Solo 401(k) vs SEP-IRA: which fits your operator profile

The Solo 401(k) two-stack structure

Solo 401(k) (also called Individual 401(k) or one-participant 401(k); the IRS has no preferred name) is a 401(k) plan sponsored by a sole-proprietor or owner-only S-Corp / partnership where the only eligible participant is the owner (and optionally a spouse who works in the business). Per IRC §401(k) and IRS Publication 560, the contribution structure has two distinct legal sources:

Solo 401(k) max contribution (2025 limits) =

  Employee elective deferral:
      $23,500 flat
        + $7,500 catch-up if age 50+
        + $11,250 super-catch-up if age 60-63
            (per SECURE 2.0 §109)

  + Employer-side contribution:
      25% of net SE earnings (post-SE-tax adjustment)
      OR 25% of W-2 wages if running an S-Corp

  Combined cap (per IRC §415(c)):
      $70,000 in 2025
      $77,500 with age-50+ catch-up
      $81,250 with age-60-63 super-catch-up

The employee elective deferral is independent of income. You can earn $30,000 of net SE income and contribute the full $23,500 employee piece (subject to: contribution cannot exceed earned income for the year). That is the magic of Solo 401(k). For low-to-mid income self-employed operators, that flat amount stacks on top of the 25%-of-net-earnings employer side and dramatically increases tax-deferred contribution capacity vs SEP-IRA.

The SEP-IRA single-stack mechanic

Per IRC §408(k) and IRS Pub 560, a SEP-IRA (Simplified Employee Pension Individual Retirement Account) allows employer-only contributions of up to 25% of net SE earnings, capped at the same IRC §415(c) overall limit. There is no employee-side elective deferral.

SEP-IRA max contribution (2025 limits) =

  Employer contribution only:
      25% of net SE earnings (post-SE-tax adjustment)
      OR 25% of W-2 wages if running an S-Corp

  Capped at IRC §415(c):
      $70,000 in 2025

  No catch-up contributions (one of the few
  retirement plans without them)

The 25%-of-net-SE-earnings translates to roughly 20% of gross SE income after the SE-tax adjustment shrinks the base. So the operator who earns $100K of net Schedule C income contributes ~$18,587 to a SEP-IRA, while the same income contributes ~$42,087 to a Solo 401(k) ($18,587 employer + $23,500 employee). That $23,500 delta is the fundamental case for Solo 401(k) at low-to-mid income.

Side-by-side at three income benchmarks

Self-employed, age 45, no W-2 employees, computing “net SE earnings” per IRS Pub 560 worksheet (Schedule C net minus ½ SE tax):

The Solo 401(k) advantage is constant at ~$23,500 (the employee elective amount) up to the income level where SEP-IRA itself hits the §415(c) cap, then converges to zero. For most operators earning $50K–$300K, that constant $23,500 is meaningful tax- deferred space—at a 32% federal + 9% state marginal rate, $23,500 of additional contribution saves ~$9,635 in current-year tax. Over ten years that compounds.

Three cases where SEP-IRA wins anyway

1. Tax-filing-deadline establishment. SEP-IRA can be established AND funded as late as your tax filing deadline including extensions (October 15 for a calendar-year solo operator on extension). Solo 401(k) plan documents must be established by December 31 of the contribution year (per IRC §401(b)(2) and IRS Notice 2020-68); only contributions can be made up to the filing deadline. So if it is March and you are looking at a high prior-year income with no plan in place, SEP- IRA is the only option to capture prior-year deferral. (Note: SECURE 2.0 §317 narrowed this gap somewhat for solo owner-only plans, but the operational rule is still: plan document must exist by year-end for Solo 401(k) employee elective deferrals.)
2. Hate-paperwork operator profile.SEP-IRA has no annual filings. Solo 401(k) is filing-free until plan assets exceed $250,000 (per IRC §6058(a)), at which point you owe annual Form 5500-EZ. Many self-employed operators hit that threshold within 5–7 years and the 5500-EZ adds $200–$500/yr of CPA work or 30–60 minutes of self- filing. Real cost is small but not zero. SEP-IRA never triggers a 5500.
3. Multiple employer-businesses on the same employer tax ID. If you have multiple businesses (a freelance Schedule C plus a side rental that materially participates), the 401(k) controlled-group rules can require you to cover all employees in all entities. SEP-IRA also has cross- employee coverage rules but they trigger less aggressively in common solo-operator setups. If you have employees in any related business, talk to an ERISA-aware CPA before opening either plan.

The Roth question

Solo 401(k) supports Roth elective deferrals (the $23,500 employee piece can be Roth post-tax instead of pre-tax) at most providers—Vanguard, Fidelity, Schwab, E*TRADE, and most of the third-party admins (TPAs). SECURE 2.0 also added Roth treatment for the employer-side contribution as of 2024, though provider adoption has been uneven. Check your provider's plan document.

SEP-IRA had no Roth option historically. SECURE 2.0 §601 added Roth SEP capability effective 2023, but as of mid-2026 most custodians still do not support it operationally— Fidelity, Vanguard, and Schwab have all signaled Roth-SEP roadmap but no firm GA dates as of this writing. Operators who want Roth treatment today should default to Solo 401(k).

For lower-income early-career operators who expect to be in a higher tax bracket later, Roth treatment of the $23,500 employee piece is worth real money. The Roth Solo 401(k) is a first-class tool for that.

Loan provisions, rollovers, and other practical features

• Plan loans:Solo 401(k) plans can include a loan provision (per IRC §72(p))—you borrow up to 50% of the vested balance up to $50K, repaid within 5 years on a level-amortization schedule. Useful as emergency capital access without triggering early-withdrawal penalties. SEP-IRA has no loan provision.
• Rollover destination:Both plans accept rollovers from prior 401(k)s, IRAs, and similar tax-deferred accounts. Solo 401(k) is somewhat preferred as a rollover destination because consolidating in a 401(k) preserves the ability to defer RMDs from the rolled-in account beyond age 73 if you are still working (per IRC §401(a)(9) “still- working” exception). IRAs (including SEP-IRAs) do not get this exception.
• Backdoor Roth interaction: SEP-IRA balances count toward the IRA pro-rata rule per IRC §408(d)(2), which can taint backdoor Roth conversions. Solo 401(k) balances do not, because they are not IRAs. If you actively do backdoor Roth conversions and need to keep your IRA pro-rata balance at zero, Solo 401(k) is operationally cleaner. (Some operators roll old SEP-IRA balances into Solo 401(k) to clear the pro-rata problem.)

Operator decision framework

Default to Solo 401(k) if:

• Net SE income is under $300K (most solo operators)
• You want Roth treatment
• You want a loan provision
• You do backdoor Roth conversions
• You are operating consistently year-over-year and can stick to December 31 plan-establishment timing

Default to SEP-IRA if:

• Income is >$300K and the contribution math collapses to the same number anyway
• You missed Solo 401(k) plan-establishment for the year and need tax-filing-deadline flexibility
• You want zero annual filing burden (no 5500-EZ ever)
• You expect plan complexity to cause more pain than the extra $23,500 of contribution capacity is worth

Optionally combine both:there is no rule against having a Solo 401(k) and a SEP-IRA simultaneously, but the combined IRC §415(c) cap still applies, so the second plan adds zero net contribution capacity. The reason to have both is operational—some operators run a Solo 401(k) at one custodian for current-year contributions and keep an old SEP-IRA balance at another. Most operators consolidate over time.