To master the Mega-Backdoor Roth, you must first distinguish between the three types of contributions available in a modern 401(k). Most beginners confuse "Roth 401(k) contributions" with "After-Tax 401(k) contributions." While they sound similar—both involve money that has already been taxed—they are treated very differently by the IRS and your plan administrator.

The Three-Bucket System

Imagine your 401(k) is divided into three distinct physical buckets. Each bucket has its own rules for how money goes in, how it grows, and how it is taxed when it comes out.

The Pre-Tax Bucket: This is the traditional 401(k). You put money in before taxes are taken out of your paycheck. It grows tax-deferred. When you take it out in retirement, every penny (contribution plus growth) is taxed as ordinary income .
The Roth Bucket: You put money in after taxes are taken out. It grows tax-free. When you take it out in retirement, nothing is taxed, provided you meet the 5-year rule and are over 59½ .
The After-Tax (Non-Roth) Bucket: This is the "Mega" bucket. Like the Roth bucket, you put money in after taxes. However, unlike the Roth bucket, the earnings in this bucket are generally taxable as ordinary income upon withdrawal .

The "Backdoor" part of the strategy is the process of moving money from Bucket #3 (After-Tax) into Bucket #2 (Roth) as quickly as possible. Why? Because you want the growth on that money to be tax-free, not just tax-deferred .

Understanding the Section 415(c) Limit

The IRS places two different caps on your 401(k). The first is the 402(g) limit, which is the $23,500 (for 2025) that you can choose to defer from your salary into the Pre-tax or Roth buckets . Most people stop here because they believe this is the "max."

The second cap is the 415(c) limit, which is the "Total Annual Additions" limit. For 2025, this is $70,000 . This limit includes:

Your $23,500 elective deferrals (Pre-tax or Roth)
Your employer’s matching contributions
Any employer profit-sharing contributions
Your after-tax contributions

Case Study: The "Mega" Math

Let’s look at Sarah, a 35-year-old engineer earning $200,000 a year.

Step 1: Sarah maxes out her Roth 401(k) deferral at $23,500.
Step 2: Her employer provides a 5% match, which totals $10,000.
Total so far: $33,500.
The Gap: The IRS allows a total of $70,000. Sarah’s "Mega" capacity is $70,000 - $33,500 = $36,500.

Sarah can contribute an additional $36,500 into her 401(k) using the after-tax bucket. If she then converts that $36,500 to her Roth 401(k) or a Roth IRA, she has effectively put $60,000 into a tax-free Roth environment in a single year ($23,500 deferral + $36,500 mega backdoor) .

The "Tax Trap" of Unconverted After-Tax Dollars

It is critical to understand that simply making after-tax contributions is not enough. If Sarah makes that $36,500 contribution but leaves it in the after-tax bucket for 20 years, the growth will be taxed.

Scenario A (No Conversion): Sarah’s $36,500 grows to $100,000. When she retires, she pays ordinary income tax on the $63,500 of gain.
Scenario B (Mega Backdoor): Sarah converts the $36,500 to Roth immediately. It grows to $100,000. When she retires, the entire $100,000 is tax-free .

This is why the "conversion" step is the most important part of the process. Without it, you are essentially investing in a taxable account that has the liquidity restrictions of a retirement plan—the worst of both worlds.

Comparison of Contribution Types (2025)

Feature	Pre-Tax 401(k)	Roth 401(k)	After-Tax 401(k)
Tax Break Today?	Yes	No	No
Tax-Free Growth?	No (Deferred)	Yes	No (Earnings Taxed)
Tax-Free Withdrawals?	No	Yes	Only Contributions
2025 Limit	Part of $23,500	Part of $23,500	Up to $70,000 (Total)
Income Limits?	None	None	None

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Frequently Asked Questions: After-Tax Basics

Q: Can I make after-tax contributions if I haven't maxed out my $23,500 deferral?
A: Technically, yes, but it’s usually not advisable. You should generally prioritize the $23,500 limit first because those contributions are either tax-deductible (Pre-tax) or immediately tax-free on growth (Roth). After-tax contributions should be the "overflow" once the first $23,500 is exhausted .

Q: Does my employer match count against the $23,500 limit?
A: No. The employer match only counts toward the $70,000 total limit . This is a common misconception that prevents people from saving more.

Q: What if I am over 50?
A: If you are 50 or older, you get a "catch-up" contribution ($7,500 in 2025). This catch-up is added to the $70,000 limit, bringing your total potential annual additions to $77,500 .