In this step-by-step guide, I will walk through the steps to set up a Mega Backdoor Roth with Fidelity. Specifically, I will outline the steps to set up an Mega Backdoor Roth using an “in-plan conversion” (as opposed to the tedious and tax-ineffective “ in-service conversion”).
Additionally, if you don’t know the differences between the regular Backdoor Roth and the Mega Backdoor Roth conversions, read my blog post first!
Your employer’s 401(k) plan needs to allow after-tax contributions. Without this option, you can’t do a mega backdoor Roth. You can check our List of 401(k) Benefits by Company to see if your employer supports after-tax contributions, or verify with your employer’s HR team.
Your employer’s 401(k) plan allows for “in-plan” rollovers to a Roth 401(k) or “in-service” withdrawals to a Roth IRA.
If neither is offered, you can’t touch your after-tax contributions in your 401(k). Therefore, your earnings would grow tax-deferred, not tax-free! In that case, I’d recommend avoiding the Mega Backdoor Roth altogether. It’s actually worse to invest after-tax dollars in a traditional 401(k) than a regular brokerage account because 401(k) withdrawals are taxed as ordinary income, not (long-term) capital gains!
Prerequisite #2: While there are 2 ways to set up a Mega Backdoor Roth , I will focus only on using the “in-plan conversion” method, which is the fully-automated approach and doesn’t come with the tax headaches that the “in-service conversion” method does.
Interactive Tool: Calculate Your Max Contribution
Planning your payroll deductions? Use our free 2026 Mega Backdoor Roth Calculator to determine your exact remaining after-tax space, employer match cap, and per-paycheck contribution schedule.
Click here and login using your Fidelity.com credentials. You should land on a page that asks you to select which account you want to modify. Select your company’s 401(k) plan from the dropdown.
a) You’ll be taken to a page with a section that says, “Manage Your Contribution Amount.” Click the link called “Contribution Amount.” You’ll find yourself on a page that allows you to change your “Contribution Amount per Pay Period”:
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Note
b) Here’s where the magic happens.
The Pretax Deferral section is for your normal pre-tax traditional 401(k). The max contribution limit here is $24,000 in 2026.
The Roth Deferral section is for normal post-tax Roth 401(k) deferrals. I don’t contribute anything here because I prefer maxing out my pre-tax traditional 401(k) to lower my taxable income today.
The After Tax Deferral section is where you set up your post-tax Mega Backdoor Roth contributions!
How much you can contribute is calculated using the following formula:
In 2025: Max Mega Backdoor Roth contributions = $70,000 - (employee 401(k) contributions) - (employer match contributions)
In 2026: Max Mega Backdoor Roth contributions = $71,000 - (employee 401(k) contributions) - (employer match contributions)
For example, in 2026, if you max out your traditional 401(k) at $24,000 and your employer match contributions are $15,000, then your max Mega Backdoor Roth contributions would be $32,000. That is, $71,000 - $24,000 - $15,000 = $32,000.
This step is simple. Just call Fidelity Workplace Planning at 800-557-1900, and tell them you’d like to add “Automatic Roth In-Plan Conversion” to your account. If the person who answers happens to not know how to do that, just call them again later.
Warning
a) After you receive your next paycheck, log back into fidelity.com .
b) Click on your 401(k) account.
c) In the main area of the page, you will see an “I Want to…” section. Click on “View Balances.”
d) You should see a pie chart called “Sources.” For example:
If you see “Roth In-Plan Conversion” in the legend, you’re all set! If not, be sure to double check all previous steps. Call Fidelity if you’re still having trouble.
That’s it. You’ve successfully set up a Mega Backdoor Roth ! Now you can start watch your post-tax contributions grow tax-free!
If you’re interested in learning about another way to grow even more of your investments tax-free, read my other step-by-step guide to do a regular Backdoor Roth ! I’ve also created a guide on how to setup automatic investing , as well.
Disclaimers
Quick answers to common questions, technical edge cases, and tax rules.
Yes, as long as your employer 401(k) plan allows after-tax contributions (distinct from Roth 401(k)) and permits in-service withdrawals or in-plan Roth rollovers. While the user interface and phone numbers differ by plan custodian (like Vanguard or Empower), the core IRS rules under Section 415(c) remain identical.
Roth 401(k) contributions share the standard employee elective deferral limit ($24,000 in 2026). After-tax contributions, by contrast, sit in a separate non-elective bucket that can be funded up to the overall Section 415(c) plan limit ($71,000), after accounting for employee deferrals and employer match dollars.
If earnings accumulate in the after-tax bucket before conversion, the gains are treated as taxable ordinary income upon conversion. Calling Fidelity to activate automated daily in-plan conversion ensures after-tax contributions convert immediately upon payroll settlement, preventing taxable earnings from compounding.
Yes. The IRS Section 415(c) annual additions ceiling ($71,000 in 2026) encompasses employee elective deferrals, employer matching or profit-sharing contributions, and after-tax dollars combined. Use our Mega Backdoor Roth Calculator to calculate your exact headroom.
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