Fidelity Solo 401k: The Complete Mega Backdoor Roth Setup & Upgrade Guide
Watch: How to set up or upgrade your Fidelity Solo 401k for full Mega Backdoor Roth access and beyond
Whether you’re a solopreneur looking to open a brand-new Solo 401k for 2025, or you already have a basic Fidelity brokerage plan and want to unlock powerful features like the Mega Backdoor Roth, this complete guide covers everything you need to know. From contribution deadlines and eligibility rules to retroactive upgrades and tax credits, My Solo 401k Financial walks you through each step in detail.
Can You Still Open a Solo 401k for 2025?
One of the most frequently asked questions is whether it is too late to establish a Solo 401k for the 2025 tax year. The short answer: it depends on your business entity type and the contribution you want to make.
Sole Proprietors: You Still Have Time
If your business is taxed as a sole proprietorship (and you didn’t establish a solo 401k on or before 12/31/2025), you can still open a Solo 401k in 2026 and make all types of 2025 contributions — employee (salary deferral), employer, and voluntary after-tax contributions (used for the Mega Backdoor Roth).
Employee contributions (salary deferrals) for 2025 must be made by April 15, 2026 — even for sole proprietors who file a timely extension on their Form 1040. The plan must be opened and accounts established in time to meet this deadline. Act now as third party financial institution processing times to open accounts increases as we approach the deadline.
If you file a timely extension for your 2025 Form 1040, you have until October 15, 2026 to open the plan, establish your brokerage account, and make employer and/or voluntary after-tax (Mega Backdoor Roth) contributions for 2025.
S-Corps and Partnerships
For businesses taxed as an S-Corporation, or Partnership (and you didn’t establish a solo 401k on or before 12/31/2025), it is too late to open a new solo 401k plan in 2026 and make employee contributions for 2025. However, if you filed a timely extension:
- You may still open or upgrade a Solo 401k and make employer and/or voluntary after-tax contributions for 2025 by your extended deadline.
What Is the Mega Backdoor Roth Solo 401k Strategy?
The Mega Backdoor Roth strategy allows self-employed individuals to contribute voluntary after-tax dollars to their Solo 401k — far beyond the standard employee salary deferral limit — and then convert or transfer those funds into a Roth account for potential tax-free growth.
Unlike a traditional Roth IRA (which has strict income limits and low contribution ceilings), the Mega Backdoor Roth through a Solo 401k has no income limit and allows contributions up to the plan’s overall annual limit.
Suppose you are a sole proprietor under age 50 with at least $70,000 of self-employment compensation for 2025 (i.e. line 31 of Schedule C less one-half of self-employment tax). You max out your employee salary deferral of $23,500 and make an employer profit-sharing contribution of $10,000. The remaining room up to the $70,000 overall limit — in this case $36,500 — can be contributed as voluntary after-tax dollars and then converted to Roth. That $36,500 can grow tax-free inside your Solo 401k Roth account.
Why a Basic Fidelity Solo 401k Cannot Do This
Fidelity’s standard (prototype) Solo 401k plan — the free brokerage plan available directly through Fidelity — does not allow voluntary after-tax contributions. This means Mega Backdoor Roth contributions are simply not possible without upgrading to a plan that uses a custom, third-party plan document — such as the one offered by My Solo 401k Financial.
Retroactive 2025 Plan Upgrade Schedule
If you already have a basic Solo 401k at Fidelity (or another brokerage) that was established on or before December 31, 2025, you can still upgrade that plan to the My Solo 401k Financial plan document and retroactively make 2025 Mega Backdoor Roth contributions — as long as the upgrade is completed in time to meet your 2025 contribution deadline.
If your business is a sole proprietorship or C-Corp and you want to make all 2025 contributions — including Mega Backdoor Roth — by the standard April 15, 2026 deadline without filing an extension, you need to act now. The plan upgrade and brokerage account setup takes time. Filing a timely extension gives you until October 15, 2026.
Top Reasons to Upgrade Your Fidelity Solo 401k
Upgrading your basic Fidelity brokerage Solo 401k to the plan offered by My Solo 401k Financial unlocks a wide range of features that the standard Fidelity prototype plan simply does not provide.
SECURE ACT Tax Credits: Upgrade Pays for Itself
My Solo 401k Financial was the first Solo 401k plan provider to offer a Solo 401k plan document that makes self-employed business owners eligible to claim the SECURE Act tax credits. These credits apply not only to brand-new plans — they are also available when you upgrade an existing basic Solo 401k plan.
- $500 per year for three consecutive years = $1,500 total
- These are tax credits, not deductions — meaning a dollar-for-dollar reduction in your tax liability
- Applies to both new plans and retroactive upgrades of existing plans
- My Solo 401k Financial charges $650 initially ($525 establishment fee + first $125 annual fee), then $125/year — making the $1,500 in credits more than cover fees for the first 7+ years
How to Get Started With My Solo 401k Financial
Whether you are opening a brand-new Solo 401k or upgrading an existing basic plan at Fidelity, the process with My Solo 401k Financial is fast and fully supported:
- Submit your order at mysolo401k.net
- Receive your plan documents — typically prepared within the same business day
- Open your Fidelity investment-only non-prototype account using the plan documents (My Solo 401k Financial provides hands-on assistance throughout)
- Initiate rollovers in parallel if applicable — no need to wait for the new account to be fully open
- Make contributions — employee, employer, and/or voluntary after-tax for Mega Backdoor Roth purposes
Whether you are opening a new plan or upgrading your existing Fidelity Solo 401k, My Solo 401k Financial can help you set up the right plan structure — same-day plan documents, hands-on support, and full Mega Backdoor Roth access.
Next Steps: Get Started Today
