Buried in the fine print of many corporate 401(k) plans is a feature that lets high earners stuff up to $46,000 extra into tax-free retirement accounts each year.
It's called the mega backdoor Roth, and the people who use it are mostly the ones already maxing out every other retirement account they have.
The IRS caps your standard 401(k) salary deferrals at $23,500 in 2025, plus a catch-up if you're 50 or older.
But the total cap on all contributions to a defined contribution plan — yours, your employer's match, and any after-tax money — sits much higher, at $70,000.
That gap between the two numbers is the loophole.
You contribute after-tax dollars, then convert them to Roth, either inside the plan or by rolling them into a Roth IRA.
The catch is that your employer has to allow it.
Plenty of plans don't offer after-tax contributions at all, and some that do restrict in-service conversions.
You can't just call Fidelity and demand the feature.
So the first move is checking your plan's summary description, usually a PDF buried in your HR portal that almost nobody reads.
The second catch is that this is not free money.
You're setting aside income you've already paid tax on, and you'll owe tax again on any earnings that accumulate before the conversion happens.
If your plan only lets you convert once a year, a year of market gains gets taxed at your ordinary income rate — which for the target audience here is often 32% or higher.
Run the numbers before assuming this is a slam dunk.
Then there's the question of who this is actually for.
If you're not already maxing a traditional 401(k) and an IRA, and you don't have an emergency fund, this is a distraction.
The mega backdoor is the last floor of a very tall building.
Financial advisors who charge a percentage of assets have a reason to love it — more assets under management, more fees — but you can usually set it up yourself with a few phone calls.
Roth conversions have survived multiple tax law overhauls, but Congress has repeatedly floated rules targeting large Roth balances.
Nothing has passed, and nobody credible is predicting a specific change.
Still, building a seven-figure Roth on the assumption the rules stay frozen forever is a bet, not a plan.
One more thing worth knowing: the paperwork is annoying.
Some plans require you to call every pay period to convert.
If your plan makes it painful, the friction may not be worth the tax benefit for smaller contributions.
For households earning $200,000 or more with a plan that supports it, the math can work out well over a 20-year horizon.
For everyone else, the standard advice — max the match, then a Roth IRA, then a taxable brokerage — still wins most of the time, and it wins without a single phone call to HR.
The mega backdoor Roth isn't a scam, but it's also not the secret the internet makes it sound like.
Final Thoughts
It's a narrow tool for a narrow group, and the loudest voices promoting it usually have something to sell you.