Background

The mega backdoor Roth is one of the most powerful tax maneuvers available to someone whose 401(k) plan allows after-tax contributions and in-service withdrawals. The basic idea is straightforward: you contribute after-tax dollars to your 401(k), then move that money into a Roth IRA, where it grows tax-free forever. No income limits. No conversion drama.

What most skip over is what happens when your after-tax bucket has been sitting long enough to accumulate investment gains. At that point you are no longer dealing with one but two pools of money: the original principal (already taxed, eligible for Roth) and the earnings on that principal (not yet taxed, must go somewhere else). That distinction drives the entire process described in this post.

An in-service withdrawal sounds simple on paper. When there are earnings mixed in with your principal, the process gets more involved — but it is absolutely doable.

I recently went through this myself with Voya as my 401(k) provider and Fidelity as my IRA home. Here is exactly how it works.

How the Two-Check System Works

When you request an in-service withdrawal from an after-tax 401(k) that has gains, the plan administrator is required to split the distribution. They will issue two separate checks:

• Check 1 — Principal: Your after-tax contributions. This goes to a Roth IRA as a direct rollover.
• Check 2 — Earnings: The gains on those contributions. This must go to a rollover IRA, because the earnings were never taxed.

One thing worth knowing before you call: the value of your account will fluctuate until the moment Voya processes the withdrawal. If the market moves against you between the time you request the distribution and when it settles, the gains portion shrinks. That difference does not disappear, it gets pulled from your existing rollover 401(k) balance, if you have one. If you do don’t, you might consider requesting a distribution for 80-90% of both principal and gains to avoid issues with your plan administrator. In my case, I had rollover 401(k) funds that would have covered any differences with market swings. It is a small but real consideration if markets are volatile on the day you call.

Step-by-Step: The Full Process

1. Confirm your plan allows in-service withdrawals

Not every 401(k) permits this. Call your plan administrator or check your Summary Plan Description before assuming you can move forward. You need two features: after-tax contributions and in-service distributions.

2. Open a rollover IRA before you call

You will need a traditional rollover IRA at your brokerage to receive the earnings check. If you already have one, great. If not, open it ahead of time. In my case, I used Fidelity. This account needs to exist and be ready to receive funds before the checks arrive.

3. Call your plan administrator and request the in-service withdrawal

Tell them you want an in-service withdrawal of your after-tax balance and that you want two checks: one for the principal (direct rollover to Roth IRA) and one for the earnings (direct rollover to a rollover IRA). Have your account numbers ready for both accounts. Also, make sure both checks are made to the financial institution receiving the funds FBO (for the benefit of) your name. In my case, both checks were sent to me and I was able to deposit them using Fidelity’s mobile app.

4. Deposit Check 1 into your Roth IRA

When the checks arrive, deposit the principal check into your Roth IRA. If you’re using Fidelity, select “rollover” and “direct rollover” as the contribution type. This is your mega backdoor Roth. That money is now in a Roth IRA and will grow tax-free.

5. Deposit Check 2 into your rollover IRA — and leave it in cash

Deposit the earnings check into your rollover IRA. Do not invest this money. Leave it sitting in cash or a money market. This is critical. The reason is explained in the next section.

6. Wait for funds to settle, then initiate the reverse rollover

Once the funds have settled in your rollover IRA (typically a few business days), call Fidelity and request a reverse rollover, meaning you want to move the funds from the rollover IRA back into your 401(k). Give them the receiving account details: Your 401(k) Administrator Plan FBO [Your Name], [Your 401(k) rollover pre-tax account number]. Fidelity can coordinate the outgoing transfer directly; however, Voya is very particular and requires a list of documents accompanying the check to process the reverse rollover. As a result, Fidelity sent the check to my home address so that I could finalize the package to then send to Voya.

Why the Rollover IRA Funds Must Go Back to the 401(k)

This step is not optional if you also do the regular backdoor Roth IRA contribution. Here is why it matters.

The IRS uses what is called the pro-rata rule when calculating taxes on a Roth conversion. If you have any money sitting in a traditional or rollover IRA at the end of the year, the IRS treats all your IRA money as a blended pool and taxes your conversion proportionally.

By sending the earnings check back into your 401(k) through a reverse rollover, you eliminate the IRA balance entirely. Your rollover IRA ends the year at zero, the pro-rata rule has nothing to grab onto, and your backdoor Roth conversion is clean. Also, consider completing the reverse rollover first before doing the regular backdoor Roth IRA contribution.

⚠ DO NOT INVEST THE ROLLOVER IRA FUNDS: If you invest the earnings that land in your rollover IRA and the value changes before the reverse rollover is complete, you create a mismatch. Keep it in cash or a money market until the transfer to your 401(k) is confirmed.

Summary at a Glance

Money	Where It Comes From	Where It Goes	Tax Treatment
After-tax principal	After-tax 401(k)	Roth IRA	Tax-free growth going forward
After-tax earnings	After-tax 401(k)	Rollover IRA → 401(k) Rollover pre-tax	Tax-deferred

Final Thoughts

The mega backdoor Roth with gains is one of those things that sounds intimidating and complicated until you have done it once. The two-check system is a pain but exist to keep the IRS happy. After-tax dollars go to Roth, pre-tax earnings go somewhere pre-tax. The reverse rollover at the end is what keeps your annual backdoor Roth contributions clean for the pro-rata rule.

If your plan allows it and you have after-tax contributions accumulating, this process is worth the two phone calls. The long-term compounding inside a Roth IRA is hard to beat.

📋 DISCLAIMER: This post reflects my personal experience and is for informational purposes only. It is not tax or financial advice. Everyone’s situation is different — consult a CPA or financial advisor before making moves with retirement accounts.