You contribute after-tax money, it grows tax-free, and you withdraw it tax-free in retirement.
The catch is the contribution limit—just $7,000 in 2025, or $8,000 if you're 50 or older.
But there's a lesser-known move that lets high earners sock away far more.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the standard backdoor Roth that high earners use to skirt income limits.
This one is bigger, and it lives inside your workplace 401(k).
In 2025, the total amount you and your employer can put into a 401(k) is $70,000.
That includes your $23,500 employee deferral, any employer match, and—critically—after-tax contributions.
Many plans allow you to contribute after-tax dollars beyond the normal deferral cap, up to that $70,000 ceiling.
If your employer kicks in, say, $10,000, you could still have roughly $36,500 of room for after-tax money.
Then you convert those after-tax dollars into a Roth account, either inside the plan or by rolling them into a Roth IRA.
Any earnings that sneak in before the conversion are taxed, which is why speed matters.
A 40-year-old who shields an extra $30,000 a year in Roth space could retire with hundreds of thousands more in tax-free income, depending on market returns.
That's not a promise—markets swing, and your results will vary—but the math on tax-free compounding over decades is hard to ignore.
Not every 401(k) plan offers after-tax contributions, and even fewer allow in-plan Roth conversions or "spillover" elections.
You need to check your plan's summary description or call HR.
If you're self-employed, a solo 401(k) can unlock the same strategy.
If you're a high earner at a big company, your plan may or may not cooperate.
This strategy is built for people who already max out their regular 401(k) and still have money left to save.
The IRS has blessed the mechanics, but rules are technical.
Conversions trigger tax forms, and a mistake can create a taxable event you didn't plan for.
Many people hire a CPA the first year they try it.
If you're switching jobs, a mega backdoor move in your final year can get messy.
Coordinate with a tax pro before you roll anything. **Our take:** The mega backdoor Roth is one of the few legal ways to buy tax-free growth at scale, and it's wasted on nobody.
Final Thoughts
If your plan allows it and your budget supports it, the question isn't whether to use it—it's why you'd leave that space empty while the IRS keeps the door open.