SEP IRA vs. solo 401(k): choose by employees, savings capacity and workload

For an owner-only business, the SEP IRA vs. solo 401(k) decision usually comes down to contribution mechanics and administrative work. A solo 401(k) can let the owner contribute in two roles, employee and employer, while a SEP IRA accepts employer contributions only. A SEP can cover employees, but its equal-percentage rule can raise the cost of saving for the owner once staff are eligible.

Both are tax-advantaged retirement-plan structures for self-employed people and small-business owners. The useful comparison is your workforce, compensation calculation, target contribution, and the provider’s plan terms.

SEP IRA vs. solo 401(k) at a glance

• Who can use it: A solo 401(k), also called a one-participant or individual 401(k), is for a business with no employees other than the owner and possibly a spouse. A SEP IRA, short for Simplified Employee Pension IRA, can be used by businesses with employees.
• Where contributions come from: In a solo 401(k), an owner may make employee elective deferrals, meaning compensation directed into the plan at the employee’s election, plus employer contributions. A SEP IRA uses employer contributions only.
• Catch-up contributions: A solo 401(k) may allow additional catch-up contributions for eligible older participants, subject to applicable annual limits. A SEP IRA does not allow them.
• Roth and loans: A solo 401(k) may offer Roth employee deferrals and participant loans if its plan document allows them. SEP IRAs do not permit participant loans. Roth treatment for SEP employer contributions may depend on provider and plan availability, so confirm the feature before relying on it.
• Paperwork: SEP IRAs generally have lighter setup and maintenance requirements and no annual filing. A solo 401(k) requires more plan documentation and recordkeeping; Form 5500-EZ is required once plan assets exceed $250,000.
• Employees: A SEP generally requires the employer to contribute the same percentage of compensation for every eligible employee. Adding non-owner employees to a solo 401(k) can end its owner-only arrangement and bring the responsibilities of a regular 401(k) plan.

The contribution difference that changes the comparison

Both plans can share the same overall annual contribution ceiling in a given tax year. That headline limit does not show how much an owner can contribute at a particular income.

A SEP contribution is based on the employer side alone. In a solo 401(k), the employee-deferral layer may be added before the employer contribution, subject to annual rules. That structure can produce a larger retirement contribution at many moderate income levels.

Illustrative math, not a tax calculation

Assume an incorporated owner receives $100,000 of compensation, chooses a $20,000 employee deferral to a solo 401(k), and has an allowable employer contribution equal to 25% of compensation. This example ignores annual caps and other facts that may alter a real result.

• SEP IRA: 25% × $100,000 = $25,000 employer contribution.
• Solo 401(k): $20,000 employee deferral + ($100,000 × 25%) employer contribution = $45,000.
• Difference: $45,000 − $25,000 = $20,000.

The point is the contribution structure, not the dollar amount. The SEP uses one employer contribution. The solo 401(k) can combine two contribution sources. Annual IRS limits still apply, and employer contributions are generally limited to 25% of compensation.

The calculation can be less straightforward for a sole proprietor. Self-employment taxes and the deduction for the employer contribution affect the compensation base, so applying a flat 25% to a business’s headline profit can overstate the permitted amount. Get individualized tax guidance before acting on a calculation.

Start with your employee and hiring plans

If the business has non-owner employees, a solo 401(k) generally is not the owner-only plan to use. A SEP IRA can include eligible employees, so it may fit a business that already has staff or expects to add them.

A SEP can still create a meaningful employer expense. If the owner contributes 15% of compensation, eligible employees generally must receive 15% of their compensation as well. A growing payroll can raise the cost of the plan.

An owner who expects to remain owner-only may put more weight on the solo 401(k)’s two contribution sources, potential catch-up contributions, possible Roth deferrals, and possible loan feature. A business that values lighter ongoing administration, or needs a plan that can cover employees, may put more weight on a SEP IRA.

A practical choice checklist

1. Count eligible workers: Is the owner, with or without a spouse, the only employee? If not, review employee-coverage rules before choosing a plan.
2. Map hiring plans: A near-term hire can change the compliance burden of an owner-only 401(k).
3. Estimate the actual contribution: Compare employer-only SEP capacity with the employee-deferral-plus-employer structure of a solo 401(k). Use current-year IRS limits.
4. Identify account features you need: Check whether the particular solo 401(k) supports Roth deferrals or loans. Verify any Roth SEP feature directly with the provider.
5. Price the administration: Compare setup fees, ongoing account costs, recordkeeping, and the possibility of a future Form 5500-EZ filing.
6. Check the entity-specific math: Compensation rules can differ by business structure, particularly for unincorporated owners.

A solo 401(k) can be a strong fit for a one-person business seeking more saving capacity at a given income. A SEP IRA can appeal for its simpler employer-only design and ability to cover employees. Calculate the contribution under your own structure, then weigh that result against employee obligations and administrative burden.

Frequently asked questions

What happens to a solo 401(k) when you hire an employee?

A solo 401(k) is intended for an owner-only business, with a spouse potentially covered. Adding a non-owner employee can end that status and bring added responsibilities associated with a traditional 401(k), including potential testing and administrative requirements. Review the plan and employee-eligibility rules before hiring.

When does a solo 401(k) need to file Form 5500-EZ?

The supplied sources state that Form 5500-EZ is required once solo 401(k) plan assets exceed $250,000. A solo 401(k) also requires more documentation and recordkeeping than a SEP IRA, so check current filing instructions for the relevant year.

Can a SEP IRA have Roth contributions?

The research sources do not support a universal answer. One source says recent law may permit Roth treatment of SEP employer contributions, while another says SEP IRAs have no Roth option. Confirm current availability and the provider’s plan terms before relying on Roth SEP treatment.

Sources

• Solo 401(k) vs. SEP-IRA: How Solopreneurs Should Choose — www.employeefiduciary.com
• SEP IRA and Sole Proprietor 401k Plan Comparison — institutional.fidelity.com
• Solo 401(k) vs. SEP IRA: Self-Employed Retirement Options — www.thrivent.com
• Solo 401(K) vs SEP IRA: Which Is Right for You? | PLANADVISER — www.planadviser.com