Two Hats, Two Contributions
A solo 401(k) lets a self-employed person contribute as both employee and employer — the only retirement plan that does. As employee, you make elective deferrals up to $24,500 for 2026 ($32,500 at 50+, with a special $35,750 for ages 60–63). As employer, your business adds profit-sharing of up to 20% of net self-employment earnings (25% of W-2 wages for S-corps). Combined, everything must fit under the $72,000 total cap for 2026. No other plan for the self-employed stacks contributions this way — it is the structural reason the solo 401(k) usually wins.
Eligibility is simple: you must have self-employment income and no full-time employees other than yourself and a spouse. Freelancers, consultants, sole proprietors, single-member LLCs, and owner-only S-corps all qualify. Hire your first full-time employee and the solo 401(k) must convert to a regular 401(k) — a good problem to have.
The Employee Piece: $24,500 Off the Top
The elective deferral is the solo 401(k)'s superpower at moderate incomes. It comes straight off your compensation before the employer math even starts, and — unlike profit-sharing — it does not scale with income. Earn $60,000 and you can still defer the full $24,500; a SEP IRA at the same income allows only about $12,000. That flat $24,500 is why the solo 401(k) dominates for six-figure-and-under earners.
Deferrals can be pre-tax or Roth (designated Roth account), and you can split between them. The Roth option is unavailable in SEP IRAs entirely — another structural edge. Catch-up contributions at 50+ ($8,000 standard; $11,250 for the 60–63 "super catch-up" under SECURE 2.0) apply to the employee piece only.
The Employer Piece: 20% (or 25%)
Profit-sharing is where entity type matters. Sole proprietors (and partnerships, single-member LLCs) may contribute up to 20% of net adjusted self-employment income — the 20% figure already accounts for the self-employment tax deduction circularity, so just use 20% of net profit. S-corps may contribute up to 25% of W-2 wages paid to the owner.
The S-corp wrinkle: profit-sharing is based on your salary, not total profit. Pay yourself a $60,000 salary on $200,000 of profit and the employer piece caps at $15,000 — the remaining profit generates no contribution room. This is the hidden cost of an aggressively low S-corp salary: you save payroll tax but shrink your retirement contribution ceiling. Model both before setting salary.
The $72,000 Cap and How It Binds
Employee plus employer cannot exceed $72,000 for 2026 ($80,000 at 50+ with catch-up; $83,250 for 60–63). At high incomes the cap binds and the employer piece gets squeezed: with $300,000 of net earnings at 55, the math wants $32,500 employee + $60,000 employer = $92,500, so the employer piece is cut to $39,500 to land exactly on $72,000. The calculator applies this automatically — the employee deferral is never reduced; only profit-sharing gives way.
Solo 401(k) vs SEP IRA: The Honest Comparison
A SEP IRA allows only the employer piece — 20% of net (25% of W-2) up to the same $72,000 cap — with no employee deferral, no Roth option, and no loans. Below roughly $300,000 of income, the solo 401(k)'s extra $24,500 deferral means it allows strictly more contributions at every income level. The comparison table above shows your exact gap.
When does the SEP win? Simplicity and timing. A SEP can be opened and funded up to the tax filing deadline (including extensions) — even for the prior year, retroactively. A solo 401(k) must be established by December 31 (though contributions can come later), and elective deferrals must be elected during the plan year. The SEP also has essentially zero administration. If you need dead-simple and you are already past year-end, the SEP is the fallback.
Deadlines That Actually Matter
Establish the plan by December 31 of the tax year — this is the one hard deadline, and missing it costs you the entire year's employee deferral. Elective deferrals must be contributed by your tax filing deadline. Employer profit-sharing can be contributed up to the filing deadline including extensions. If you are reading this in December and have no plan yet: open one this week; funding can follow.
Roth vs Pre-Tax: Which Deferral?
Pre-tax deferrals cut this year's bill — valuable at high marginal rates. Roth deferrals cost tax now but grow tax-free forever, with no RMDs on the Roth portion for the owner. The classic heuristic: Roth in low-income years, pre-tax in peak years. Early-career freelancers with volatile income often benefit from Roth; established consultants in high brackets usually take the deduction. You can also hedge — split the $24,500 between both.
Loans and Other Solo 401(k) Perks
Unlike any IRA, a 401(k) — solo included — may allow loans up to $50,000 or 50% of the balance. It is not free money (you pay yourself back with interest, and unpaid loans become taxable distributions), but as emergency liquidity it beats raiding the account. Solo 401(k)s also enjoy stronger creditor protection under federal law than IRAs in most states — relevant for litigious professions.
Common Mistakes
Mistake one: no plan by December 31 — the year's deferral room evaporates. Mistake two: S-corp salary too low, starving the 25%-of-wages employer piece. Mistake three: contributing as both — maxing a solo 401(k) and a day-job 401(k) beyond the combined $24,500 employee limit; the employee cap applies across all plans. Mistake four: forgetting Form 5500-EZ once assets exceed $250,000 — a simple filing, but the penalties for skipping it are not simple.
Hiring Your Spouse: Doubling the Plan
A solo 401(k) covers the owner and a spouse who works in the business — and that second participant doubles the household's contribution room. Each spouse gets their own $24,500 employee deferral (plus catch-ups), while the shared $72,000 cap applies per person, not per plan. A couple netting $200,000 can shelter up to $144,000 between two solo 401(k) accounts — a number no IRA combination touches.
The spouse must earn legitimate compensation from the business — real work, reasonable pay, documented. Their employee deferral cannot exceed their own earnings, and profit-sharing follows the same 20%/25% rules on their share. Done properly, this is the highest-octane retirement setup available to a family business; done sloppily (a "salary" with no work behind it), it is an audit flag. Pay for real work, keep timesheets, and enjoy the doubled ceiling.
Putting It All Together
Pick your entity, enter income and age, and read the split: employee deferral first, profit-sharing second, capped at $72,000. Compare against the SEP to see your margin. Then act on the deadlines — establish by December 31, fund by filing. For the self-employed, the solo 401(k) is the highest-octane retirement vehicle in the code; the only mistake is leaving its contribution room on the table.
Five Solo 401(k) Mistakes That Cost Thousands
First, missing the establishment deadline. You must open the solo 401(k) by December 31 of the tax year — unlike a SEP IRA, which can be opened at filing time. Miss New Year's Eve and the year's employee deferral room is gone forever. Second, botching the S-corp calculation: S-corp owners can only base contributions on W-2 wages, not total profit. Paying yourself too little salary to "save on payroll tax" directly shrinks your allowable 401(k) contribution — run both numbers before setting salary.
Third, forgetting the 5500-EZ. Once plan assets exceed $250,000, you must file Form 5500-EZ annually; the penalty for not filing starts at $250 per day. Fourth, double-counting limits across jobs. The $24,500 employee deferral cap applies across ALL 401(k)s combined — if you also contribute to an employer's plan at a day job, coordinate the total. Fifth, ignoring the Roth option. Many solo 401(k) providers now offer a Roth sub-account; if you're in a low-income year (early business, grad school side gig), Roth deferrals can beat pre-tax. Review these five every December and the plan stays clean.