SEP IRA vs Solo 401(k) 2026: Which Self-Employed Retirement Account Wins for You

The Core Difference: Two Sources of Contributions

SEP IRA has one contribution source: employer profit-sharing. Since you're the employer of yourself, you decide how much of your business income to contribute, up to ~20% of net SE income (calculated formula caps the headline 25% to about 20%).

Solo 401(k) has two contribution sources: employee deferral + employer profit-sharing. The employee deferral is a flat $23,500 in 2026 (or $31,000 if 50+). The employer profit-sharing is the same ~20% as SEP IRA. You add them together to get your total.

This single difference explains most of the comparison. The employee deferral is a flat dollar amount that doesn't depend on income — meaning a moderate earner gets a larger contribution percentage in a Solo 401(k) than in a SEP IRA.

Worked Comparison by Income Level

Here's the side-by-side at common net SE income levels for a 2026 contributor under 50:

The two converge above $300K of net SE income because both hit the $70,000 IRS cap. Below that, Solo 401(k) is generally better — sometimes dramatically so for lower-earners.

When to Pick SEP IRA Anyway

Three reasons to choose SEP IRA despite the Solo 401(k)'s typically larger contribution capacity:

Reason 1: Simplicity. SEP IRA is essentially a souped-up traditional IRA. Most discount brokers (Fidelity, Schwab, Vanguard) let you open one in 10 minutes. No annual filings, no plan documents, no IRS Form 5500 (until balance exceeds $250K). Solo 401(k) needs an actual plan document (provided by your broker) and Form 5500-EZ once the balance hits $250K.

Reason 2: Backdoor Roth IRA strategy. If you do annual backdoor Roth IRA conversions, having an existing SEP IRA balance triggers the pro-rata rule — your backdoor Roth conversions become partially taxable based on the pro-rata share of pre-tax IRA money. SEP IRA balances count toward this. Solo 401(k) balances don't.

A workaround: maintain only a Solo 401(k) for self-employment savings and zero traditional/SEP/SIMPLE IRA balances. This keeps backdoor Roth IRA conversions clean.

Reason 3: Tax-filing-deadline flexibility. SEP IRA can be opened AND funded up to your tax filing deadline (April 15, or October 15 with extension). Solo 401(k) must be established by December 31 of the contribution year — though contributions can still be made until the filing deadline. If you're scrambling to reduce your prior-year tax bill in March and don't have a Solo 401(k) already, SEP IRA is your only option.

When to Pick Solo 401(k)

Reason 1: Maximum contributions at moderate income. As shown above, anything under $300K of net SE income gets meaningfully more Solo 401(k) room.

Reason 2: Roth option. Solo 401(k) allows Roth contributions on the employee deferral portion. SEP IRA does not (technically Roth SEP was added by Secure Act 2.0, but most providers haven't implemented it yet). For high earners who want tax-free retirement growth, the Roth Solo 401(k) is a meaningful advantage.

Reason 3: Loan provision. Solo 401(k) plans can allow you to borrow up to 50% of the balance (max $50K) from your own plan, repaid with interest to yourself. SEP IRA has no loan provision — early withdrawal triggers tax + 10% penalty. The Solo 401(k) loan can be a useful liquidity option, though most financial advisors discourage borrowing from retirement.

Reason 4: Mega backdoor compatibility. Some Solo 401(k) providers (Fidelity, Schwab) now offer after-tax contributions with in-plan conversion — enabling mega backdoor Roth for self-employed people. SEP IRA has no equivalent.

Reason 5: Better for couples. If your spouse works in the business, both spouses can be Solo 401(k) participants with separate $23,500 deferrals + their own profit-sharing. SEP IRA caps the spouse on the same ~20% of their SE income with no additional deferral capacity.

The Net SE Income Calculation

Both plans cap contributions at a percentage of "net self-employment income," which is NOT your gross revenue. The IRS formula:

1. Start with net profit from Schedule C (or your equivalent business income)
2. Subtract one-half of self-employment tax (the deductible portion you claim on Schedule 1)
3. The result is your "net SE income" for retirement plan purposes
4. For SEP IRA / Solo 401(k) profit-sharing, multiply by 20% (the IRS formula reduces the headline 25% to 20% for self-employed)

Example: gross revenue $150,000, business expenses $30,000, net profit $120,000. Self-employment tax is roughly 15.3% on 92.35% of net profit = $16,950. Half of that ($8,475) is deductible. Net SE income for retirement = $120,000 - $8,475 = $111,525. Maximum SEP IRA contribution = $111,525 × 0.20 = $22,305.

This is why the "25% of income" headline number always under-estimates the actual cap. The 20% effective rate is the right mental model for self-employed.

Setup Mechanics

What About SIMPLE IRA?

A third option, the SIMPLE IRA, is sometimes mentioned in the same conversation. Skip it for solo self-employed:

• Lower contribution limit ($16,500 for 2026)
• Mandatory employer match (3% of compensation)
• 25% early withdrawal penalty in first 2 years (vs 10% for SEP/Solo 401k)

SIMPLE IRA only makes sense for businesses with employees who want a simpler-than-401(k) plan. For solo self-employed, SEP or Solo 401(k) is always better.

What About a Defined Benefit / Cash Balance Plan?

For very high-income self-employed people (typically $300K+ of net SE income with consistent earnings), a defined benefit pension plan can allow $100K-$300K/year in contributions — far beyond SEP or Solo 401(k). These have meaningful complexity (actuarial calculations, required contributions, IRS qualification rules), and typically need a third-party administrator at $1,500-$3,000/year.

Not appropriate for most. But worth knowing about if you're a high-income consultant, doctor, or professional who can sustain large contributions for 5-10+ years.

A Note on Roth Solo 401(k)

The Roth Solo 401(k) is increasingly the right choice for high-earning self-employed people. The math:

• Roth contributions are made with after-tax dollars; growth and qualified withdrawals are tax-free
• Traditional/pre-tax contributions reduce current-year tax; growth and withdrawals are taxed as ordinary income at retirement

If you expect to be in the same or higher tax bracket in retirement, Roth wins. Most self-employed people in their 30s-50s should heavily weight toward Roth contributions on the employee deferral portion — the certainty of tax-free growth in a high-tax future is valuable.

The employer profit-sharing portion is pre-tax only in a Solo 401(k) (Roth profit-sharing wasn't enabled until Secure Act 2.0, and most providers still don't support it). So a typical mixed strategy: Roth on the employee deferral, pre-tax on the profit-sharing. This gives you both tax diversification and current-year tax savings.

Related Calculators and Guides

• Solo 401(k) Calculator — exact contribution limit for your income
• SEP IRA Calculator — SEP-specific contribution math
• Roth IRA Calculator — for the smaller backdoor strategy
• Mega Backdoor Roth Guide — pair with Solo 401(k) for highest-impact retirement savings
• Retirement Planning Guide

Sources: IRS Section 408(k) (SEP), Section 401(k), and Section 415(c). IRS Revenue Procedure 2025-32 (2026 inflation adjustments). This guide is for educational purposes and does not constitute tax or investment advice. Self-employment retirement plan rules are complex; consult a CPA for situations involving multiple businesses, employees, or unusual income patterns.