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 one-participant 401(k) is for an eligible self-employed owner with no common-law employee other than a working spouse; 2026 employee and employer contributions can reach a $72,000 annual-additions ceiling before catch-ups. A Roth IRA is a personal account with a $7,500 limit ($8,600 at 50+), compensation and MAGI gates, and tax-free qualified withdrawals. The contribution ledgers are separate, but shared employee deferrals, plan compensation and the owner deduction still connect them.

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 the plan offers it. This person-wide §402(g) limit generally aggregates 401(k), 403(b), SIMPLE and SARSEP deferrals. If a day-job 401(k) already takes $15,000, only $9,500 of employee-deferral room remains for the side-business plan.
• Employer contribution: up to 25% of W-2 plan compensation — or generally a 20% reduced rate for a sole proprietor after the required adjustments. SECURE 2.0 permits optional vested Roth employer contributions, but the plan document and provider must support them and the amount is included in current income.
• 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 — with separate gates

The one-participant 401(k) and regular IRA use separate contribution limits, so funding the plan does not directly consume the $7,500 IRA cap. Actual room still depends on business compensation, other employee deferrals and the $72,000 annual-additions ceiling. The owner's plan deduction can reduce self-employed IRA compensation and MAGI, while direct Roth eligibility depends on modified AGI. Being covered by the plan separately affects the Traditional IRA deduction.

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.

How a Solo 401(k) can affect 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) balances are outside the IRA denominator. If the exact plan accepts the exact source, eligible pre-tax IRA money may be rolled into it; nondeductible IRA basis cannot. Confirm the plan document, any SIMPLE two-year restriction, investment gains and the projected December 31 Traditional/SEP/SIMPLE balance before treating the conversion as nontaxable. The Backdoor Roth Diagnostic walks through those conditions.

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. Above the direct-Roth phase-out: a Roth deferral inside the plan may still be available. Any separate IRA-to-Roth conversion needs the pro-rata and receiving-plan checks.

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.

The broader sequence — entity compensation, day-job deferrals, employees, deadlines and pro-rata coordination — is mapped in Roth IRA for self-employed people.