Solo 401(k) vs SEP IRA: Which Fits Your Income, Entity, and Plans to Hire?

Both plans are popular with the self-employed, and both let you contribute a large amount compared with an IRA. The differences are in how contributions are built, when you can act, and what happens when your business grows. This comparison focuses on decisions rather than definitions. For details on each plan, see the solo 401(k) guide and the SEP IRA guide.

The core structural difference

A SEP IRA has one contribution source: the employer. The contribution is a percentage of compensation up to a cap.

A solo 401(k) has two sources: employee deferrals (your own salary reduction) and employer contributions. The combination means a fixed dollar amount is available from deferrals regardless of how large your income is, up to your compensation, then the employer portion adds to it.

That structure is why solo 401(k) plans often produce a higher total at moderate income levels. At high incomes, both plans can reach the overall annual dollar limit, and the difference shrinks.

Illustrative comparison

Imagine two owners, one earning $60,000 net and another earning $300,000 net, without employees. A solo 401(k) can allow the first owner to contribute a large share of income by deferring salary and then adding an employer amount, while a SEP allows roughly one-fifth of net earnings. For the second owner, both plans can reach the limit or close to it. These comparisons are simplified. Specific numbers depend on entity type, self-employment tax, and annual limits.

Side by side

Deadline advantage

The SEP's biggest practical advantage is timing. If you realize in March that you want a deduction for last year, a SEP can generally be opened and funded by the tax filing deadline (with extension). A solo 401(k) requires timely adoption for employee deferrals, though recent legislation gives sole proprietors more time in the first year. Employer contributions to a solo 401(k) can still be made by the filing deadline once the plan exists. If you are already past the year-end and have no plan, a SEP may be the only way to get a deduction for the prior year, and later you can switch to a solo 401(k) for future years.

Hiring plans

If you expect to hire employees soon, both plans can become costly: the SEP requires equal-percentage contributions for eligible employees, and a solo 401(k) becomes a regular 401(k) with testing and coverage rules. Options for growing employers include the SIMPLE IRA, a safe harbor 401(k), or a plan designed by a third-party administrator. Consider the timeline and the mix of employees. Part-time or long-tenure rules can matter.

Backdoor Roth IRA considerations

High earners who use the backdoor Roth IRA strategy face the pro rata rule, which looks at all traditional, SEP, and SIMPLE IRA balances. A large SEP balance can make a backdoor conversion mostly taxable. A solo 401(k) is not included in the pro rata computation, so it does not create this problem. Some owners prefer a solo 401(k) partly for this reason.

Roth features

If you want Roth deferrals, a solo 401(k) is more likely to offer them through mainstream providers. Roth SEP contributions became possible under recent legislation, but adoption and availability vary. If Roth flexibility matters, confirm the provider's plan document.

Loans and flexibility

A solo 401(k) can allow loans up to statutory limits. That can be valuable as an emergency liquidity source, though it is not a substitute for an emergency fund and has risks. A SEP does not offer loans. Neither plan avoids penalties for early withdrawal (with exceptions).

Administration and cost

SEP: nearly no administration. Solo 401(k): some ongoing tasks, including tracking contributions and filing Form 5500-EZ when assets are above the threshold. Providers offer low-cost solo 401(k) plans with support. If you want the lowest hassle, a SEP wins. If you want maximum contribution and features, the solo 401(k) is often worth the extra step.

Entity considerations

• Sole proprietors and single-member LLCs: both plans work. The solo 401(k) tends to produce a higher contribution at lower profit.
• S-corp owners: both are based on W-2 wages. The solo 401(k) deferral portion lets you contribute up to the annual limit as long as wages are sufficient, even when the 25 percent employer portion would be limited by a low salary. See retirement plans for S-corp owners.
• Partnerships: partners are treated as self-employed for plan purposes.

Which to choose: quick guide

Lean toward a solo 401(k) if:

• You have no employees and are not likely to hire soon.
• You want the higher contribution potential at moderate income.
• You want Roth deferrals or the possibility of a loan.
• You use or may use backdoor Roth contributions.

Lean toward a SEP IRA if:

• You want the simplest setup and no annual filing.
• You want to open a plan after the year has ended and still deduct for that year.
• Your income is high enough to reach the cap either way.
• You are comfortable with employer-only contributions.

Consider a third option if you want to contribute more than either allows. High-income owners with stable profits sometimes add a cash balance plan.

Common mistakes

1. Picking based on marketing rather than owner facts.
2. Ignoring the impact on backdoor Roth strategy.
3. Not planning for employee hires.
4. Assuming contributions are limited only by income, without checking the formulas.
5. Missing the solo 401(k) adoption deadline.
6. Contributing to both plans without understanding the combined limits. If you have both a SEP and a 401(k) with the same employer, the overall limits apply across the plans.

If you would like a numerical comparison using your own income and entity type, AE Tax Advisors can assist, and a discovery call is a good first step.