A freelancer earning $60,000 in net self-employment income can stash $12,000 in a SEP-IRA. Same income, same year, in a solo 401(k): roughly $36,000. That is a $24,000 difference in tax-sheltered savings, and it comes from one choice on a setup form.
The retirement plan you pick as a one-person business is one of the highest-leverage tax decisions you will make. It is not glamorous, it is rarely urgent, and it is almost always made by defaulting to whatever the brokerage's homepage advertises. That default is usually wrong for low-to-middle earners, and it gets even more wrong for 2026, when SECURE 2.0's Roth catch-up rules, super catch-up contributions, and a $72,000 combined limit reshape the math.
This guide walks through how to choose between a solo 401(k) and a SEP-IRA based on your income level, your hiring plans, your business entity, your age, and your administrative tolerance. By the end, you should know which plan fits your situation and what setup deadlines you cannot afford to miss.
The Big Picture: Two Plans, Two Philosophies
Both the solo 401(k) and the SEP-IRA are tax-advantaged retirement plans designed for the self-employed. Both share the same overall annual contribution ceiling of $72,000 for 2026. Past that, they diverge sharply.
A SEP-IRA treats you exclusively as an employer. You contribute on behalf of yourself (and any eligible employees) using a single bucket: up to 25% of compensation, capped at $72,000.
A solo 401(k) treats you as both employer and employee. You wear two hats:
- Employee deferral: up to $24,500 in 2026 (or $32,500 if age 50 or older).
- Employer profit-sharing: up to 25% of compensation (about 20% of net self-employment income for sole proprietors).
- Combined cap: $72,000 (plus catch-up if eligible).
The employee deferral piece is the magic ingredient. It lets you contribute a flat dollar amount regardless of how high your income is, which dramatically helps anyone whose net business income sits below the threshold where 25% reaches the ceiling.
The Math: When Each Plan Wins
The single most important question is your net self-employment income (gross revenue minus business expenses, minus half of self-employment tax). Run the numbers at three income levels to see how the gap looks.
Income: $60,000 net self-employment
- SEP-IRA: roughly 20% of $60,000 = $12,000
- Solo 401(k): $24,500 employee deferral + ~$12,000 employer = $36,500
The solo 401(k) shelters three times as much. Not even close.
Income: $120,000 net self-employment
- SEP-IRA: roughly 20% of $120,000 = $24,000
- Solo 401(k): $24,500 + ~$24,000 = $48,500
The solo 401(k) is still about double. The employee deferral keeps doing heavy lifting.
Income: $300,000 net self-employment
- SEP-IRA: capped at $72,000 (25% of comp tops out)
- Solo 401(k): $24,500 + ~$47,500 = $72,000
At this income level, both plans hit the same combined ceiling. The solo 401(k) advantage disappears.
The crossover point is roughly $175,000 to $200,000 of net self-employment income for a sole proprietor. Below it, the solo 401(k) almost always wins on raw contribution capacity. Above it, the two plans contribute essentially the same amount.
The Hiring Question: One Rule That Changes Everything
A solo 401(k) earns its "solo" in the name. As soon as you hire a full-time non-spouse employee (defined as someone working more than 1,000 hours a year, dropping to a long-term-part-time threshold under SECURE 2.0), the plan no longer qualifies.
You have two options at that point:
- Upgrade to a full small-business 401(k) with discrimination testing, an annual Form 5500, and likely a third-party administrator. Costs jump from roughly $0 to $2,000+ per year.
- Terminate the plan and roll the assets to an IRA.
A SEP-IRA, by contrast, scales with hiring. You add eligible employees to the plan, and you contribute the same percentage of compensation for each one. That sounds fair, but it is also expensive: if you give yourself 20%, you owe every eligible employee 20%.
Decision rule:
- Definitely staying solo (or just adding a spouse): solo 401(k).
- Already have or plan to hire 1–3 employees: SEP-IRA, but model the cost of matching their percentages.
- Plan to scale to 4+ employees in the next few years: skip both and look at a SIMPLE IRA or a traditional small-business 401(k).
The Roth and Catch-Up Rules: SECURE 2.0 Is Live in 2026
Starting January 1, 2026, two SECURE 2.0 provisions reshape catch-up contributions for the self-employed.
1. Mandatory Roth Catch-Ups for High Earners
If your prior-year FICA wages exceed $145,000 (indexed; effectively $150,000 for 2026), every dollar of your age-50+ catch-up contribution must go into a Roth (after-tax) bucket rather than pre-tax.
This affects:
- S-corp and C-corp owners taking W-2 wages above the threshold. Their Solo 401(k) catch-ups must be Roth in 2026.
- Sole proprietors and single-member LLCs: Generally exempt because self-employment earnings are not FICA wages. Sole proprietor solo 401(k) participants can still make pre-tax catch-up contributions, regardless of income.
If you run an S-corp and pay yourself a salary above $150,000, this matters. Check with your plan administrator that they support Roth catch-ups before January 2026, or your catch-up contribution may be disallowed.
2. Super Catch-Up for Ages 60–63
A new "super catch-up" applies to participants ages 60, 61, 62, and 63 (only those years).
- Regular catch-up at age 50+: $8,000 for 2026
- Super catch-up at ages 60–63: $11,250 for 2026
The super catch-up applies only to plans that allow elective deferrals, which means solo 401(k) participants can use it. SEP-IRA participants cannot—SEP-IRAs do not allow catch-up contributions at all.
If you are in your early 60s with strong business income, the solo 401(k) is dramatically more powerful: up to $35,750 in employee deferrals alone, plus the employer profit-sharing piece.
Setup, Deadlines, and Paperwork
Procrastination kills these contributions. Each plan has different rules for when you must establish the plan and when you must fund it.
SEP-IRA
- Establish: As late as the due date of your tax return for the year, including extensions.
- Fund: Same deadline. For a sole proprietor with a 2026 extension, that means as late as October 15, 2027.
- Paperwork: Single-page IRS Form 5305-SEP or prototype document from your custodian.
- Annual filing: None.
This is the SEP-IRA's killer feature. You can make a 2026 contribution decision in October 2027, when you finally see how the year actually shook out.
Solo 401(k)
- Establish (the plan document itself): By December 31, 2026, for sole proprietors making 2026 elective deferrals. (SECURE Act 1.0 extended the deadline for employer contributions, but the elective deferral piece still needs the plan in place by year-end for most owners.)
- Make elective-deferral election: By December 31, 2026 (you must commit to an employee deferral by year-end even if you fund it later).
- Fund employee deferrals: By your tax filing deadline (April 15, 2027, or October 15 with extension for sole proprietors).
- Fund employer profit-sharing: By your tax filing deadline, including extensions.
- Annual filing: Form 5500-EZ once plan assets exceed $250,000 at year-end.
The biggest unforced error: opening a solo 401(k) on January 5, 2027, "for the 2026 tax year." That works for the employer profit-sharing piece, but you lose the entire employee deferral. With the deferral worth up to $24,500, that is an expensive oversight.
Other Practical Considerations
Beyond the headline numbers, several smaller-but-real differences can sway the decision.
Loans
Solo 401(k) plans can offer participant loans (up to 50% of vested balance, capped at $50,000). SEP-IRAs cannot. If you anticipate ever needing access to retirement funds for a business cash crunch, the loan feature is a meaningful safety valve.
Roth Option
Most solo 401(k) plans now support designated Roth employee deferrals, and SECURE 2.0 also permits Roth employer contributions. SEP-IRAs have traditionally been pre-tax only, though SECURE 2.0 opened the door to Roth SEPs—but most custodians have not yet implemented them. If Roth flexibility matters to you, the solo 401(k) is significantly more developed.
Backdoor Roth Compatibility
This trips up high earners. SEP-IRA balances count toward the IRS's pro-rata rule for backdoor Roth conversions, which can poison your ability to do tax-free backdoor Roths. Solo 401(k) balances do not count. If you are over the Roth IRA income limit and want to keep doing backdoor Roth contributions, the solo 401(k) is the cleaner choice.
Administrative Burden
A SEP-IRA is the simplest qualified retirement plan in the U.S. tax code. There is no ongoing paperwork, no testing, no reporting until you take distributions. A solo 401(k) is more complex: you must adopt a written plan document, track elective deferrals separately, and file Form 5500-EZ once you cross $250,000. For most one-person businesses, that paperwork is still light—but it is not zero.
The Bookkeeping Connection
Both plans hinge on a single number: net self-employment income. That is the figure on Schedule C line 31 (or Schedule K-1 box 14 for partnerships) minus half of self-employment tax. Get that number wrong, and your contribution is wrong—either you under-contribute and leave tax savings on the table, or you over-contribute and trigger a 6% annual excise tax until you fix it.
Keeping clean, current books throughout the year is the only way to know what your contribution capacity actually is before deadlines pile up. If you are running on shoeboxes-of-receipts bookkeeping and only finding out your true profit at tax filing, you have already lost the ability to plan retirement contributions optimally. Cash-flow visibility lets you stagger contributions through the year, smooth taxes, and avoid the December scramble.
This is especially important for higher earners considering Roth versus pre-tax decisions. If you can see month by month that you are tracking toward a $300,000 year, you might decide to load up on Roth contributions in a year when your marginal rate is lower than you expect retirement to be. If you discover that in April after the fact, you cannot change last year's choice.
A Simple Decision Framework
Run through these questions in order:
-
Will I have any non-spouse employees this year or next?
- Yes → SEP-IRA (or a small-business 401(k) if more than a handful).
- No → continue.
-
Is my net self-employment income above roughly $175,000?
- Yes → both plans hit the same cap. Choose based on administrative preference, loan needs, and backdoor Roth considerations. Solo 401(k) still has a slight edge.
- No → continue.
-
Am I willing to do slightly more setup paperwork and file a Form 5500-EZ once I cross $250,000 in plan assets?
- Yes → solo 401(k) (it will shelter substantially more income at your level).
- No → SEP-IRA is a fine choice, but you are leaving real money on the table below $175K.
-
Am I 60–63 with strong income?
- Solo 401(k), to capture the super catch-up.
-
Do I run an S-corp paying myself >$150K in W-2 wages?
- Either plan works, but confirm your solo 401(k) custodian supports Roth catch-ups for 2026 compliance.
Common Mistakes to Avoid
- Defaulting to a SEP because it is simpler. Simpler is fine if you are above $175K. Below that, you are giving up tens of thousands in tax-deferred space every year.
- Opening a solo 401(k) in January for the prior year. You lose the employee deferral piece entirely.
- Forgetting that SEP-IRA pro-rata rules block backdoor Roths. This catches a lot of high-earning consultants.
- Hiring an employee and not realizing your solo 401(k) just disqualified itself. Audit your employee count every year.
- Contributing the same percentage to a SEP for "fairness" without modeling the cost. If you contribute 20% for yourself, you owe 20% of compensation for every eligible employee. This adds up fast.
- Missing the contribution deadline because you filed an extension and forgot. The SEP extension deadline saves many people; the solo 401(k) elective-deferral deadline does not extend.
Keep Your Finances Organized from Day One
Your retirement contribution capacity is only as accurate as your books. Whether you choose a solo 401(k) or SEP-IRA, you need a clear, current picture of net self-employment income before deadlines hit—not after. Beancount.io provides plain-text accounting that gives you complete transparency and control over your financial data, with version-controlled records and AI-ready exports for your tax preparer. Get started for free and see why developers, consultants, and freelancers are switching to plain-text accounting to keep their numbers retirement-ready year-round.