The mega backdoor Roth lets a self-employed person push up to $72,000 into a Roth bucket in 2026 — but only if the solo 401(k) plan document permits voluntary after-tax contributions and in-service Roth conversions. The free prototype plans from Fidelity, Schwab, and E*Trade do not.
That blocks the highest-leverage tax-advantaged move available to solo operators. The fix is a paid third-party plan administrator. The four that actually support the strategy charge anywhere from $125 a year to $499 setup plus $348 a year — roughly a 4x spread for plans that look identical on paper.
For most self-employed DIY investors, My Solo 401k Financial is the right pick: $650 the first year, $125 a year after, with assets custodied at Fidelity or Schwab. Carry wins for anyone who values polished UX and automated conversions over the ~$174 in annual savings. IRA Financial is the right call when alternative assets matter. Nabers Group (Solo401k.com) is the most expensive option for what it delivers and hard to justify for the standard mega backdoor use case.
The full comparison covers 2026 fees, where the assets actually sit, and the specific friction reported in the community for each option.
Why the Free Solo 401(k)s Are a Dead End for This Strategy
The mega backdoor Roth requires two plan provisions. The plan must allow voluntary after-tax contributions (a third money-type, separate from pre-tax and Roth deferrals). And it must allow in-service Roth conversion — either an in-plan Roth rollover or an in-service distribution to a Roth IRA — so the after-tax dollars convert before they accumulate taxable gains.
The free prototype plans from the major brokerages don’t include both. Fidelity’s standard solo 401(k) plan document doesn’t allow voluntary after-tax contributions. Neither does Schwab’s, E*Trade’s, or the Ascensus (formerly Vanguard) plan. They all permit pre-tax and Roth deferrals, but stop there.
That’s the gap a lot of self-employed people miss until they’ve already opened the wrong account. A doctor on r/whitecoatinvestor summarized the trap directly: “whether a Mega Backdoor Roth works or not usually comes down to the actual plan design, not just the IRS rules. You need a plan that allows after-tax contributions and in-plan or in-service rollovers to Roth… and a lot of Solo 401(k) providers simply don’t support that. When I was shopping providers for my Solo 401(k), I found that some ‘big name’ plans looked fine on paper but didn’t allow the features needed for a Mega Backdoor at all.”
The IRS rules themselves are generous. For 2026, the 415(c) overall contribution limit is $72,000 per unrelated employer ($80,000 with the age-50 catch-up), with the employee elective deferral capped at $24,500 ($32,500 with catch-up). Everything between the two limits is available for the after-tax bucket — for a sole proprietor with enough net self-employment income, that’s the entire mega backdoor Roth opportunity. The plan document is what decides whether the reader can use it.
The fix isn’t to wait for Fidelity or Schwab to update their prototype plans. They haven’t, and the 2026 product pages confirm they still won’t. The fix is a custom plan document from a third-party administrator. The reader’s investments can still sit at Fidelity or Schwab — the third-party administrator simply opens a “non-prototype” or “investment-only” brokerage account that runs under their own plan document. Same brokerage UI, different plan rules.
Four administrators support the full mega backdoor mechanics for 2026. The rest of the article ranks them.
The Four Providers That Actually Support the Mega Backdoor Roth
| Provider | Setup fee | Annual fee | Where assets custody | MBR automation |
|---|---|---|---|---|
| My Solo 401k Financial | $525 (bundled into $650 first-year total) | $125/year | Fidelity or Schwab | Manual (paperwork + brokerage transfer) |
| Carry | $0 | $299/year (Core) | Carry’s own brokerage stack | Automated in-app |
| IRA Financial | ~$300-$500 (varies by tier) | $299-$1,000 range | Self-directed custody (alt-asset friendly) | Manual |
| Nabers Group / Solo401k.com | $499 | $29/month ($348/year) | Self-directed via partner custodian | Manual |
All four file Form 5500-EZ on the reader’s behalf once total plan assets cross $250,000 (the IRS reporting threshold). All four include plan-document amendments as the IRS updates regulations. The differences that matter are price, custody, and how much manual work each mega backdoor cycle takes.
My Solo 401k Financial — Best for Cost-Minimizing DIY Investors
My Solo 401k Financial charges a $650 first-year fee that covers establishment plus the first 12 months of plan-document services, then $125/year ongoing. The annual fee includes Form 5500-EZ preparation when assets cross $250,000, Form 1099-R prep on distributions, and ongoing plan-document amendments as the IRS changes the rules.
The structural advantage: assets custody at Fidelity or Schwab brokerage under the My Solo 401k Financial plan document. The reader gets the brokerage experience they already know — Fidelity’s interface for trades, Schwab’s customer service if they prefer it — without paying the third-party for a separate platform.
For a DIY-minded self-employed investor whose goal is “buy VTI and let the mega backdoor compound,” this is the cheapest viable option by a wide margin. Across ten years, the cost difference vs. Carry is ~$1,740 — a real number for a strategy whose entire purpose is tax efficiency.
The tradeoff is friction. Contributions go through the brokerage account workflow: deposit cash into the brokerage settlement fund, then move it to the after-tax bucket via the administrator’s process (a form, sometimes a phone call). For Schwab custody specifically, a Bogleheads reader described the per-contribution workflow plainly: “With Schwab I have to open a brokerage account in addition to the Roth and Traditional solo 401k accounts. I have to transfer money into that account, wait until it settles, and then call Schwab to let them know what contributions I’m making and to which account before making them so they can note it right in their system.” A separate Schwab user confirmed it isn’t a setup-only quirk: “A transmittal form has to be filled out every single time and funds must come from a schwaab brokerage.”
Fidelity custody is materially cleaner. A solo 401(k) holder on r/Bogleheads noted: “your miles may vary but I’ve had to call Fidelity twice this year for different reasons, not too hard to deal with them. I probably could have opened an account online and it would have been easy enough to do, contributing was easy to set up too.” For readers picking My Solo 401k Financial, defaulting to Fidelity custody removes most of the per-contribution friction.
This is the right provider for the cost-minimizing DIY investor who’d rather save $174/year and accept one extra step per contribution. Anyone planning to use this for self-directed solo 401k consolidation of stranded old workplace 401ks also benefits — My Solo 401k Financial handles incoming rollovers as part of the annual fee.
Carry — Best for Hands-Off Operators Who Value UX
Carry charges $0 setup and $299/year for the Core tier. The mega backdoor Roth conversions are automated inside the app — once the reader sets up after-tax contributions, the conversion to Roth happens without manual triggers. The Core tier bundles the solo 401(k) with a Traditional IRA, Roth IRA, and taxable brokerage account on the same platform.
For someone whose time is worth more than the $174/year delta vs. My Solo 401k Financial, the math is straightforward. The full mega backdoor Roth contribution cycle through a third-party administrator with brokerage custody can take an hour or two of paperwork per cycle. Carry compresses that to a few in-app clicks. For a busy consultant pulling $20K-$50K/year into the after-tax bucket, paying $299 to make the strategy actually happen — rather than aspirationally exist on the to-do list — is usually correct.
The tradeoff is platform lock-in. Carry custodies assets in its own stack, not at Fidelity or Schwab. That’s fine for index-fund-only investing, but anyone who wants the broader Fidelity or Schwab brokerage capabilities (specific mutual funds not on Carry’s menu, direct Schwab customer service, an existing relationship with a brokerage rep) is paying for convenience and giving up the familiar UI.
Carry is also a newer company — founded 2021, well-funded but without the multi-decade track record of established plan administrators. For most readers that’s a non-issue (plan assets sit at the custodian, not at Carry itself), but it deserves to be named.
Pick Carry if the reader genuinely will skip the mega backdoor Roth when faced with form-filling friction. Pick something else if the reader is OK with one quarterly contribution workflow.
IRA Financial — Best for Alternative Assets
IRA Financial’s pricing varies by tier; its own 2026 fee guide places self-directed solo 401(k) specialists in a $299-$1,000/year range for ongoing fees, with setup fees layered on top. The annual fee includes Form 5500-EZ preparation, plan-document amendments, and tax-consulting support.
The reason to pay more than the My Solo 401k Financial rate is alternative-asset capability. IRA Financial’s plan structure is built for self-directed solo 401(k) investing in real estate, private credit, private equity, precious metals, and crypto held inside the 401(k) wrapper. For a self-employed investor who wants to allocate part of the after-tax bucket to a rental property or a private lending position, IRA Financial’s plan document and operational support are tuned for it. The cheaper providers technically permit the same investments — a solo 401(k) is a solo 401(k) — but the operational lift on the reader is much higher.
For a reader whose only goal is buying index funds inside the mega backdoor Roth bucket, IRA Financial is overpaying. The capabilities that justify the higher fee go unused.
Nabers Group (Solo401k.com) — Most Expensive, Hard to Justify
Nabers Group charges $499 setup plus $29/month ($348/year), for a first-year total of $848 and ongoing $348/year. The plan supports the full mega backdoor Roth mechanics — voluntary after-tax contributions and in-service Roth conversions. The company markets itself as the largest non-prototype solo 401(k) administrator by sign-ups and assets under administration.
The price is roughly 2.8x My Solo 401k Financial’s ongoing fee with no meaningfully different feature set for the standard mega backdoor use case. The marketing emphasizes alternative-asset support (real estate, crypto, private placements) — capabilities IRA Financial already provides, often at a comparable or lower total cost depending on tier.
Skip unless a specific Nabers feature — a particular custodian relationship, a specific alternative-asset structure, a customer-service preference — actually matters. For a vanilla mega backdoor Roth into a brokerage account, the price isn’t earned.
Pick by Profile (the Verdict)
| Reader profile | Provider | Reason |
|---|---|---|
| Cost-minimizing DIY investor; index funds at Fidelity | My Solo 401k Financial | $125/year ongoing; cleanest custody at Fidelity |
| Hands-off operator; values UX; willing to pay for it | Carry | $0 setup, $299/year, automated MBR conversions |
| Alternative-asset investor (real estate, private credit) | IRA Financial | Operational support for non-public assets inside the 401(k) |
| Standard mega backdoor Roth, index funds | Not Nabers Group | 2.8x the cost of My Solo 401k Financial without a matching feature gap |
The standard advice in r/Bogleheads — fill the standard solo 401(k) employee deferral first, then the employer contribution, then layer the mega backdoor on top — assumes the plan permits after-tax contributions. Reordering or modeling the long-run impact across these accounts is what retirement planning tools like Boldin and ProjectionLab are built to do, and worth running before committing $20K-$50K/year into the after-tax bucket.
For most self-employed DIY investors reading this, the answer is My Solo 401k Financial with Fidelity custody. For most others, it’s Carry.
What Actually Breaks in Practice (Community Evidence)
Three patterns recur across the Bogleheads forum, r/Bogleheads, and r/whitecoatinvestor threads on this topic — each one a reason the choice between providers actually matters.
Schwab custody adds a transmittal-form workflow tax. Multiple Bogleheads readers reported the same per-contribution friction with Schwab as a custodian for third-party solo 401(k) plans. The cleanest description: contributions land in a Schwab brokerage account first, then require a transmittal form to allocate to the right solo 401(k) sub-account. Convenient features like online transfers between accounts don’t apply — one reader put it directly: “A transmittal form has to be filled out every single time and funds must come from a schwaab brokerage.” For readers picking My Solo 401k Financial or any administrator that lets the reader choose custody, this argues for defaulting to Fidelity.
Fidelity custody contributions work cleanly once set up. The same threads consistently describe the Fidelity workflow as low-friction once the account exists. A reader summarized: “contributing was easy to set up too” with no need for ongoing phone calls. There’s an initial setup paperwork burden (mail-in forms for the third-party plan adoption), but per-contribution friction is minimal.
Migrating later is genuinely painful. A separate cautionary tale on r/Bogleheads described the Ascensus (former Vanguard) migration: “They would send a paper check via snail mail to account holder first who had to then send to the new brokerage. It was a convoluted process that Ascensus made difficult. I am still stuck in Ascensus.” Solo 401(k) plans can be amended or terminated, but the operational rollover involves paper checks and manual handoffs. The implication: pick well the first time. Paying $125-$299/year for the right provider beats paying nothing for the wrong one and rebuilding the account a year later.
The plan-document gap is the silent killer. Across multiple threads, the most common pattern is: a self-employed person opens a free brokerage prototype solo 401(k) intending to do the mega backdoor Roth, then discovers months later that voluntary after-tax contributions aren’t permitted. One r/Bogleheads reader noted picking Fidelity “because they allow everything legal. Roth (fidelity only recently got this), loans, mega backdoor, options” — a conflation that’s instructive. Fidelity custody supports those mechanics. Fidelity’s free prototype plan document doesn’t grant them. For mega backdoor purposes, those are different things. The reader who wants the strategy needs a plan document that permits it — and the only plan documents that do for solo 401(k)s are the four paid third-party administrators in this comparison.
Frequently Asked Questions
Why can’t I just use Fidelity’s free solo 401(k) for the mega backdoor Roth?
Fidelity’s prototype plan document doesn’t permit voluntary after-tax contributions — the money-type that makes the mega backdoor Roth work. The same applies to Schwab, E*Trade, and the Ascensus (former Vanguard) prototype. The workaround is a custom plan document from a third-party administrator, with assets still custodied at Fidelity or Schwab brokerage.
How much does the mega backdoor Roth actually save in taxes?
It doesn’t reduce current-year taxes — voluntary after-tax contributions go in with after-tax dollars. The savings are on the conversion side: post-conversion, that bucket grows tax-free and withdraws tax-free in retirement, beyond the standard $24,500 deferral limit. Lifetime savings depend on the gap between the reader’s current and retirement marginal rates and the holding period. For a self-employed person stuffing $20K-$50K/year into the bucket for 20+ years, the lifetime advantage over a taxable brokerage account is well into six figures. Modeling the exact number for a specific situation is what tools like Boldin or ProjectionLab are built for.
Can I switch providers later if I pick wrong?
Yes — solo 401(k) plans can be amended or fully terminated and rolled over to a new administrator. Multiple Bogleheads readers describe doing exactly this when Vanguard sold its solo 401(k) business to Ascensus. The migration involves administrative friction (forms, sometimes paper checks via snail mail), but it isn’t a tax event when handled correctly. The community lesson is consistent though: pick well the first time.
Do I have to file Form 5500-EZ?
Only once total plan assets cross $250,000. All four providers in this comparison include 5500-EZ preparation in their annual fee at that threshold. Below $250K, no filing is required. For solo 401(k) plans that terminate, a final 5500-EZ is required regardless of asset level.
Is it worth paying $125-$299/year just to do this?
A self-employed investor contributing $20,000/year to the after-tax bucket, assuming 7% real return over 20 years and a 15% long-term capital gains rate on the taxable alternative, captures a tax-advantage delta in the five-to-six-figure range over the holding period. A $125-$299 annual fee is a rounding error against that number. The fee question is only close if the reader won’t actually contribute meaningfully — in which case neither the strategy nor the provider matter.
Do I need an LLC or S-corp to open a solo 401(k)?
No. Sole proprietor status with 1099 income qualifies, including for the mega backdoor Roth. Multiple Bogleheads readers confirmed this in practice — direct check deposits to a personal checking account, then contributions to the solo 401(k) from there, are an acceptable workflow. An LLC or S-corp is a separate decision driven by liability and tax-bracket considerations, not by solo 401(k) eligibility.
What if I have both W-2 and 1099 income?
The 415(c) limit ($72,000 in 2026) applies per unrelated employer, so a reader with a W-2 day job and a separate 1099 business can theoretically use the mega backdoor Roth in both plans — once at the W-2 employer (if the W-2 401(k) supports it) and once in the solo 401(k). The $24,500 employee elective deferral limit is shared across all plans, but the after-tax bucket is per-plan. This is why the strategy is especially valuable for self-employed side businesses operated alongside a W-2 role.
Bottom Line
Most self-employed DIY investors should pick My Solo 401k Financial with Fidelity custody — $125/year ongoing, one workflow per contribution, the cheapest viable path to the full mega backdoor Roth in 2026. Anyone who genuinely values polished UX and would otherwise skip the strategy should pick Carry at $299/year. IRA Financial earns its higher fee only when alternative assets matter. Nabers Group rarely earns its 2.8x premium over My Solo 401k Financial.
Open the account before the business-tax-return deadline (April 15, 2027 with extension for sole proprietors) — the IRS permits retroactive solo 401(k) establishment. Voluntary after-tax contributions still need to land within the calendar year to count, so accounts opened in early 2027 cover the 2027 tax year, not 2026.
The mega backdoor Roth is the largest legal Roth contribution the tax code allows a solo operator. The administrative fee is the toll.
References
- r/Bogleheads — Schwab vs Fidelity for ease of Solo 401k opening/administration — https://reddit.com/r/Bogleheads/comments/1obrff3/schwab_vs_fidelity_for_ease_of_solo_401k/
- r/Bogleheads — Can someone explain to me why the advice is to max out 401K before mega backdoor Roth? — https://reddit.com/r/Bogleheads/comments/1qd14t9/can_someone_explain_to_me_why_the_advice_is_to/
- r/whitecoatinvestor — Mega backdoor Roth vs solo 401k — https://reddit.com/r/whitecoatinvestor/comments/1pvddn6/mega_backdoor_roth_vs_solo_401k/
- r/Fire — Brokerage Contributions vs. Mega Backdoor Roth 401k — https://reddit.com/r/Fire/comments/1ta7ncl/brokerage_contributions_vs_mega_backdoor_roth_401k/
- My Solo 401k Financial — Solo 401k pricing page — https://www.mysolo401k.net/solo-401k/solo-401k-pricing/
- My Solo 401k Financial — Making Year 2026 Annual Solo 401k Contributions (2026 limits) — https://www.mysolo401k.net/making-year-2026-annual-solo-401k-contributions-pretax-roth-and-voluntary-after-tax-a-k-a-mega-backdoor/
- bestsolo401k.com — Carry Solo 401k Review (2026): $299/yr, Auto Mega Backdoor — https://bestsolo401k.com/providers/carry-solo-401k-review
- IRA Financial — Solo 401(k) Provider Fees Explained — 2026 Guide — https://www.irafinancial.com/blog/solo-401k-provider-fees-2026-guide/
- IRA Financial — Top Solo 401(k) Providers of 2026 — https://www.irafinancial.com/blog/top-solo-401k-providers-of-2026/
- The College Investor — Comparing The Best Solo 401k Providers In 2026 — https://thecollegeinvestor.com/18174/comparing-the-most-popular-solo-401k-options/
- My Solo 401k Financial — Does E*Trade’s Solo 401k Support Mega Backdoor Roth? — https://www.mysolo401k.net/does-etrades-solo-401k-support-mega-backdoor-roth-complete-guide-2025/
- My Solo 401k Financial — Fidelity Solo 401k: The Complete Mega Backdoor Roth Setup & Upgrade Guide — https://www.mysolo401k.net/fidelity-solo-401k-the-complete-mega-backdoor-roth-setup-upgrade-guide/
- Fidelity Investments — What is a mega backdoor Roth? — https://www.fidelity.com/learning-center/personal-finance/mega-backdoor-roth
- Bogleheads.org wiki — Mega-backdoor Roth — https://www.bogleheads.org/wiki/Mega-backdoor_Roth
