If you've maxed out your 401(k) and still have money sitting in a savings account earning pennies, there's a legal loophole worth knowing about.
It's called the mega backdoor Roth, and it lets some Americans stash tens of thousands more into tax-free retirement accounts each year.
Your employer's plan has to allow it, and most don't.
In 2025, the total amount you can put into a 401(k) from all sources — your paycheck plus any employer match — is $70,000, or $77,500 if you're 50 or older.
The regular employee contribution cap is only $23,500.
If your plan permits after-tax contributions and in-service withdrawals, you can fill that gap and then convert the money to a Roth, where it grows tax-free.
A regular Roth IRA caps you at $7,000 a year, and high earners are locked out entirely.
The mega backdoor route sidesteps that income limit because you're going through your workplace plan, not an IRA.
For someone in their 40s with a decent salary, this could mean hundreds of thousands of extra tax-free dollars by retirement.
Industry surveys suggest only around a fifth of 401(k) plans offer both features you need: after-tax contributions and the ability to move that money out while still employed.
Big tech companies and a handful of large employers tend to offer it.
Small businesses and many mid-size firms don't.
Your first move is simple — log into your plan portal or call HR and ask two questions: "Do you allow after-tax contributions?" and "Can I do an in-service withdrawal or conversion?" If the answer is yes, watch the details.
Some plans let you convert automatically with each paycheck, which keeps the taxable growth near zero.
Others require you to call or click every quarter.
There can also be fees per conversion, and a few plans limit how many times a year you can do it.
If you convert after the money has grown, you'll owe income tax on that gain.
One more caution: the word "mega" makes it sound like free money.
You still need the cash flow to fund it, and that money is locked up until retirement age unless you pay penalties.
It also only makes sense after you've grabbed any employer match and maxed out tax-advantaged accounts you already have.
Treat it as a step for people who are already saving aggressively.
A Roth balance has no required minimum distributions, so it can sit and compound for decades.
Withdrawals in retirement are tax-free, which gives you flexibility when tax rates might be higher later.
For a household already maxing everything else, it's one of the few remaining places to shelter serious money.
My take: this isn't a trick for everyone, and that's fine.
Ask your HR department the two questions above — it takes five minutes and costs nothing.
If your plan says yes, you've found one of the better deals in the tax code.
Final Thoughts
If it says no, you've lost nothing but a phone call.