Solo 401(k) vs SEP IRA: Which Retirement Plan Is Best?

I meet with online business owners every week who make excellent income, $100K, $200K, sometimes half a million, but have zero retirement savings. They're building thriving businesses, yet financially they're living paycheck to paycheck. Why? Because they've never established a retirement plan. A retirement plan, either a SEP IRA or solo 401(k), changes the math entirely: you contribute a portion of income before taxes, so you build wealth and cut your tax bill in the same motion.

Here is a simple version. You earn $150,000 from your online course business. Instead of paying taxes on the full amount, you contribute $45,000 to a solo 401(k) first, dropping your taxable income to $105,000. At a 24 percent federal marginal rate, that saves around $10,800 in income tax this year (the contribution does not reduce self-employment tax) while building $45,000 in retirement wealth . That is the entire point of a retirement plan: it solves two problems at once.

Understanding the Tax Deduction and the Long-Term Payoff

Retirement contributions are an "above-the-line" deduction: they reduce your adjusted gross income (AGI) directly, no itemizing required, which matters because AGI feeds your tax bracket and your eligibility for other deductions and credits (the deduction reduces income tax, not self-employment tax). Example: a freelance designer earning $175,000 contributes $40,000 to a SEP IRA, dropping taxable income to $135,000. At a 24 percent federal marginal rate, that saves her roughly $9,600 in income tax this year (the contribution does not reduce self-employment tax), and the $40,000 keeps growing tax-sheltered on top of that .

The bigger payoff is compounding. A 28-year-old online course creator who consistently contributes $45,000 annually to a solo 401(k) could build roughly $7.2 million in tax-deferred retirement wealth by age 65, assuming a 7 percent average annual return; the same dollars invested outside a retirement plan, after paying tax on the gains each year, would grow to roughly $3.5 million instead (illustrative projection, not a promised return, results vary).

A SEP IRA (Simplified Employee Pension IRA) is the retirement plan designed for simplicity. You can literally open one in 30 minutes online. There's no annual compliance filing, no complex rules, and no required contributions every year, you contribute what you can, when you can. For a freelancer with inconsistent income or someone who just wants to keep things straightforward, a SEP IRA is the obvious choice.

How Much Can You Contribute?

The stated rule is 25% of compensation, up to $72,000 for 2026, with a $360,000 compensation cap and an $800 minimum-compensation eligibility threshold . But for an unincorporated owner (sole proprietor or single-member LLC), "25% of compensation" is not 25% of your net profit. The Pub. 560 worksheet mechanic works out to approximately 20% of net self-employment earnings, after subtracting the deductible half of SE tax . The 25% figure is correct only when it's applied to W-2 wages, which is the case for S-corp owners and common-law employees, not for sole proprietors.

Think of it this way: your unincorporated business earned $100,000 in profit. Applying the roughly 20% effective rate gives you a SEP contribution of about $18,600 for the year . At $200,000 in profit, your SEP limit is roughly $37,200. At very high income, you hit the 2026 annual cap of $72,000 regardless of income .

Why "Employer Contributions" Matter

Here's a distinction that confuses people: SEP IRA contributions are always classified as "employer contributions." This doesn't mean your business is different from a 401(k). What it means is that the money comes directly from your business profit, not from a reduction in your salary. You're not setting aside part of a paycheck, the entire contribution comes from business earnings. This actually makes SEP IRAs more flexible than 401(k)s, because you can adjust contributions year-to-year based on actual profit, not a predetermined salary amount.

Roth SEP: Now Available, Not Yet Universal

SECURE 2.0 authorized Roth SEP contributions starting in 2023 . As of 2026, custodian and payroll-system support is still uneven, several major custodians still do not offer it. If a Roth SEP matters to your plan, ask your custodian directly before assuming it's available; do not assume every SEP provider supports it. A Roth SEP contribution gets no deduction today but grows and withdraws tax-free later, the same trade-off as any other Roth account.

The Real Advantage: Simplicity in Variable Income Years

The SEP IRA's biggest strength is flexibility for unpredictable income. Imagine you're a freelance copywriter. Year one you earn $120,000, so you contribute roughly $22,300 to your SEP. Year two, a major client goes under and you earn only $60,000, so you contribute less, roughly $11,200, or nothing at all if cash is tight . There's no penalty for contributing less than the maximum, and there's no requirement to make a contribution in every year. A solo 401(k) has its own flexibility here too (see Section 3), so this is not a reason to default to a SEP by itself.

SEP IRA Limitations to Consider

The most important limitation: at income levels between $75,000 and $200,000, SEP IRA contributions fall well short of what a solo 401(k) allows. That's because the solo 401(k) has an additional contribution bucket, the employee deferral, that a SEP IRA doesn't offer. We'll explore this advantage in the next section. But if your income fluctuates significantly, or if you just want maximum simplicity, the SEP IRA's flexibility and ease of setup can outweigh the contribution limits.

I had a course creator walk into my office asking about retirement plans. She earned $200,000 annually and couldn't believe the difference between plans. "A solo 401(k) lets me contribute over $61,000," she said, "but a SEP IRA caps me at about $37,000. That's a $24,500 difference." She was right. A solo 401(k) is designed specifically to give self-employed people access to more retirement savings room. It does this by splitting contributions into two separate buckets: employee deferrals and employer contributions.

Understanding the Two Contribution Buckets

This is the fundamental concept that makes solo 401(k)s more generous than SEP IRAs. In a solo 401(k), you wear two hats: employee and employer. As the employee, you can contribute up to $24,500 of your own compensation to the plan in 2026 . As the employer (your business), you can contribute on top of that, roughly 20% of net earnings if you're an unincorporated sole proprietor, or 25% of W-2 wages if you're an S-corp owner-employee . These two buckets add together, with a combined limit of $72,000 annually in 2026 .

Concrete Examples: How the Two Buckets Work

Let's walk through three different income levels to show how solo 401(k) contributions actually work. The first two are unincorporated sole proprietors, who do NOT run payroll or pay themselves W-2 wages; both buckets come straight off net self-employment earnings. The third is an S-corp owner-employee, where W-2 salary is the correct anchor.

Solo 401(k) Additional Features

Beyond the two-bucket advantage, solo 401(k)s offer features a SEP IRA simply doesn't have. You can include a Roth component, allowing after-tax contributions that grow tax-free; see whether those deferrals should be traditional or Roth for how to decide, or 401(k) vs Roth IRA if you're weighing the solo 401(k) against a Roth IRA instead. You can borrow from your plan balance, up to 50% or $50,000, useful for emergencies . And if you're 50 or older, catch-up contributions ($8,000 extra in 2026, or $11,250 if you're 60-63) give you more room to save . The solo 401(k) is also the retirement chassis that makes a mega backdoor Roth possible for the self-employed, since it can be built with after-tax (non-Roth) employee contributions and in-plan conversion language that a SEP IRA structurally cannot support.

Now that you understand how each plan works, let's compare them directly across the features that actually matter. This table shows every meaningful difference: setup complexity, contribution limits, flexibility, and long-term features like borrowing and Roth options. Figures are 2026 limits .

One question I get constantly: "Which plan should I choose?" The answer depends significantly on your income level. A solo 401(k) isn't always better, at very high incomes the two plans converge because both hit similar caps. But at every level below the cap, the solo 401(k) advantage is essentially the full employee deferral bucket, which a SEP IRA structurally cannot offer. Here's exactly how the contributions compare, using 2026 limits for an unincorporated sole proprietor :

Income: $50,000 Net

At lower income levels, the gap already matters. You're just starting out or supplementing a W-2 job.

Income: $100,000 Net

This is where many online business owners land. The solo 401(k) advantage becomes compelling.

Income: $150,000 Net

Solid six-figure income. The decision becomes clearer: solo 401(k) is likely worth the added setup.

Income: $200,000 Net

At this level, you're likely thinking seriously about optimization. The solo 401(k) advantage is substantial.

Want your own numbers instead of these illustrative tiers? Run them through our solo 401(k) contribution calculator or our self-employment tax calculator to see the full picture, not just the retirement piece.

Many online business owners eventually elect S-Corp status to save on self-employment taxes. If you go this route, retirement plan calculations change fundamentally. Instead of being based on total business profit, your contributions now depend on the W-2 salary you pay yourself. This is important to understand because it affects which retirement plan makes sense.

S-Corp + Solo 401(k) Example

S-Corp + SEP IRA Example

Compare the two: solo 401(k) gives you $47,000 in retirement contributions, while a SEP IRA caps you at $22,500. That's a $24,500 difference, exactly the size of the employee deferral bucket. The S-corp structure actually makes the solo 401(k) even more valuable, because you keep the employee deferral bucket even though a chunk of your income comes out as a distribution instead of salary.

Everything above assumes your self-employment income is your only source of retirement plan eligibility. A lot of readers here are not in that position, they have a W-2 day job with its own 401(k), 403(b), or 401(a), plus 1099 or solo business income on the side. Moonlighting physicians are the single biggest group we see this from, but it applies to anyone with a day job and a side hustle.

Two Different Limits, Not One

The elective deferral limit (the $24,500 employee bucket) is a per-PERSON limit that applies across every employer you defer into during the year, whether that's a hospital 403(b), a government 401(a), or your own solo 401(k) . If you already max out your $24,500 employee deferral at your day job, you generally cannot also make an employee deferral into your solo 401(k) the same year.

The overall $72,000 combined limit (415(c)), on the other hand, applies separately to each UNRELATED employer . That means a hospital-based 403(b)/401(a) plan and your own solo 401(k) on moonlighting income can each build toward their own $72,000 ceiling, as long as the businesses are not related to each other. There's an important wrinkle here: 403(b) plans have their own aggregation quirk where the 415(c) limit can be treated as shared with a business the participant controls in certain circumstances , so this is a case where a quick check with your CPA before you fund both plans aggressively is worth the ten minutes.

If this describes your situation, whether you're a physician moonlighting between hospital shifts and locum work, or any W-2 employee with a side business, this is worth a real conversation, not a guess. See our CPA for physicians service and our locum tenens tax guide for the fuller picture, and see 403(b) vs 401(k) and 401(a) vs 401(k) if you're not sure which type of plan your employer offers.

So which plan should you actually choose? After working through the details, here's the honest answer: it depends on your income level, your risk tolerance for complexity, and your long-term plans. But we can make this straightforward.

The Real Decision: Simplicity vs. Wealth Building

Here's what really matters: a SEP IRA takes 30 minutes to open and requires minimal maintenance. A solo 401(k) takes a few hours to set up with a plan document, and requires Form 5500-EZ once plan assets exceed $250,000 . For a sole proprietor there is no payroll cost at all, payroll only applies if you're an S-corp owner running W-2 wages through a payroll system, which is a separate decision from which retirement plan you pick. The trade: solo 401(k) owners typically save an extra $20,000 to $25,000 per year in retirement contributions at these income levels.

Do the math on that. An extra $24,500 annually, compounded over 20 years at a 7 percent assumed return, is roughly $1 million more at retirement (illustrative projection, not a promised return, results vary with actual returns and contribution timing). The setup cost is a few hours of paperwork and a modest plan-document fee. The ROI on that complexity is hard to argue with.

The Deadlines: What Changed Under SECURE 2.0

This is the section most other sites still get wrong. The old rule was "solo 401(k) must be established by December 31 or you're locked out." That rule is out of date. The SECURE Act and SECURE 2.0 Section 317 changed the deadlines meaningfully .