Solo 401(k) vs SEP IRA for Business Owners

For a business owner with no employees, the retirement account choice is worth more than almost any other tax decision available in a single year. At $100,000 of net profit, a SEP IRA allows roughly $20,000 of contribution and a solo 401(k) roughly $44,000 — and the difference is not a rounding error in a tax return.

The gap exists because the two accounts are built differently. A SEP receives only employer contributions. A solo 401(k) receives an employee deferral as well, and that deferral is not a percentage of profit.

In this article, we set out where the difference comes from, the deadlines that decide which is available to you this year, and the cases where the simpler account is still the right one.

1. Where the Difference Comes From

Both accounts allow an employer contribution calculated as a percentage of compensation or net self-employment earnings. On that component alone, the two are broadly equivalent.

The solo 401(k) adds a second component. As the employee of your own business you may also defer a flat dollar amount, which does not scale with profit — so at modest profit levels it dominates the calculation entirely. The IRS describes the structure in its guidance on one-participant 401(k) plans.

The consequence is counterintuitive: the lower your profit, the larger the advantage. A SEP at $100,000 of profit is capped by the percentage; a solo 401(k) adds the deferral on top and roughly doubles the result.

An age-based addition applies to the 401(k) side as well. Participants above a set age may make a further catch-up deferral each year, which widens the gap again for an owner in their fifties or sixties. The SEP has no equivalent, because it has no employee component to add to.

Both accounts also permit a business to run them alongside other savings. Neither one prevents a personal IRA contribution, though deductibility of that contribution may be restricted once a workplace plan exists.

2. The Comparison in Practice

The two accounts differ on more than the headline number, and several of those differences matter for an owner-managed business.

As the comparison shows, the solo 401(k) wins on capacity and flexibility while the SEP wins on simplicity. The SEP's real constraint is employees: contributions must generally be made for eligible staff at the same rate as for the owner, which can make it expensive as a business grows.

A spouse working in the business is the quietest advantage of the solo 401(k). Where a spouse is genuinely employed and paid, they may contribute in their own right, which can double the household's annual capacity without any change to the business itself.

Entity type also shapes the employer component. For an S corporation owner, the contribution is calculated from W-2 wages rather than from total profit, so a low salary limits the retirement contribution as well as the payroll tax — a trade-off we set out in our article on S corp and LLC choices.

3. Deadlines Decide What You Can Use

The two accounts differ sharply on timing, and this catches owners more often than the contribution maths.

A SEP can generally be established and funded up to the tax filing deadline including extensions, which means a decision made in the spring can still apply to the prior year. A solo 401(k) has historically required the plan to exist before the year ends to capture the employee deferral, so a December decision and an April decision are not equivalent.

Rules in this area have been amended in recent years and continue to move, so verify the current position with a CPA rather than acting on a remembered deadline. The practical instruction is simpler: if you think a solo 401(k) might suit you, open it before 31 December, even if you fund it later.

One administrative threshold is worth diarising at the outset. Once a solo 401(k) exceeds a set level of assets, an annual information return becomes due for the plan. It is a straightforward filing, but missing it carries penalties out of proportion to the effort, so note it when the account is opened rather than discovering it years later.

4. The Objection: Locking Money Away

However, some argue that a business owner's best return is the business itself, and that money in a retirement account is capital that cannot be deployed.

For a business genuinely constrained by capital, that is a serious argument. Two considerations sit against it. The first is diversification: an owner whose income, savings and pension all depend on one company is concentrated in a way most would not accept in any other portfolio. The second is that a solo 401(k) commonly permits loans, so a portion remains reachable in a way a SEP does not.

There is also a cash flow point that pairs with the quarterly calendar. A deductible contribution reduces the tax owed for the year, so the decision interacts directly with the instalments described in our article on estimated tax for New York owners.

For New York City owners there is a further point. A deductible retirement contribution reduces federal and state taxable income, but city business taxes are computed on business income and are not reduced by a personal retirement deduction in the same way. Model the saving on the layers it actually reaches, as our article on the unincorporated business tax describes.

5. Final Thoughts

For an owner with no employees, the solo 401(k) is usually the stronger account: roughly twice the capacity at moderate profit, a Roth option, and loan access. The SEP earns its place where simplicity matters most or where a late decision is being made after year end.

Where staff exist or are planned, the calculation changes again, because a SEP obliges contributions for eligible employees and a solo 401(k) stops being available at all.

Contribution limits, deadlines and rules change annually, so confirm current figures with the IRS and a US CPA before opening either. If you are setting up a New York company and want the entity, payroll and retirement decisions taken together rather than in sequence, speak with our team.

Reinvent NY provides business consulting, operational support, and coordination services. Legal advice and immigration filings are handled by independent licensed attorneys. Real estate services are provided through licensed professionals and applicable brokerage relationships. This article is for informational purposes only and does not constitute legal or investment advice.

More for business owners: NYC Unincorporated Business Tax vs S Corp, S Corp vs LLC for a New York Business, Estimated Tax for New York Business Owners.