A little-known feature tucked inside many workplace retirement plans is letting high earners move tens of thousands of dollars into tax-free accounts every year — and most people who qualify have never heard of it.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the regular backdoor Roth conversion most finance nerds already know.
This one rides on top of your 401(k), using a separate limit that can dwarf the standard $23,000 employee contribution.
For 2024, the total amount that can flow into a 401(k) from all sources — you and your employer — is capped at $69,000, or $76,500 if you're 50 or older.
If your employer match is modest, that leaves a big gap between what you contribute and the overall ceiling.
After-tax contributions fill that gap, and then you convert them to a Roth.
Say you already max out your pre-tax contributions and get a $5,000 match.
You could still sock away roughly $41,000 in after-tax money, convert it, and let it grow tax-free for decades.
Do that for a few years and the numbers get serious.
Your plan has to allow after-tax contributions and either in-plan conversions or in-service withdrawals.
Vanguard's 2024 How America Saves report found only about 22% of plans offer the feature, though adoption is climbing fast.
When you convert after-tax dollars, any earnings that piled up before the conversion are taxable.
Move the money quickly — ideally the same day or week — and that slice stays tiny.
Let it sit for a year and you could owe real money on the growth.
The IRS also applies a pro-rata rule to conversions if you hold a traditional IRA, which can muddy the math and create unexpected taxes.
And once the money is in the Roth bucket, the usual five-year rules apply before earnings come out tax-free.
For workers at big tech firms, hospitals, and law offices, this is often the single biggest legal tax break available.
For everyone else, it's a reminder that the tax code rewards people who read the fine print — and quietly punishes those who don't.
Check your plan's summary description or call your 401(k) administrator and ask two questions: Do you allow after-tax contributions, and do you allow in-plan Roth conversions?
If the answer to both is yes, talk to a tax professional before pulling the trigger.
The gap between savers who know about this and those who don't will compound for decades.
Final Thoughts
That's not a loophole problem — it's an information problem, and it's one you can fix this week with a single phone call.