What Is a Solo 401(k)?

Employees get 401(k)s, sometimes with matching. Self-employed people get something arguably better: the Solo 401(k), where you’re both the employee and the employer, and you get to contribute wearing both hats.

What it is

A Solo 401(k) is a regular 401(k) designed for businesses with no full-time employees other than the owner (and their spouse). Same tax shelter as the corporate version, double the contribution opportunities.

Who qualifies

Two requirements: you have self-employment income, and your business has no full-time employees besides you (a spouse who works in the business can participate too). Part-time workers under 1,000 hours a year are fine. Freelancers, consultants, sole proprietors, and S-corp owners all qualify.

The two-hat math (2026 limits)

• As employee: contribute up to $24,500 ($32,000 if 50+).
• As employer: contribute up to 25% of compensation.
• Combined cap: $72,000 ($80,000 if 50+).

Example: $100,000 in net self-employment income lets you contribute roughly $24,500 as employee plus about $18,500 as employer, about $43,000 sheltered in a single year. Higher earners can hit the full $72,000.

S-corp vs. sole proprietor: the math differs

The calculation depends on your business structure. Sole proprietors use net self-employment income (after the employer half of self-employment tax). S-corp owners use W-2 wages paid to themselves, which means the salary you set directly affects how much you can contribute. Set too low a salary and you limit your 401(k) room; set it too high and you pay unnecessary payroll taxes. This is one area where a tax professional earns their fee. The Solo 401(k) works for both structures, but the optimal salary and contribution strategy differ.

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The Roth option and the mega backdoor

Many Solo 401(k) providers allow Roth contributions on the employee side, giving you the same Roth-vs-traditional choice employees get. And some providers go further: they allow after-tax contributions with in-plan Roth conversions, which means you can run the mega backdoor Roth inside your own Solo 401(k). Not every provider supports this, so check before you choose one. It’s a meaningful tiebreaker.

Solo 401(k) vs. SEP IRA

The SEP IRA only lets you wear the employer hat (25% of compensation, no employee deferral), so at the same income the Solo 401(k) shelters more. That extra $24,500 employee contribution is pure upside. The SEP has simpler paperwork, but if you qualify for the Solo, it usually wins.

Three Solo 401(k) mistakes to avoid

First, missing the December 31 setup deadline. You can fund employer contributions until tax day, but the account itself must exist by year-end. No account, no deduction. Second, choosing a provider without checking Roth and mega-backdoor support, then discovering the limitation years later when switching is a hassle. Third, contributing as both employee and employer to a Solo 401(k) while also maxing a separate employer’s 401(k). The $24,500 employee limit applies across ALL 401(k)s combined. The $72,000 total cap is per plan, but the employee portion is shared. Track it carefully if you have W-2 and self-employment income in the same year.

Deadlines and setup

Open the account by December 31 of the tax year you want it to cover. Employee deferrals must be made by year-end; employer contributions have until your tax filing deadline. Every major brokerage offers Solo 401(k)s. Compare them on Roth support and mega-backdoor compatibility, not just fees.

Where to open one and what to ask

Fidelity, Schwab, Vanguard, and E-Trade all offer Solo 401(k)s with no setup fees. Before choosing, ask three questions. One: do you support Roth employee contributions? Two: do you allow after-tax contributions with in-service Roth conversions (for the mega backdoor)? Three: what are the investment options and expense ratios? The answers vary more than you’d expect. A provider that checks all three boxes is worth more than one with slightly lower fees but missing features you’ll want later.

Bottom line: if you’re self-employed with no employees, the Solo 401(k) is the highest-capacity retirement shelter available, up to $72,000 a year, with Roth options on top. It’s the closest thing the tax code offers to a cheat code, and it’s completely legal. (For the IRA-side equivalent, see the backdoor Roth.)