Most Americans know the backdoor Roth, the trick of funding a Roth IRA through a conversion when your income is too high to contribute directly.
A lesser-known cousin does the same thing on a much bigger scale.
It's nicknamed the mega backdoor Roth, and for a slice of workers, it moves tens of thousands of dollars a year into tax-free territory.
The catch is that you can only use it if your employer's 401(k) plan allows it.
You need two features: the ability to make after-tax contributions beyond the standard pretax and Roth limits, and the option to convert that money to Roth, either inside the plan or by rolling it to a Roth IRA.
For 2025, the total amount that can go into a 401(k) from all sources, including your contributions and your employer's match, is $70,000, or $77,500 if you're 50 or older.
Your own elective deferrals are capped at $23,500, plus a $7,500 catch-up if you're 50 or older.
If your employer kicks in, say, $10,000, that leaves roughly $36,500 of room you could fill with after-tax money and then convert.
The payoff is that future growth comes out tax-free in retirement.
Unlike a taxable brokerage account, you don't owe capital gains tax on decades of compounding.
Unlike a traditional 401(k), you don't owe income tax on withdrawals.
The trade-off is that you pay tax now on the conversion, so it stings most in high-income years.
First, many plans simply don't offer after-tax contributions, and some that do limit them to a small percentage of pay.
Ask your HR department for the plan's summary description and look for language about "after-tax" or "employee after-tax" contributions.
Highly compensated employees can have after-tax contributions refunded if the plan fails nondiscrimination tests, which can create an unwanted tax bill on earnings.
Third, if you convert inside the plan, some providers only allow it once a year or charge a fee per conversion.
The often-overlooked bonus is that after-tax money can be rolled to a Roth IRA, where it escapes required minimum distributions and gives you a wider menu of investments.
Be careful about the order of operations, though.
If you have pretax money in a traditional IRA, the pro-rata rule can make part of any backdoor Roth conversion taxable, so check your balances before you move anything.
If you're carrying high-interest credit card debt, don't have an emergency fund, or can't max out a normal 401(k) match, those come first.
The mega version is the last layer, not the first.
One more thing worth knowing: starting in 2026, catch-up contributions for people earning more than $145,000 must go into Roth accounts, which nudges high earners further down this path.
If your plan has the right features, the mechanics are simple enough that a payroll election and a phone call can set it up.
The mega backdoor Roth is one of the few remaining ways to shelter serious money from taxes, but it rewards people whose employers happen to offer the right plan features.
Final Thoughts
That's a gap worth closing, and until Congress does, it's a reminder that the tax code hands the biggest breaks to those who read the fine print.