The final step in the Mega-Backdoor Roth is the conversion. This is the "backdoor" itself—the transition of funds from a taxable state (after-tax bucket) to a tax-free state (Roth bucket). The timing and execution of this step determine how much you will owe the IRS today and how much you will save in the future.

The Mechanics of the Conversion

When you move money from the after-tax bucket to a Roth account, the IRS looks at two components: the principal (the money you contributed) and the earnings (the interest or market gains that money made while sitting in the bucket) .

Tax on Principal: Since you already paid taxes on this money before it went into the 401(k), there is no tax due when you convert the principal to Roth .
Tax on Earnings: Any growth that occurred before the conversion is considered pre-tax. When you convert these earnings to Roth, they are taxed as ordinary income in the year of the conversion .

Example: The Cost of Waiting

Let’s say you contribute $10,000 to the after-tax bucket.

Scenario 1 (Immediate Conversion): You convert the $10,000 the next day. There are $0 in earnings. Tax bill: $0.
Scenario 2 (Delayed Conversion): You wait six months. The $10,000 has grown to $11,000. When you convert, the $1,000 in earnings is added to your taxable income for the year. If you are in the 32% tax bracket, you owe $320 in taxes.

This is why "Automatic In-Plan Conversions" are so valuable—they minimize the time the money spends in the after-tax bucket, effectively reducing the tax on earnings to nearly zero .

Two Paths for the Conversion

Depending on your plan, you generally have two choices for where to send your after-tax dollars.

Path A: In-Plan Roth Conversion (To Roth 401k)

This is the simplest method. You move the money from the "After-Tax" bucket to the "Roth 401(k)" bucket within the same employer plan.

Pros: Easy to manage; often can be automated; allows for 401(k) loans .
Cons: You are limited to the investment options within your employer's plan.

Path B: In-Service Distribution (To Roth IRA)

You move the money out of the 401(k) and into a personal Roth IRA at a brokerage like Fidelity or Schwab.

Pros: Unlimited investment choices; easier to access for early retirement (using the Roth IRA contribution withdrawal rules) .
Cons: More administrative legwork; cannot be used for 401(k) loans; may involve "pro-rata" complexities if not handled correctly.

The "Split" Rollover Strategy

If you have significant earnings in your after-tax bucket and want to avoid a big tax bill during the conversion, you can use a "split" rollover. The IRS allows you to send the after-tax contributions to a Roth IRA (tax-free) and the earnings to a Traditional IRA (tax-deferred) .

This keeps the earnings from being taxed today, but it does mean those earnings will eventually be taxed when you withdraw them from the Traditional IRA in retirement. Most experts suggest that if the earnings are small, it is better to just pay the tax now and get everything into the Roth IRA for permanent tax-free growth .

Step-by-Step Execution Guide

If your plan does not offer automation, follow this manual process:

Calculate Your Space: Determine your 415(c) limit ($70,000 minus your deferrals and employer match) .
Adjust Your Payroll: Log into your benefits portal and set your "After-Tax" contribution percentage.
Monitor the Bucket: Once the funds land in the after-tax bucket (usually every payday), call your plan administrator.
Request the Conversion: Ask for an "In-plan Roth conversion of my after-tax source."
Repeat: Do this as frequently as your plan allows to minimize taxable earnings.

Summary Checklist for Success

Maxed out the initial $23,500 deferral (Pre-tax or Roth).
Confirmed the plan allows "After-Tax" (non-Roth) contributions.
Confirmed the plan allows "In-Plan Roth Conversions" or "In-Service Distributions."
Set up "Automatic Conversions" if available.
Verified HCE status and checked for potential nondiscrimination refunds.
Tracked any converted earnings for tax reporting (Form 1099-R).

The Mega-Backdoor Roth is a sophisticated tool, but for those who have the right plan and the right income, it is the single most effective way to build a massive tax-free fortune within the framework of a standard workplace retirement plan . By understanding the "three-bucket" system and the power of the 415(c) limit, you can move beyond the "standard" max and unlock the true potential of your 401(k).