Solo 401(k) vs. Roth IRA: What’s the Difference — and Can You Have Both?

A Solo 401(k) and a Roth IRA differ mainly in scale and structure. A Solo 401(k) is a one-participant workplace plan for the self-employed with no employees: you contribute as both employee (up to $24,500 in 2026, pre-tax or Roth) and employer (up to 25% of compensation), to a combined $72,000 cap — with no income limit. A Roth IRA is a personal account: after-tax contributions up to $7,500 ($8,600 at 50+), tax-free qualified withdrawals, anytime access to contributions, and a MAGI limit to contribute directly. The limits are separate — most self-employed savers can fund both.

What’s the real difference between a Solo 401(k) and a Roth IRA?

Start with what each account is. A Solo 401(k) (also sold as an individual 401(k) or one-participant 401(k)) is a full 401(k) plan scaled down to a business of one — a sole proprietor, single-member LLC, S-corp owner, or independent contractor with no employees other than a spouse. Because you are both the company and the worker, you contribute from both sides: an employee deferral and an employer contribution. That dual role is where the enormous capacity comes from.

A Roth IRA is a personal account with no connection to your business at all. The dollars go in after-tax, grow tax-free, and come out tax-free in retirement — and it stays the most flexible account you can own: contributions are withdrawable anytime, there are no required minimum distributions for the owner, and you can hold it at any brokerage with any investments, no plan documents required.

So the comparison isn't really “which is better” — it's a high-capacity business plan vs. a flexible personal account, and they're designed to stack.

Solo 401(k) vs. Roth IRA at a glance (2026)

2026 figures per IRS Notice 2025-67 (employee deferral $24,500; §415(c) combined cap $72,000; catch-ups $8,000 / $11,250 per SECURE 2.0 §109; Roth IRA $7,500/$8,600). Roth 401(k) lifetime RMDs were eliminated for 2024 and later by SECURE 2.0 §325.

How much can you actually put in a Solo 401(k)?

The headline numbers stack like this for 2026:

• Employee deferral: up to $24,500 — pre-tax, or Roth if your plan offers it. One caution: this limit is shared across every 401(k) you have. If a day job's 401(k) already takes $15,000 of deferrals, only $9,500 of deferral room is left for the side business's solo plan.
• Employer contribution: up to 25% of compensation — always pre-tax under current rules at most providers. For sole proprietors the effective rate works out to roughly 20% of net self-employment earnings after the required adjustments; the calculation is the fiddly part of solo-401(k) life.
• Combined cap: $72,000 (IRC §415(c)) — plus catch-ups on top: $8,000 at 50+, or $11,250 at ages 60–63, for an all-in $80,000–$83,250.
• The spouse multiplier: a spouse who genuinely works in the business is the one employee a solo plan may cover — with their own employee-plus-employer capacity. A married business can shelter well over $140,000 a year at full throttle.

Against that, the Roth IRA's $7,500 looks tiny — but capacity isn't the whole story.

Can you have both? (Yes — and the limits don’t touch)

The Solo 401(k) limit is a workplace-plan limit; the Roth IRA limit is an individual IRA limit. They never interact. A 35-year-old freelancer with a strong year could put $72,000 into a Solo 401(k) and $7,500 into a Roth IRA — $79,500 of tax-advantaged space. Solo 401(k) participation doesn't shrink your Roth IRA limit, and it doesn't affect Roth eligibility either: only your modified AGI does. (Being covered by a workplace plan changes the Traditional IRA deduction — a different rule people regularly confuse with this one.)

One quiet edge for high earners: the Solo 401(k)'s Roth deferral option has no income limit. A consultant earning $400,000 is far past the Roth IRA phase-out, but can still put the full $24,500 of employee deferrals into the Roth side of a solo plan — no backdoor required.

The backdoor-Roth superpower: a Solo 401(k) can fix the pro-rata problem

This is the interaction that makes the two accounts genuine teammates. High earners doing backdoor Roth conversions get tripped by the pro-rata rule (§408(d)(2)): the IRS counts all traditional, SEP, and SIMPLE IRA balances when taxing a conversion, so old pre-tax IRA money makes the backdoor partly taxable.

401(k) plans are not part of that calculation. If your Solo 401(k) accepts incoming rollovers (most do), you can roll your pre-tax IRA balances into the plan, leave a $0 pre-tax IRA footprint by December 31, and convert future nondeductible contributions essentially tax-free. For self-employed high earners, this is often the single best structural reason to open a solo plan — it simultaneously adds $72,000 of capacity and unlocks a clean annual backdoor Roth.

Where the Roth IRA still wins

Raw capacity isn't everything, and the small account holds its own:

• Anytime access: Roth IRA contributions come out tax- and penalty-free at any age. Solo 401(k) money is generally locked until 59½ (a plan loan — up to 50% of the balance or $50,000 — is the workaround, and it must be repaid).
• No RMDs, ever, automatically: the Roth IRA has no lifetime RMDs by design. In a solo plan, pre-tax money faces RMDs at 73/75 (Roth solo-401(k) balances escaped RMDs starting in 2024, and many savers eventually roll them to a Roth IRA anyway).
• Zero administration: no plan document, no restatements, no Form 5500-EZ filing once assets pass $250,000, no deadline mechanics. You open it and fund it.
• Tax-free certainty: every dollar of growth in the Roth IRA is already settled with the IRS. The Solo 401(k)'s pre-tax side is a deferral — the tax bill arrives in retirement.

Which should you fund first?

A common framework for the self-employed — educational, not personalized advice:

1. Start with the Roth IRA if your income allows direct contributions — it's free to open, takes minutes, and buys flexibility and tax-free certainty with the first $7,500.
2. Open the Solo 401(k) as business income grows — the employee deferral ($24,500) typically comes first, then the employer side scales with profit.
3. High earners flip the order: past the Roth IRA phase-out, the solo plan becomes the main event (Roth deferrals with no income limit + the pro-rata cleanup that re-opens the backdoor).

The deeper now-vs-later tax question — pre-tax deferrals or Roth contributions inside the plan — is the same trade-off as the Roth vs. Traditional decision: it hinges on your tax bracket today versus in retirement.