Solo 401(k) vs SEP-IRA: The Math That Settles the Debate for Good

If you are self-employed and not maxing out a retirement account, you are leaving one of the most powerful tax deductions on the table. A Solo 401(k) or SEP-IRA lets you shelter $50,000–$70,000+ per year from income tax — more than a traditional employer 401(k) in many cases.

The question is which account to use. The answer, for solo operators, is almost always the Solo 401(k). Here is why, with the actual 2026 contribution limits and formulas.

2026 Contribution Limits at a Glance

Feature	Solo 401(k)	SEP-IRA	SIMPLE IRA
Employee deferral	$23,500	None	$16,500
Employer contribution	20% of net SE earnings (sole prop) or 25% of W-2 (S-Corp)	Same formula	3% match
Total maximum	$70,000	$70,000	$23,500
Catch-up (age 50-59, 64+)	+$7,500	None	+$3,500
Super catch-up (age 60-63)	+$11,250	None	+$3,850
Roth option	Yes	No	No

The critical difference: Solo 401(k) has an employee deferral component. SEP-IRA does not. This means the Solo 401(k) lets you contribute $23,500 regardless of income level, then add the employer contribution on top. SEP-IRA only has the employer contribution — which is calculated as a percentage of earnings.

Why Solo 401(k) Wins at Every Income Level

Under $100,000 Net Profit

At lower income levels, the employer contribution percentage produces modest amounts. The employee deferral is what makes the Solo 401(k) dominant.

Example: $80,000 net profit as sole proprietor

Net SE earnings for contribution = $80,000 × 0.9235 = $73,880 (after SE tax deduction adjustment)

Solo 401(k): $23,500 employee deferral + 20% × $73,880 employer = $23,500 + $14,776 = $38,276
SEP-IRA: 20% × $73,880 = $14,776

The Solo 401(k) lets you contribute $23,500 more per year. At a 24% marginal rate, that is $5,640 in additional tax savings every single year.

At $150,000 Net Profit

Solo 401(k): $23,500 + 20% × ($150,000 × 0.9235) = $23,500 + $27,705 = $51,205
SEP-IRA: $27,705

The Solo 401(k) advantage: $23,500 — the full employee deferral amount.

At $350,000+ Net Profit

Both accounts hit the $70,000 total cap, but Solo 401(k) reaches it faster because of the employee deferral component. The SEP-IRA only reaches $70,000 when net SE earnings hit approximately $379,000.

For most freelancers earning between $60,000 and $350,000, the Solo 401(k) provides strictly more contribution room. Run your exact retirement numbers →

The S-Corp Salary Connection

If you have elected S-Corp status, your employer contribution is based on your W-2 salary (25% of salary, not 20% of net SE earnings). This creates a critical interaction between your salary level and your retirement contribution ceiling.

Example: $200,000 S-Corp profit, $85,000 salary

Employee deferral: $23,500
Employer contribution: 25% × $85,000 = $21,250
Total Solo 401(k): $44,750

Same profit, $110,000 salary:

Employee deferral: $23,500
Employer contribution: 25% × $110,000 = $27,500
Total Solo 401(k): $51,000

The higher salary unlocks $6,250 more in employer contributions. At a 32% marginal rate, that is $2,000 per year in additional tax savings from the retirement account alone — before considering the QBI deduction effects of salary level.

This is why salary optimization for S-Corp owners is a multi-variable problem. The salary that minimizes SE tax is not the salary that maximizes retirement contributions, and both interact with the QBI deduction. Talivero's optimizer handles all three simultaneously.

When SEP-IRA Makes Sense

SEP-IRA has two genuine advantages:

1. Simpler administration. Opening a SEP-IRA takes 15 minutes at Fidelity, Schwab, or Vanguard. There is one form (IRS Form 5305-SEP), no annual Form 5500-EZ filing, and no plan document to maintain. Solo 401(k) requires a plan adoption agreement, annual filing of Form 5500-EZ when assets exceed $250,000, and slightly more administrative overhead.

2. Deadline flexibility. SEP-IRA contributions can be made up to the tax filing deadline (April 15, or October 15 with extension). Solo 401(k) employee deferrals must be elected by December 31 of the tax year. If you are deciding in March whether to make a prior-year contribution, SEP-IRA gives you more flexibility.

However, these advantages are marginal. The $23,500 employee deferral in the Solo 401(k) dwarfs the convenience savings of a SEP-IRA. For most freelancers, the 30 minutes of additional paperwork per year is worth $5,000–$7,500 in annual tax savings.

The Roth Angle

Solo 401(k) plans can include a Roth option. SEP-IRAs cannot.

Roth contributions do not reduce your current-year taxable income, but withdrawals in retirement are completely tax-free. If you believe your future tax rate will be higher than your current rate — a reasonable assumption for younger freelancers with growing incomes — the Roth Solo 401(k) is uniquely valuable.

You can split your employee deferral between traditional (pre-tax) and Roth (after-tax). The employer contribution is always pre-tax. This gives you flexibility to optimize your current-year tax bill while building a tax-free retirement bucket.

Catch-Up Contributions: The Age 60-63 Super Bonus

Starting in 2025, individuals aged 60–63 qualify for an enhanced "super catch-up" contribution of $11,250 (instead of the standard $7,500 catch-up for ages 50–59 and 64+). This only applies to 401(k) plans, not SEP-IRAs.

For a 61-year-old freelancer:

Solo 401(k): $23,500 + $11,250 catch-up + employer contribution = up to $81,250
SEP-IRA: employer contribution only = up to $70,000

The Solo 401(k) advantage jumps to $11,250+ in the super catch-up years. If you are approaching this age range, the Solo 401(k) is not just preferable — it is essential.

Practical Setup Guidance

Opening a Solo 401(k)

Most major brokerages (Fidelity, Schwab, Vanguard) offer free Solo 401(k) plans. The process:

Establish the plan by December 31 of the tax year (for employee deferrals)
Complete the plan adoption agreement
Fund the employee deferral by December 31
Fund the employer contribution by the tax filing deadline (April 15 or October 15 with extension)
File Form 5500-EZ annually once plan assets exceed $250,000

The SIMPLE IRA Trap

If you previously set up a SIMPLE IRA, you cannot contribute to a Solo 401(k) or SEP-IRA in the same calendar year. You must terminate the SIMPLE IRA (effective January 1 of the following year) before switching to a Solo 401(k). Plan this transition carefully to avoid a gap year with reduced contribution limits.

How Entity Choice Affects Retirement Strategy

Your entity structure determines which contribution formula applies, how much you can contribute, and how salary interacts with retirement limits. For a full analysis of how your entity choice (sole prop, LLC, S-Corp) intersects with retirement planning and state entity costs, read our S-Corp crossover analysis.

When planning retirement contributions, consider how they fit into your overall financial picture. For vehicle purchases that qualify for business deductions under Section 179, tools like DriveDecision can help you model the total cost of ownership alongside the tax benefits.

The Verdict

For solo self-employed individuals, Solo 401(k) dominates SEP-IRA at every income level. The $23,500 employee deferral, Roth option, catch-up provisions, and loan feature make it strictly superior. The only trade-off is slightly more paperwork.

If you have not opened a Solo 401(k), do it before December 31. Every year you wait costs you $5,000–$7,500 in foregone tax deductions.

Calculate your optimal retirement contribution →