If you're self-employed and trying to decide between a Solo 401(k) and a SEP-IRA in 2026, the math has quietly shifted in favor of the Solo 401(k) for most people. The combination of higher elective deferrals, the SECURE 2.0 super catch-up for ages 60–63, Roth sub-accounts, mega-backdoor strategies, and participant loans now gives the Solo 401(k) capabilities that the SEP-IRA simply cannot match. The catch: a Solo 401(k) involves more paperwork, an annual Form 5500-EZ filing once assets cross $250,000, and stricter setup deadlines. For high earners with no employees, the trade-off is almost always worth it.
This guide walks through how to choose between the two plans, how to compute the maximum deductible contribution for each one in 2026, when the Solo 401(k)'s employee deferral plus profit sharing beats a 25% SEP contribution, how to layer catch-ups and the new super catch-up, how to build a Roth sub-account with the mega-backdoor strategy, and the filing obligations once your plan assets grow.
Why This Choice Matters More Than Most Tax Decisions
A single contribution decision can change your retirement balance by hundreds of thousands of dollars over a working life. Consider a 45-year-old freelance software developer earning $150,000 of net self-employment income. Under a SEP-IRA, the maximum 2026 contribution is roughly $27,872 (after the special 20% calculation). Under a Solo 401(k), the same person can defer the full $24,500 employee deferral plus the same employer contribution, for a total of about $52,372 — nearly double.
Over 20 years at a 7% return, that extra $24,500 of annual contribution compounds to more than $1 million in additional retirement savings. The two plans look similar on the surface, but the structural advantages of the Solo 401(k) are enormous for anyone making enough to fully fund the employer portion.
The 2026 Contribution Landscape
Before comparing strategies, anchor on the numbers. The IRS publishes annual limits indexed to inflation, and 2026 brought meaningful increases plus a new SECURE 2.0 tier.
Overall Section 415(c) Annual Additions Limit
Both plans share a common ceiling: the Section 415(c) annual additions limit, which is $72,000 in 2026. This is the absolute cap on combined employee deferrals, employer contributions, and after-tax contributions to a defined contribution plan.
Solo 401(k) Limits for 2026
- Employee elective deferral: $24,500
- Age 50+ catch-up: an additional $7,750 (total deferral $32,250)
- SECURE 2.0 super catch-up (ages 60–63): an additional $11,250 in place of the regular catch-up (total deferral $35,750)
- Employer profit-sharing contribution: up to 25% of compensation
- Total Section 415(c) limit: $72,000 (plus catch-up on top), reaching $79,750 with regular catch-up and $83,250 with super catch-up
SEP-IRA Limits for 2026
- Maximum employer contribution: 25% of compensation, capped at $72,000
- No employee deferral
- No catch-up contributions at any age
Notice what's missing from the SEP-IRA column. There is no way to add an employee deferral, no catch-up, no Roth, and no after-tax bucket. That structural simplicity is a feature for some, a ceiling for most.
The 20% vs. 25% Confusion
One of the most common mistakes self-employed savers make is plugging the wrong percentage into their contribution calculation. Both plans say "25% of compensation," but the math depends on your business structure.
S-Corporation Owners Use 25% of W-2 Wages
If you draw a W-2 salary from your S-corporation, the 25% applies directly to that wage. Shareholder distributions do not count as earned income for retirement plan purposes. Only your W-2 box 1 wages drive the calculation.
So an S-corp owner with a $100,000 W-2 salary can contribute up to $25,000 from the employer side. To reach the full $72,000 ceiling through employer contributions alone, you need W-2 wages of $288,000 (since $288,000 × 25% = $72,000).
Sole Proprietors and Single-Member LLCs Use 20% of Net Self-Employment Income
If you file Schedule C or you're a single-member LLC taxed as a sole proprietor, your "compensation" is net self-employment income reduced by half of self-employment tax and the retirement plan contribution itself. After the algebra, this simplifies to roughly 20% of net Schedule C earnings (before the contribution).
So a Schedule C filer with $150,000 of net earnings has a maximum employer/SEP contribution of approximately $27,872, not $37,500. This is the single most common error in DIY retirement planning for the self-employed.
When Solo 401(k) Beats SEP-IRA
The Solo 401(k) wins whenever the employee deferral plus the profit-sharing piece exceeds what a SEP could contribute alone. Because the SEP has no deferral component, the Solo 401(k) is dramatically better at lower income levels.
A Practical Crossover Example
A consultant nets $80,000 from Schedule C. Under a SEP-IRA, the maximum contribution is roughly $14,866. Under a Solo 401(k), the same person can defer $24,500 (employee) plus $14,866 (employer profit-sharing), for a total of $39,366. The Solo 401(k) more than doubles the savings.
The advantage shrinks only at very high income. At W-2 wages of $288,000 or net Schedule C earnings of about $360,000, both plans hit the same $72,000 cap. Below that level, the Solo 401(k) wins by the full $24,500 deferral amount.
When the SEP-IRA Still Makes Sense
The SEP-IRA isn't obsolete. It still has a niche when:
- You haven't decided whether you want a retirement plan and need to wait until you file your taxes. SEPs can be opened and funded up to the extended tax return deadline. Solo 401(k)s must generally be established by December 31 of the plan year (though SECURE Act 2019 allows establishing by the tax deadline for employer contributions only — not employee deferrals).
- You want absolutely zero administrative burden. No annual filing, ever, regardless of assets.
- You're a partner in a partnership and want pooled employer contributions without the complexity of a 401(k) plan document.
For everyone else, the structural advantages of the Solo 401(k) are decisive.
SECURE 2.0 Super Catch-Up for Ages 60–63
SECURE 2.0 Section 109 introduced a new "super catch-up" that adds meaningful capacity for savers in their early sixties. For 2026, participants who reach ages 60, 61, 62, or 63 by the end of the calendar year can contribute up to $11,250 in catch-up contributions, replacing the regular $7,750 catch-up. Once you turn 64, you revert to the standard catch-up.
The super catch-up is a Solo 401(k) feature only — SEP-IRAs have no catch-up provision of any kind. For a 61-year-old with W-2 wages of $200,000 and a properly structured plan, total 2026 contributions can hit $83,250.
The Roth Catch-Up Mandate
Starting in 2026, catch-up contributions for participants whose prior-year W-2 wages exceeded $145,000 (indexed) must be made on a Roth basis. This applies to both the regular age-50 catch-up and the super catch-up. Your Solo 401(k) plan document must allow Roth deferrals to keep the catch-up door open. Many vanilla brokerage-template Solo 401(k) plans do not support Roth contributions, so this is worth verifying before December 31.
The Mega-Backdoor Roth: Solo 401(k)'s Hidden Superpower
Self-employed business owners with strong cash flow can use a Solo 401(k) to make voluntary after-tax contributions and immediately convert them to Roth. This is the mega-backdoor Roth strategy, and it's available only in plans that explicitly support after-tax contributions and in-service distributions.
How It Works
- Make your regular employee deferral (pre-tax or Roth) of up to $24,500.
- Make your employer profit-sharing contribution.
- Fill the remaining Section 415(c) headroom — up to $72,000 total — with voluntary after-tax contributions.
- Convert those after-tax dollars to a Roth sub-account, ideally the same day or the next day, to avoid taxable earnings buildup.
For a freelancer with $200,000 of net Schedule C income who has already maxed deferrals and profit sharing, this can move an additional $10,000–$20,000 per year into a Roth bucket that grows tax-free.
Plan Document Requirements
The standard Solo 401(k) plans offered by Fidelity, Schwab, and Vanguard generally do not support voluntary after-tax contributions or in-service Roth conversions. You need a non-prototype or "checkbook control" plan from a specialty third-party administrator. Setup fees typically run $300–$1,000 plus modest annual fees, and the strategy needs to be worth the cost.
Participant Loans: An Underrated Feature
Solo 401(k) plans can offer participant loans of up to 50% of the vested balance, capped at $50,000. This is not retirement strategy advice — borrowing from retirement is rarely optimal — but it provides a liquidity backstop that SEP-IRAs cannot. Loans must be repaid within five years (longer for primary residence purchases), with interest paid back into your own account. Default treats the loan as a taxable distribution.
SEP-IRA holders have no loan option. Early withdrawals before age 59½ trigger ordinary income tax plus a 10% penalty.
Form 5500-EZ: The One Filing Obligation You Cannot Ignore
Solo 401(k) plans must file Form 5500-EZ annually once total plan assets reach $250,000 at year-end. The filing is due by July 31 of the following year, with an automatic extension to October 15 available via Form 5558. SEP-IRAs have no equivalent filing.
Form 5500-EZ is short, free, and filed through the DOL's EFAST2 system. Penalties for late filing are punitive — $250 per day up to $150,000 — but the IRS offers a delinquent filer voluntary compliance program at $500 per return (capped at $1,500) if you discover a missed filing before the IRS notices.
What to Report on Form 5500-EZ
- Total plan assets at year-end
- Contributions made during the year
- Distributions and rollovers
- Plan administrative information
Most third-party plan administrators include 5500-EZ preparation in their annual service fees. If you run a self-administered plan, set an annual calendar reminder for July 31.
Bookkeeping Discipline for Self-Employed Retirement Contributions
Whether you choose a Solo 401(k) or SEP-IRA, clean bookkeeping is what makes the contribution calculation defensible. The retirement plan deduction sits on Schedule 1, Line 16 for self-employed people, and the audit pattern is simple: the IRS wants to see that the contribution does not exceed the legal maximum tied to net self-employment earnings or W-2 wages.
Three habits keep your records clean throughout the year:
- Separate the contribution from operating cash flow. Move the contribution amount into a clearly labeled retirement plan transfer in your books on the day you fund it. Mixing it with general business expenses muddies your trial balance.
- Reconcile W-2 box 1 wages monthly if you run an S-corp. Your year-end retirement contribution depends on accurate payroll. A miscoded reimbursement or fringe benefit can change your maximum contribution by thousands of dollars.
- Document the calculation each year. Keep a one-page worksheet showing net Schedule C earnings, the 92.35% adjustment for self-employment tax, the half-SE-tax deduction, and the resulting maximum contribution. If the IRS asks, you have a paper trail.
For S-corp owners, the W-2 reasonable compensation question deserves separate attention. The IRS treats salary that's too low (to dodge payroll tax) and salary that's too high (to inflate retirement contributions) as audit triggers. Industry comparables, hours worked, and qualifications all factor into a defensible salary.
Stacking the Plans With Other Tax Strategies
The Solo 401(k) and SEP-IRA don't exist in isolation. Layering them with other strategies can compound the benefit.
The Solo 401(k) Plus Backdoor Roth IRA
A Solo 401(k) holds employer-side dollars and pre-tax employee deferrals. Separately, you can still contribute up to $7,000 ($8,000 if 50+) to a backdoor Roth IRA each year, assuming no pre-tax IRA balances that would trigger the pro-rata rule. SEP-IRA balances do count against the pro-rata rule and can spoil the backdoor — another reason to favor the Solo 401(k).
Self-Employed Health Insurance Deduction
Health insurance premiums paid by a self-employed person — including for an S-corp owner whose premiums flow through W-2 Box 1 — are deductible above the line, separate from the retirement plan deduction. Both deductions reduce the same adjusted gross income.
Augusta Rule Rental Income
S-corp owners using the Section 280A(g) Augusta Rule to rent their personal residence to the business for up to 14 days a year can layer that with retirement contributions, as long as the Augusta payment is reasonable and supported by comparable venue quotes.
Choosing the Right Plan: A Decision Framework
A simple decision tree captures the choice for most self-employed savers:
- Will you have employees other than your spouse? If yes, a Solo 401(k) is off the table. Consider a SEP-IRA or SIMPLE IRA.
- Do you want catch-up contributions, Roth options, or loan access? If yes, choose the Solo 401(k).
- Are you setting up the plan after December 31 of the contribution year? A SEP-IRA can still be opened up to your extended tax deadline; a Solo 401(k) can only accept employer contributions for the prior year if established by the tax deadline (no employee deferrals).
- Will plan assets stay under $250,000 indefinitely? Both work; the Solo 401(k)'s 5500-EZ filing only kicks in above that threshold.
- Do you want to use the mega-backdoor Roth? Only the Solo 401(k) supports it, and only with a specialty plan document.
For most freelancers, consultants, and small business owners earning between $50,000 and $300,000, the Solo 401(k) is the answer. The setup is more work — the right plan document, an annual filing once you cross $250,000 in assets, and a bit of payroll discipline — but the contribution capacity, Roth flexibility, and loan access are worth it.
Common Mistakes That Trigger IRS Attention
A few errors come up repeatedly in IRS examinations of self-employed retirement plans:
- Contributing on shareholder distributions instead of W-2 wages. S-corp owners must use W-2 box 1 only.
- Using 25% instead of 20% on Schedule C earnings. The special calculation reduces the effective rate.
- Funding the employee deferral late. Sole proprietor deferrals must be made by the tax filing deadline including extensions; S-corp deferrals must be deposited as soon as administratively feasible.
- Missing the December 31 plan establishment date for employee deferrals. SECURE Act 2019 only extended the deadline for employer contributions.
- Forgetting Form 5500-EZ once assets cross $250,000. Penalties accrue quickly.
- Letting SEP-IRA balances spoil a backdoor Roth. Pre-tax IRA dollars trigger pro-rata taxation.
Keep Your Retirement Contributions Audit-Ready
As you build a retirement strategy around a Solo 401(k) or SEP-IRA, the contribution calculation is only as good as the underlying books. Beancount.io provides plain-text accounting that gives you complete transparency over self-employment income, W-2 wages, and the contribution math behind your retirement plan deduction — no black boxes, no vendor lock-in. Get started for free and see why developers and finance professionals are switching to plain-text accounting that's version-controlled and AI-ready.