Solo 401(k) or SEP IRA: Which Wins for the Self-Employed in 2026?

The 60-second rule

If you want to save more than ~20% of your net self-employment income, open the Solo 401(k). The SEP’s ceiling is the employer share alone — roughly a fifth of what you net — and nothing you do raises it. The Solo adds a $24,500 employee deferral on top of that same fifth, which is the entire ballgame: at $100,000 of net income that’s $18,587 vs $43,087 of room. If your savings goal fits under the SEP’s cap, take the SEP and skip the paperwork. And if you have W-2 employees, the Solo isn’t an option at all — it legally requires a business of one (plus a spouse). Everything below is the evidence for that rule. Run your own numbers in the Solo 401(k) vs SEP IRA calculator.

How the SEP IRA really works

The SEP IRA is the minimalist’s plan. There is exactly one kind of contribution — the employer contribution — capped at 25% of compensation up to the annual limit ($72,000 in 2026). For the self-employed, that 25% is computed on a reduced base (your net earnings minus half the SE tax minus the contribution itself), which is why everyone quotes it as “about 20% of net.” At $100,000 of Schedule C net: the base works out to $92,935, and 20% of that is $18,587. That’s the whole plan — no employee deferral, no catch-up contributions, no Roth option, no loans. Setup takes minutes at any brokerage, there are no annual filings, and contributions can be made up to the tax-filing deadline including extensions — the procrastinator’s plan in the best sense. The fine print that matters: the day you hire eligible W-2 employees, the SEP must cover them too, at the same percentage you give yourself. The simplicity scales — expensively.

How the Solo 401(k) really works

The Solo 401(k) — officially the one-participant 401(k) — lets you wear two hats. Hat one, the employee: you defer up to $24,500 in 2026 from your compensation, plus catch-ups from age 50. Hat two, the employer: the business contributes the same ~20%-of-net the SEP allows. Both hats share one $72,000 combined ceiling. At the same $100,000 of net: $24,500 + $18,587 = $43,087 of room — $24,500 more than the SEP, by construction. The extras the SEP can’t touch: the employee deferral can be designated Roth (after-tax now, tax-free later — the employer share is always pre-tax), and the plan can offer loans up to $50,000 or 50% of the balance. The costs: the plan must exist by December 31 for that year’s employee deferrals to count (miss New Year’s Eve and the $24,500 is gone for that tax year), and once assets cross $250,000 you file a short Form 5500-EZ annually. And the hard rule: no full-time W-2 employees besides a spouse. Hire one and the Solo must be converted or terminated.

The $11,413 worked example, line by line

Take a 40-year-old freelancer with $100,000 of net self-employment earnings, a 22% marginal bracket, and a $30,000 savings goal:

1. The base both plans share: $92,935. Net earnings minus half the SE tax ($100,000 × (1 − 0.153 × 0.9235 ÷ 2)). Every contribution limit below is computed off this number, not the $100,000.
2. The employer share: $18,587. Twenty percent of $92,935. This is the SEP’s entire ceiling — and the Solo’s second hat.
3. The Solo’s employee deferral: $24,500. The 2026 limit (no catch-up at 40). The SEP has no equivalent — this line is the whole fight.
4. The ceilings: Solo $43,087 vs SEP $18,587. Both under the $72,000 combined cap, so no binding constraint yet. The Solo’s headroom is $24,500 wider — exactly the deferral.
5. The goal test: $30,000 doesn’t fit the SEP. The SEP caps at $18,587 — $11,413 short of the goal. The Solo holds all $30,000 with $13,087 to spare.
6. The prize: $11,413 of extra room, $6,600 of tax savings. The deductible $30,000 at a 22% bracket saves $6,600 in tax — but only the Solo can shelter the full amount. That’s the verdict: Solo 401(k) wins by $11,413.

Now move the inputs and watch the logic hold. Drop the goal to $15,000: both plans hold it, the extra room buys nothing, and the SEP wins on simplicity. Raise the goal to $80,000: both plans cap out ($43,087 vs $18,587), and the Solo still wins by $24,500 — the deferral gap is constant. Set the employees dropdown to yes: the Solo is off the table entirely. The calculator prices all four branches.

The age ladder: catch-ups widen the gap

The SEP has no catch-up contributions — its ceiling is the same at 35 and 65. The Solo’s employee deferral grows with age, and the gap grows with it. At the same $100,000 of net:

The 60–63 “super catch-up” — $11,250 on top of the $24,500 — is the Solo’s best trick: a 62-year-old gets $35,750 of extra room the SEP simply doesn’t offer. If you’re in the home stretch and self-employed, this table alone usually ends the debate. (All figures 2026; see the 2026 retirement contribution limits table.)

When the SEP wins anyway

The Solo wins the math whenever the goal exceeds ~20% of net. But three cases belong to the SEP. One: the goal fits. At a $15,000 goal on $100,000 of net, both plans shelter every dollar — the Solo’s extra $24,500 of headroom, Roth door, and loan feature all sit unused, and the SEP’s five-minute setup and zero filings win outright. Two: you have (or will hire) employees. The Solo requires a business of one; the SEP is built to extend to staff. If a hire is even likely within the year, start with the SEP and skip the conversion headache. Three: you value the filing deadline. The SEP can be opened and funded up to the extended tax deadline — the Solo’s employee deferral dies on December 31. Chronic procrastinators should price their own habits honestly.

The deadlines that kill procrastinators

Write these down. Solo 401(k): the plan must exist by December 31 of the tax year for employee elective deferrals to count — the $24,500 (plus catch-ups) cannot be created retroactively. The employer contribution, for both the Solo and the SEP, can be made up to the tax-filing deadline including extensions. The SEP as a whole shares that generous deadline: open it in March, fund it in September on extension, deduct it for the prior year. The practical rule: open the Solo in December even if you fund it in April — the establishment deadline is the one that bites, and it bites silently.

The Roth door (and the loan)

Two Solo features deserve their own paragraph because they change strategy, not just arithmetic. Roth elective deferrals: some or all of the $24,500 can go in after-tax and grow tax-free — the SEP offers no Roth at all, every dollar pre-tax. If you’re young, in a low bracket, or expect higher taxes later, the Roth Solo is the best deal in the self-employed retirement universe. Loans: a Solo 401(k) can let you borrow up to $50,000 (or 50% of the balance) from yourself — useful as an emergency backstop, dangerous as a habit. The SEP allows no loans, period. Neither feature shows up in the headroom math, but both show up in real life.

Three questions to ask yourself

1. Is my savings goal more than a fifth of my net? Divide your goal by your net earnings. Over ~0.20 and the SEP mathematically cannot hold it — the Solo is the answer before you read another paragraph.
2. Will I hire anyone in the next 18 months? A yes (beyond a spouse) kills the Solo. Don’t build a plan you’ll have to dismantle — the SEP, or a small-business 401(k), is the honest pick.
3. Do I want Roth, or might I need a loan? If either answer is yes, the Solo’s extra features are worth the December deadline and the 5500-EZ at $250k. If both are no and the goal fits, the SEP’s simplicity is the feature.

Three mistakes to avoid

1. Opening the Solo on January 2nd and expecting last year’s deferral. The establishment deadline is December 31 — the most expensive missed date in self-employed retirement planning. The $24,500 doesn’t roll forward.
2. Double-counting the $24,500 with a day-job 401(k). The elective-deferral limit is per person, not per plan — $24,500 total across your W-2 401(k) and your Solo. The employer-share caps are separate, but the employee number is shared. Coordinate before you over-contribute.
3. Ignoring the 5500-EZ at $250,000. Once Solo 401(k) assets cross $250,000 you file Form 5500-EZ annually — a short form, but the penalties for skipping it are not short. Set a calendar reminder at $200,000 and forget about it until then.

Bottom line: the Solo 401(k) wins on headroom ($24,500 wider by construction, more with catch-ups), on the Roth door, and on loans — the SEP wins on simplicity, deadlines, and the moment you hire. Price your exact situation in the Solo 401(k) vs SEP IRA calculator, then open the winner before December 31.