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Plan Setup vs Plan Funding
Solo 401(k)
For sole proprietors and self-employed individuals, the solo 401(k) offers the highest retirement contribution limit of any plan — stacking employee and employer contributions into a single powerful account.
A solo 401(k) — also called a one-participant 401(k), individual 401(k), or i401(k) — is a traditional 401(k) plan designed for business owners with no employees other than themselves (and a spouse). It follows the same rules as any other 401(k) but without the complexity and cost of a full employer plan.
What makes the solo 401(k) exceptional is its dual-contribution structure. As an employee of your own business, you can contribute up to $24,500 in elective deferrals (2026) — the same limit as any 401(k) participant. As the employer, you can contribute up to 25% of your net self-employment compensation on top of that. The combined limit for both contributions is $72,000 in 2026. Catch-up contributions add $8,000 for ages 50–59 or 64+ (combined limit $80,000), and $11,250 for ages 60–63 under SECURE 2.0 (combined limit $83,250).
This dual-contribution structure is what lets high-earning self-employed individuals shelter significantly more income than a SEP IRA (employer contributions only) or a traditional or Roth IRA (much lower limits) could achieve on its own.
Key Concepts
Who Qualifies
Any individual with self-employment income — a freelancer, consultant, contractor, sole proprietor, or single-member LLC — can open a solo 401(k), provided you have no full-time W-2 employees other than a spouse. Part-time employees who work fewer than 1,000 hours per year generally don't count against the eligibility rule, though exact treatment varies by plan document.
Employee Contribution
As an employee, you can contribute up to $24,500 in elective deferrals (2026) from your self-employment income — the same cap as any 401(k) participant. This is a dollar-for-dollar deduction. If you have another W-2 job with its own 401(k), your total employee deferrals across all plans combined cannot exceed $24,500 — the limit is per person, not per plan.
Employer Contribution
As the employer, you can contribute up to 25% of your net self-employment compensation (net profit minus half of self-employment tax). These employer contributions are separate from — and in addition to — your employee deferrals, and are also tax-deductible as a business expense. The combined employer + employee contribution cannot exceed $72,000 (2026).
Roth Solo 401(k)
Most major custodians now offer a Roth designation for solo 401(k) employee deferrals. Unlike a Roth IRA, there's no income limit to make Roth solo 401(k) contributions. High earners who are phased out of Roth IRA contributions can still make Roth 401(k) deferrals. Employer contributions must be pre-tax regardless — the Roth designation applies only to the employee deferral portion.
Loan Provision
Solo 401(k) plans that include a loan provision allow you to borrow up to 50% of your vested account balance, with a maximum of $50,000. The loan must be repaid (with interest, to yourself) within five years, using at least quarterly payments. This is a feature that IRAs — including SEP and SIMPLE IRAs — cannot offer.
December 31 Setup Deadline
A solo 401(k) must be established (plan document signed) by December 31 of the tax year for which you want to make contributions. You cannot open a solo 401(k) after year-end and retroactively apply it to the prior year — unlike a SEP IRA, which can be opened as late as the filing deadline including extensions. Employee deferrals must also be designated by December 31; employer contributions can be made up to the tax filing deadline.
Frequently Asked Questions
Can my spouse participate in my solo 401(k)?
Yes — a spouse who earns compensation from the business can participate as an employee and make their own elective deferrals, up to the same $24,500 limit (plus catch-up if eligible). This effectively doubles the household contribution capacity. The employer can also make employer contributions on the spouse's behalf. The plan remains a "one-participant" plan even with a spouse participating — that's the only exception to the no-employees rule.
What is the mega backdoor Roth in a solo 401(k)?
If your solo 401(k) plan document allows after-tax (non-Roth) contributions and in-service withdrawals or in-plan Roth conversions, you may be able to contribute after-tax dollars beyond the employee Roth deferral limit — then immediately convert those funds to Roth. The combined limit still applies ($72,000 total), but any room not used by the employee deferral and employer match can be filled with after-tax contributions and then converted. Not all custodians support this feature — check the plan document.
When do I need to file Form 5500-EZ?
You must file Form 5500-EZ annually when your solo 401(k) plan assets exceed $250,000 at the end of the plan year (December 31). The filing deadline is July 31 of the following year (no automatic extension for Form 5500-EZ, though you can request one). Failing to file carries significant penalties — $250 per day, up to $150,000. If you've crossed the $250,000 threshold, set a calendar reminder for the July 31 deadline.
Does 1099-NEC income qualify for solo 401(k) contributions?
Yes — any net self-employment income qualifies, including income reported on a 1099-NEC. Your eligible compensation for employer contributions is your net self-employment earnings (Schedule C profit or partnership income) minus the deductible portion of self-employment tax. Employee deferrals come from your earned income as well. A side gig producing 1099 income qualifies even if you have a full-time W-2 job — though your total employee deferrals across all plans cannot exceed $24,500.
How does a solo 401(k) compare to a SEP IRA?
Both plans are popular for self-employed individuals, but they work differently. A SEP IRA only accepts employer contributions (25% of net SE income, up to $72,000 in 2026). A solo 401(k) accepts both employee deferrals and employer contributions, which means at lower income levels a solo 401(k) can reach the $72,000 limit faster. A solo 401(k) also offers Roth options and loans; a SEP IRA does not. The SEP IRA wins on simplicity — there's no plan document, no December 31 setup deadline, and no Form 5500-EZ requirement.
Can I have a solo 401(k) if I also participate in an employer 401(k) at a W-2 job?
Yes — you can have both. However, your total employee deferrals across all plans cannot exceed $24,500 (2026). If you defer $15,000 through your W-2 employer's plan, you can only defer $9,500 as the employee into your solo 401(k). The employer contribution side of the solo 401(k) — up to 25% of net SE income — is unaffected by contributions to your W-2 employer's plan and can still be made up to the combined limit.
Also on this site
Topic Guide
SEP vs. SIMPLE IRA
How the two other major small-business retirement plans compare — limits, setup, and fit.
Topic Guide
Roth Conversions
Roth solo 401(k) deferrals and in-plan conversions are part of the same Roth planning toolkit.
Reference
2026 Contribution Limits
All IRA, 401(k), SEP, and SIMPLE limits for 2026 in one table.
Solo 401(k) rules trip up even experienced planners.
December 31 setup deadlines, after-tax contribution mechanics, spousal eligibility — the Rundown covers solo 401(k) scenarios as they come up, in plain English.
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Education-only disclaimer
This guide is for general education and information only. It does not provide individualized investment, tax, or legal advice, and does not establish a client relationship. Always consult your own tax professional, financial advisor, or legal counsel before making decisions about your accounts or retirement strategy.