Solo 401(k) vs. SEP IRA vs. SIMPLE IRA: Which Retirement Plan Is Best for Your Business?

The three most common retirement plans for small business owners are the Solo 401(k), the SEP IRA, and the SIMPLE IRA. Each has different contribution limits, employee obligations, administrative requirements, and deadlines. The right choice depends on the business owner’s income level, whether they have employees, and how much they want to contribute.

How do the plans compare at different income levels?

When is the SEP IRA the better choice?

The SEP IRA used to beat the Solo 401(k) whenever the business owner needed to establish a plan AFTER the tax year had ended. That gap is now much narrower. A Solo 401(k) adopted by the tax filing deadline, including extensions, can be treated as in place for the prior year and take the employer contribution; first-year employee deferrals are open only to an unincorporated owner who is the business’s only employee (a spouse on payroll rules this out), and the plan and the deferrals must be in by the unextended due date, April 15, 2026 for 2025. The SEP IRA can be established and funded as late as the tax filing deadline, including extensions (October 15 for calendar-year filers on extension).

A business owner who realizes in March (while preparing last year’s return) that they need a retirement deduction can establish a SEP IRA and make the full employer contribution for the prior year. A Solo 401(k) adopted by the same extended deadline can take the same employer contribution, and an unincorporated owner who is the business’s only employee and adopts it by the unextended due date can add first-year deferrals on top.

When is the SIMPLE IRA the better choice?

The SIMPLE IRA is the better choice when: (1) The business has employees and wants the lowest employer cost (the 3% match only applies to participating employees, and many lower-paid employees do not participate). (2) The business owner wants employee deferrals but can’t afford the Safe Harbor 401(k) administration costs ($1,500-$3,000/year TPA). (3) The business has 5-20 employees and the 3% match is cheaper than the Safe Harbor 401(k)‘s 3% non-elective on all eligible employees.

How do Solo 401(k), SEP and SIMPLE deadlines compare?

For 2025, SEP contributions and Solo 401(k) employer contributions are due by the business’s return due date with extensions, which is October 15, 2026 for a sole proprietor who files Form 4868. Solo 401(k) deferrals must be elected by December 31 in an existing plan. A SIMPLE IRA has to be effective by October 1 of the year it starts.

The Solo 401(k) is more flexible than it used to be. A sole proprietor with no employees, spouse included, can adopt one after year-end and still make first-year deferrals if the plan is adopted and the deferrals are in by the unextended date, April 15, 2026 for 2025. A plan adopted later, up to the extended due date, can still take the employer part. S corporations and businesses with employees don’t get the prior-year deferrals.

• SIMPLE deferrals withheld from pay are due no later than 30 days after the end of the month they’re withheld. The employer match is due by the extended return date.

• A SEP can still be set up as late as the extended due date, which keeps it simple for a late decision.

• Early Withdrawal from Retirement Accounts: Penalties, Exceptions, and SEPP Plans for Business Owners

• Retirement Plan Startup Tax Credit: Up to $15,000 for New 401(k) or SIMPLE Plans (SECURE 2.0)

• Roth IRA Conversions for Business Owners: Timing, Income Management, and the Backdoor Strategy

• Backdoor Roth IRA for High-Income Business Owners

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.