self-employed earners leave $10k+ on the table at sub-$75k income.
• Missing the Solo 401(k) adoption deadline (end of tax year for contribution, with extension for deposits).
• Ignoring backdoor Roth complications — SEP-IRA balances contaminate the pro-rata rule; Solo 401(k) doesn't.
• Contributing before filing Schedule C and miscalculating net earnings.
• Skipping the Form 5500-EZ after crossing $250k Solo 401(k) assets, triggering $10,000+ penalties.

Darren's $165K Consulting Year: SEP vs Solo 401(k)

Darren W. is a single filer in Tennessee running a solo management-consulting practice with $165,000 of net Schedule C profit in 2025. He has no employees. Between a SEP-IRA and a Solo 401(k) he could save nearly $53,000 pre-tax — but the two vehicles produce different contribution ceilings at his income, and one also allows Roth contributions.

• Net self-employment profit: $165,000 | After deductible half of SE tax: $153,348
• SEP-IRA: up to 25% of self-employment earnings (effective 20% of net profit after SE-tax adjustment)
• SEP contribution at $165K: ~$30,670
• Solo 401(k) employee deferral: $23,500 (2025 limit) + employer profit-sharing 20% of net earnings ≈ $30,670
• Solo 401(k) total: ~$54,170 — roughly $23,500 more than SEP at the same income
• Solo 401(k) also permits Roth employee deferrals ($23,500 post-tax option) — SEPs do not
• Catch-up (50+): Solo 401(k) adds another $7,500, SEP does not

For any solo business owner under about $345,000 of self-employment profit, the Solo 401(k) dominates the SEP-IRA because the employee-deferral portion is a flat dollar amount independent of income — effectively giving lower earners more headroom. Above that level, both plans converge on the overall 415(c) limit. The one knock against Solo 401(k)s: Form 5500-EZ becomes required once plan assets cross $250,000, a minor annual filing. Tennessee's lack of state income tax means Darren captures the full federal benefit with no added state complexity.

Worked Example: Basil W. Compares SEP IRA and Solo 401(k)

Basil W., single sole proprietor in Montana with $130,000 of net Schedule C income, modeled SEP IRA and Solo 401(k) for 2024. Both offer large contribution room, but the Solo 401(k) almost always wins at modest incomes because of its $23,000 employee-deferral leg.

• Net SE earnings after half SE tax: roughly $120,810.
• SEP IRA max: 20% of adjusted SE net = $24,162 (employer-only contribution).
• Solo 401(k): employee $23,000 plus employer 20% adjusted SE net = $23,000 plus $24,162 = $47,162.
• Solo 401(k) advantage: $23,000 of additional deferral room at this income.
• Solo 401(k) adds a Roth option and allows loans; SEP IRA does not.

Basil picks Solo 401(k). The SEP wins only when he is 100% certain he will not contribute employee deferrals, or at very high incomes where both plans hit the $69,000 (2024) defined-contribution cap. Publication 560 lays out both. Solo 401(k) requires a plan document and Form 5500-EZ once assets exceed $250,000; SEP IRA has neither obligation.