If you've maxed out your 401(k) and still have money left to save, there's a legal maneuver that most people have never heard of.
It's nicknamed the "mega backdoor Roth," and it lets you shovel tens of thousands of extra dollars into a tax-free retirement account each year.
The catch: it only works if your employer's plan allows it.
In 2025, you can contribute up to $23,500 to a 401(k) from your salary, or $31,000 if you're 50 or older.
The total cap on all contributions to a defined-contribution plan—including employer matches—is $70,000, or $77,500 for those 50 and up.
That gap between what you put in and the overall ceiling is where the magic happens.
Say your employer kicks in $10,000 in matching funds.
That leaves roughly $36,500 of unused room.
Some plans let you fill that space with after-tax contributions.
Then you convert that after-tax money into a Roth account—either inside the plan or by rolling it into a Roth IRA.
Because you already paid tax on it, the conversion generally isn't a taxable event.
Roth money grows tax-free and comes out tax-free in retirement.
No required minimum distributions, either.
For someone in a high tax bracket today who expects higher taxes later—or just wants a big pool of tax-free income—it's a rare opening.
First, your employer has to offer after-tax contributions and either in-plan Roth conversions or in-service withdrawals.
A 2023 survey from the Plan Sponsor Council of America found that only about a quarter of 401(k) plans allow after-tax contributions at all.
You'll need to call your HR department or read the plan document to find out.
Second, watch the math on your conversion.
If your after-tax contributions earn any investment gains before you convert, those earnings are taxable.
Many plans let you convert automatically with each paycheck, which keeps the taxable bit near zero.
If you have to convert manually, do it fast.
If you hold a traditional IRA with pre-tax money and you roll after-tax 401(k) dollars into a Roth IRA, the IRS's pro-rata rule can trigger an unexpected tax bill.
The cleanest path is converting inside your 401(k) plan itself, or clearing out traditional IRA balances first.
A big conversion can bump you into a higher bracket or affect other credits.
Even with the friction, the mega backdoor Roth is one of the few remaining ways to supercharge retirement savings beyond the standard limits.
If your plan supports it, the window is open every year.
My take: this is a niche tool built for people already saving aggressively, and it rewards readers who bother to read their plan documents instead of assuming the rules are fixed.
Final Thoughts
Before chasing the headline number, confirm your plan actually permits it—otherwise you're just doing math that never hits your account.