It sits quietly in the plan documents, rarely mentioned in the HR onboarding packet, and it lets a small group of savers move tens of thousands of dollars a year into tax-free Roth territory.
Retirement nerds call it the mega backdoor Roth.
A normal 401(k) caps your contributions at $23,000 in 2024, or $30,500 if you're 50 or older.
But that's only the part that comes out of your paycheck.
The real ceiling, counting your employer's match and other additions, sits at $69,000 total for 2024.
The gap between what you put in and that total can, in the right plan, be converted to Roth money.
If your employer kicks in $5,000 and you max out at $23,000, you have roughly $41,000 of unused space.
Some plans let you fill it with after-tax contributions, then convert that money to a Roth account, either inside the 401(k) or rolled to a Roth IRA.
Do it and the growth is yours tax-free in retirement.
The catch is that it only works if your plan allows it.
According to surveys from the Plan Sponsor Council of America, only about a fifth of 401(k) plans offer after-tax contributions, and fewer still permit the automatic conversions that make the strategy painless.
You have to check your plan's summary description or call HR and ask a specific question: "Do you allow after-tax contributions and in-plan Roth conversions?" Why bother?
First, Roth money grows tax-free and comes out tax-free after 59½, which is a rare gift if you expect higher taxes later.
Second, high earners who are locked out of normal Roth IRAs by income limits can use this route to build a pile of tax-free cash anyway.
The conversion itself can trigger taxes on any earnings that piled up before you moved the money, so converting early and often keeps that bill small.
Some plans limit you to a couple of conversions a year, and some charge fees.
And if you leave the job, you'll need to roll the after-tax piece carefully so it doesn't get lumped in with pre-tax money.
Also know the difference between this and the regular backdoor Roth.
The regular one is for people above the Roth IRA income limits who make a nondeductible IRA contribution and convert it.
The mega version is bigger, tied to your workplace plan, and far less known.
If your plan allows it and you have spare cash after maxing out other accounts, this is one of the most powerful retirement moves available.
If your plan doesn't, you can lobby HR, and some workers switch jobs partly for better plan features.
It's worth asking before you assume you're out of luck.
Our take: the mega backdoor Roth isn't for everyone, but it's criminally underused by people who could benefit.
Spend ten minutes reading your plan documents this week.
Final Thoughts
The worst outcome is finding out it's not available; the best is finding tens of thousands of dollars of tax-free room you didn't know you had.