All contribution figures reflect the 2026 tax year per IRS Notice 2025-67; this article is educational and is not tax, legal, or investment advice.
TL;DR — Quick Verdict
- The 2026 annual additions limit for a solo 401(k) is $72,000, up from $70,000 in 2025 — rising to $80,000 with the age-50 catch-up and $83,250 for ages 60 through 63.
- Fidelity, Charles Schwab, and E*TRADE all charge $0 setup and $0 annual administration. Third-party plan document providers run $299 to $847 in year one.
- Hitting the full $72,000 as a Schedule C sole proprietor takes roughly $250,000 in net profit — the employer share is capped near 20% of net self-employment earnings, not 25%.
- The new mandatory Roth catch-up rule applies to FICA wages above $150,000. Sole proprietors and partners who take no W-2 wages are outside it entirely.
- Cross $250,000 in plan assets and Form 5500-EZ becomes mandatory. Skipping it costs $250 per day, capped at $150,000 per return.
- Choose a free brokerage plan unless you need participant loans, alternative assets, or mega backdoor Roth mechanics — those three needs justify paying a third-party administrator.
A self-employed consultant earning $250,000 in net profit can shelter $72,000 from federal income tax in 2026 — more than nine times the $7,500 IRA cap, and available without any employer sponsoring the plan. The IRS raised the annual additions limit under Section 415(c) from $70,000 to $72,000 for 2026, and the employee deferral from $23,500 to $24,500, in Notice 2025-67.
Most self-employed savers never get near those numbers, and the reason is almost never income. It is arithmetic. The employer profit-sharing formula for a Schedule C filer works out to roughly 20% of net earnings — not the 25% figure that circulates in provider marketing — and the gap between those two percentages costs high earners tens of thousands in unused contribution room every year.
This analysis covers what the 2026 limits actually permit at each income level, what the setup genuinely costs across Fidelity, Schwab, E*TRADE, and the third-party administrator market, and the two compliance triggers that turn a free plan into an expensive one. Anyone weighing this against other self-employed options should also review how a SEP-IRA compares to a SIMPLE IRA before committing.
2026 Solo 401(k) Contribution Limits: The Complete Numbers
Two separate ceilings govern every solo 401(k). The elective deferral limit caps what you contribute wearing your employee hat. The annual additions limit caps everything combined — deferrals plus employer profit sharing plus any forfeiture allocations. Catch-up contributions sit above the annual additions ceiling rather than inside it, which is why the age-60 maximum exceeds $72,000 by the full catch-up amount.
Source: Internal Revenue Service, Notice 2025-67 and Retirement Topics — 401(k) and Profit-Sharing Plan Contribution Limits (IRS.gov, released November 13, 2025).
The elective deferral limit aggregates across every 401(k) you participate in. Someone with a W-2 job contributing $15,000 to an employer plan has only $9,500 of deferral room left in a solo 401(k). The annual additions limit works differently — it applies per unrelated employer, so the $72,000 ceiling on your own business is not reduced by employer matching dollars at a separate job. That asymmetry is the single most valuable feature for anyone running a side business alongside employment, and it changes the calculus in most 401(k) contribution limit comparisons.
What Your Income Actually Buys: The Employer Contribution Math
Provider marketing says 25%. IRS Publication 560 says something more complicated, and the difference is expensive.
For an S-corporation or C-corporation owner, the employer profit-sharing contribution equals 25% of W-2 wages. Clean and simple. For a sole proprietor or single-member LLC filing Schedule C, compensation is defined as net profit minus half of self-employment tax minus the employer contribution itself. Because the contribution appears on both sides of that equation, the algebra collapses to a lower effective rate: 20% of net earnings from self-employment.
Consider a freelance software developer with $150,000 in Schedule C net profit. Self-employment tax on that profit runs approximately $21,194, so the deductible half is roughly $10,597. Net earnings from self-employment land near $139,403. The employer contribution ceiling is 20% of that figure — about $27,881. Add the $24,500 elective deferral and the total reaches $52,381. That is $19,619 short of the $72,000 ceiling despite a six-figure income.
Original calculations by Real Cost Report applying the IRS Publication 560 rate worksheet method. Self-employment tax computed at 15.3% on 92.35% of net profit, capped at the 2026 Social Security wage base of $184,500 per the Social Security Administration (verify at ssa.gov). Employer share at $250,000 shown as the residual to the $72,000 ceiling. Figures rounded to the nearest dollar and are estimates, not tax advice.
Reaching the full $72,000 as a sole proprietor requires roughly $250,000 in net profit. An S-corporation owner reaches it faster in one sense — 25% of $190,000 in W-2 wages produces $47,500 — but pays payroll tax on every wage dollar to get there. Neither structure is universally cheaper, and the answer depends on how the payroll tax savings from S-corp distributions compare to the lost contribution room.
Setup and Ongoing Costs: What Providers Actually Charge
Free means free at the major brokerages. Fidelity, Charles Schwab, and E*TRADE each charge $0 to open a solo 401(k) and $0 in annual maintenance, using pre-approved prototype plan documents the IRS has already blessed. Trading inside the account runs at standard commission-free rates for U.S. stocks and ETFs.
Third-party administrators charge real money because they sell something different: a custom non-prototype plan document that permits checkbook control, alternative assets like real estate or private notes, participant loans, and after-tax contributions with in-plan conversion — the mega backdoor Roth mechanic that prototype plans generally omit.
Pricing compiled from published provider disclosures and independent provider comparisons current to mid-2026. Fees change without notice — verify directly at fidelity.com, schwab.com, us.etrade.com, carry.com, mysolo401k.net, and nabers.com before opening an account.
Over a decade, MySolo401k costs $1,775 versus Nabers Group’s $3,631 — a $1,856 spread that matters only if the two plans deliver equivalent capability for your situation. They do not always. Nabers operates as a registered investment adviser and can give guidance on alternative holdings; MySolo401k sells documents and administrative support without advisory services.
Free Brokerage Plan vs. Third-Party Administrator: Which Is Better for a Solo Practitioner?
Three features separate the tiers, and none of them involve contribution limits — a $0 Fidelity plan permits the identical $72,000 that an $847 Nabers plan permits.
Participant loans come first. A prototype plan at Fidelity or Schwab does not offer them; E*TRADE and most TPAs do. The loan provision permits borrowing up to the lesser of $50,000 or 50% of the vested balance, repaid over five years with interest paid to yourself. For a business owner whose income arrives unevenly, that access can be worth several hundred dollars a year in avoided credit costs — though the tax consequences of a defaulted loan resemble those of an early 401(k) withdrawal penalty.
Alternative assets come second. Holding rental real estate, private lending notes, or physical precious metals inside a retirement plan requires a non-prototype document with checkbook control. No free brokerage plan permits it. If your investment thesis lives outside publicly traded securities, the TPA fee is the price of admission rather than an upsell.
Mega backdoor Roth mechanics come third. Filling the gap between your $24,500 deferral and the $72,000 ceiling with after-tax dollars, then converting them in-plan to Roth, requires explicit plan document language. Carry automates this at $299 annually. For a high earner already maxing the deferral, converting $30,000 or more per year to tax-free growth dwarfs the fee — and the mechanics parallel those covered in our analysis of Roth conversion costs and timing.
Verdict
Open a free plan at Fidelity, Schwab, or E*TRADE if you invest exclusively in publicly traded securities and do not anticipate borrowing from the plan. That describes the majority of self-employed savers, and paying $299 to $847 for capabilities you will never exercise is a pure loss. Choose E*TRADE among the free options if loan access has any value to you, since it is the only no-fee plan offering both Roth deferrals and participant loans. Pay for a third-party administrator only when one of three specific conditions applies: you intend to hold alternative assets, you need after-tax contributions with in-plan Roth conversion, or you want Form 5500-EZ preparation bundled into the annual fee. Below $250,000 in plan assets and with a securities-only portfolio, the free plan wins on every measurable dimension.
What Most People Get Wrong About Solo 401(k) Rules
Five errors account for most of the damage, and four of them are irreversible after year-end.
Mistake 1: Assuming the 25% employer rate applies to Schedule C income. A sole proprietor who calculates 25% of $150,000 net profit arrives at $37,500 and contributes it. The actual ceiling is roughly $27,881. The consequence is an excess contribution subject to a 10% excise tax under Section 4972 until corrected. Correct action: use the Publication 560 rate worksheet or your provider’s calculator, and treat 20% of net earnings as the working ceiling for planning.
Mistake 2: Missing the deferral election deadline. Corporations and partnerships must have the plan adopted and the salary deferral election on file by December 31. The consequence for an S-corp owner who opens a plan in February is losing the entire $24,500 employee deferral for the prior year — only the employer portion survives. Correct action: sign plan documents by December 31 even if you fund the account later.
Mistake 3: Believing the sole proprietor extension covers everything. SECURE 2.0 Section 317 lets a Schedule C filer with no employees adopt a plan and make first-year deferrals as late as the personal return due date — but without extensions. Filing Form 4868 does not extend that window for deferrals. The consequence is discovering in October that April 15 closed the door. Correct action: for a first-year plan, complete both adoption and deferral deposit by April 15.
Mistake 4: Ignoring the Form 5500-EZ trigger. Crossing $250,000 in combined plan assets on the last day of the plan year makes the filing mandatory, and it stays mandatory in every subsequent year even if the balance falls back. The consequence runs $250 per day to a maximum of $150,000 per unfiled return. Correct action: check the December 31 balance annually and file by July 31, or use Form 5558 for an extension to October 15.
Mistake 5: Hiring an employee without amending the plan. A solo 401(k) is only a one-participant plan while it covers you and your spouse. Once a common-law employee becomes eligible, the plan falls under ERISA with nondiscrimination testing, Form 5500-SF filing, and fidelity bonding. Correct action: consult a third-party administrator before the employee’s eligibility date, not after.
The 2026 Roth Catch-Up Rule and Who It Actually Hits
Headlines announced that high earners lose the pre-tax catch-up option starting January 1, 2026. For most solo 401(k) owners, the headline does not apply.
SECURE 2.0 Section 603 requires participants aged 50 and older with prior-year FICA wages above $150,000 from the employer sponsoring the plan to make all catch-up contributions as designated Roth contributions. Treasury and the IRS finalized the implementing regulations on September 16, 2025, and Notice 2025-67 raised the threshold from the statutory $145,000 to $150,000 for 2026, measured against Box 3 of the prior-year Form W-2.
The operative term is FICA wages. Sole proprietors and partners who receive self-employment income rather than W-2 wages fall outside the rule entirely, regardless of how much they earn — a point the final regulations address directly. A Schedule C consultant netting $400,000 may still make a fully pre-tax $8,000 catch-up contribution in 2026. An S-corporation owner paying themselves $180,000 in W-2 wages in 2025 may not.
The practical consequence for affected S-corp owners is a higher current-year tax bill in exchange for tax-free growth on those dollars. At a 32% marginal rate, routing $8,000 of catch-up to Roth costs $2,560 in additional 2026 federal tax. Whether that trade favors you depends on the same variables that drive any Roth versus traditional decision by tax bracket — and for savers already in their fifties, on how the account will be drawn down. Those approaching required distributions should model this alongside RMD calculation and tax costs, since Roth 401(k) balances rolled to a Roth IRA avoid lifetime distributions entirely.
Is a Solo 401(k) Worth It? Who Should Open One and Who Should Not
Income level matters less than income source and structure.
Open one if you have self-employment income and no common-law employees beyond a spouse. The threshold for the plan making sense is low: at $60,000 in Schedule C net profit, the combined ceiling of $35,652 still exceeds what a SEP-IRA permits at the same income, because the elective deferral stacks on top of the employer contribution rather than replacing it.
Open one if you have a W-2 job with a 401(k) plus side income. The annual additions limit applies separately to each unrelated employer, so your side business gets its own $72,000 ceiling even after your day job consumes the shared $24,500 deferral limit through employer matching.
Open one if you are 60 to 63 and self-employed. The $83,250 combined ceiling represents the highest contribution capacity available to any individual in the tax code outside a defined benefit plan, and the window closes at 64 when the catch-up reverts to $8,000. Anyone in this band who is behind should compare the numbers against standard retirement savings benchmarks by age before deciding how aggressively to fund it.
Skip it if you plan to hire within twelve months. The administrative conversion when a plan loses one-participant status is expensive and disruptive; a SEP-IRA handles employee eligibility more gracefully. Skip it also if your net profit falls below roughly $30,000 and you have no other retirement account, since a Roth IRA at $7,500 captures most of the available benefit with no plan document, no Form 5500-EZ exposure, and no year-end election deadline.
The tax value is straightforward to quantify. A sole proprietor at $150,000 net profit contributing the full $52,381 pre-tax in the 24% federal bracket defers approximately $12,571 in federal income tax for 2026, before state effects. That deferral is not forgiveness — those dollars are taxed on withdrawal — but three decades of compounding on untaxed principal is the entire argument, and it is the same argument that drives retirement savings targets by age and income.
Frequently Asked Questions
Can I contribute to both a solo 401(k) and a Roth IRA in 2026?
Yes. The $72,000 annual additions limit and the $7,500 IRA limit are independent. Participation in a solo 401(k) does affect traditional IRA deductibility — the IRS phase-out range for single filers covered by a workplace plan runs $81,000 to $91,000 in 2026, and $129,000 to $149,000 for married couples filing jointly. Roth IRA eligibility depends on modified adjusted gross income rather than plan participation.
Does my spouse count as an employee?
No. A spouse earning income from the same business can participate as a second participant without triggering ERISA coverage rules, and the plan retains one-participant status. Each spouse gets a separate $24,500 elective deferral and a separate $72,000 annual additions limit, potentially doubling household capacity to $144,000. Combined plan assets still count toward the $250,000 Form 5500-EZ threshold.
What happens to the plan if my business closes?
You terminate the plan and roll the balance into an IRA or a new employer’s 401(k). A final Form 5500-EZ is required for the termination year regardless of balance — even a $50,000 plan that never triggered the $250,000 filing threshold owes a closing return. Skipping it is the most common error among owners winding down, and the $250 per day penalty applies identically.
Can I move an existing solo 401(k) to a different provider?
Yes, through a plan restatement rather than a rollover. The receiving provider issues a new plan document that adopts the existing plan, preserving its original effective date and contribution history. Most third-party administrators handle restatements within their standard setup fee — $499 at Nabers Group, $525 at MySolo401k. Confirm before initiating, since a botched restatement can create a deemed distribution.
How We Researched This Article
Every contribution limit in this article was pulled directly from primary Internal Revenue Service publications rather than secondary summaries. The 2026 elective deferral limit of $24,500, the annual additions limit of $72,000, the catch-up amounts of $8,000 and $11,250, and the $360,000 compensation cap all originate in IRS Notice 2025-67 and news release IR-2025-111, released November 13, 2025. We cross-checked each figure against the IRS Retirement Topics page on 401(k) and profit-sharing contribution limits, last updated April 8, 2026. One conflict surfaced during verification: several secondary sources listed the 2026 annual additions limit as $70,000, carrying forward the 2025 figure. The IRS page governs, and $72,000 is correct.
Contribution capacity by income level is modeled, not measured. We applied the reduced-rate methodology from IRS Publication 560, computing self-employment tax at 15.3% on 92.35% of net profit, subtracting half of that tax to reach net earnings from self-employment, then applying the 20% effective employer rate. Social Security tax was capped at the 2026 wage base of $184,500 announced by the Social Security Administration on October 24, 2025. These are estimates and will differ from an individual return, which may include state taxes, the qualified business income deduction, health insurance deductions, or other Schedule 1 adjustments that alter the base.
The Roth catch-up analysis draws on the final regulations issued jointly by Treasury and the IRS on September 16, 2025, implementing SECURE 2.0 Section 603, together with the threshold adjustment in Notice 2025-67. Plan establishment deadlines reflect SECURE 2.0 Section 317. Guidance on employer contribution deductibility comes from IRS Publication 560, Retirement Plans for Small Business.
Provider pricing carries the weakest evidentiary standard in this article, and we treat it accordingly. Fees were compiled from published provider disclosures and independent comparison research current to mid-2026. Brokerage and third-party administrator pricing changes without public notice and often varies by promotion or plan tier, so we have not treated any fee figure as fixed. Verify pricing directly with the provider before opening an account. We did not evaluate investment performance, customer service quality, or platform usability, and no provider in this article compensated us or reviewed the content. Research last conducted July 2026. All figures were verified against named primary sources before publication.