Solo 401(k) vs SEP IRA: How to Pick in Under 5 Minutes
Three questions: Do you want to defer salary? Do you want Roth? Do you want loan access? Two yeses → Solo 401(k).
Both plans target the same market, self-employed, owner-only, no W-2 employees. They top out at the same $70,000 combined cap. But below that ceiling they behave differently. Here's how to decide.
Question 1: Do you want to defer salary?
SEP IRA only allows the employer share. So at any income below about $350,000 of net SE earnings, the Solo 401(k) puts more dollars into retirement because it gives you the additional $23,500 employee deferral. Quick math at $80k SE income: SEP IRA = $14,800. Solo 401(k) = $14,800 + $23,500 = $38,300. The gap is $23,500 every single year.
Question 2: Do you want a Roth option?
SEP IRA: no Roth, period. Every dollar is Traditional pre-tax. Solo 401(k): up to $23,500 of employee deferral can be Roth (and SECURE 2.0 added Roth on the employer side too, though provider support is rare in 2026). For young high-savers expecting bracket creep in retirement, Roth matters.
Question 3: Do you want loan access?
SEP IRA does not allow loans. Solo 401(k) lets you borrow up to 50% of vested balance, max $50,000, repaid over 5 years (15 if used for a primary residence). This is often the deciding factor for freelancers worried about cash flow gaps.
The setup penalty
SEP IRA wins on simplicity: about 5 minutes at any broker, no Form 5500 ever. Solo 401(k) takes 30 minutes (you sign a plan adoption agreement) and requires Form 5500-EZ once plan assets exceed $250,000, but the form is one page and free to file yourself.
If you might hire employees
This is where SEP IRA quietly wins. Hire a W-2 employee, even part time, and SEP IRA forces you to contribute the same percentage to them. That's manageable. Solo 401(k) becomes ineligible the moment you hire a non-spouse W-2 employee for 1,000+ hours, forcing a full plan migration. If you're 12 months from your first hire, SEP IRA might save you the migration headache.
The 25-year compounding gap
For someone earning $80k net SE income at age 35, the Solo 401(k) puts $23,500 more in retirement every single year. Compounded at 7% real for 30 years, that's roughly $271,000 in extra retirement assets. Purely from the employee deferral that SEP IRA can't match.
Decision tree
Solo 401(k) if: You answered yes to any of questions 1-3 above, you don't expect to hire W-2 employees in the next year, you're comfortable with one extra setup form.
SEP IRA if: You don't care about the deferral, don't want Roth, don't need loan access, and might hire soon. Or you genuinely want zero paperwork.
For 95% of one-person businesses, the answer is Solo 401(k). For the other 5%, SEP IRA is the right pick, especially if you're already running an employee 401(k) at the day-job and using the side-business plan only for the employer share. See the full comparison.