A retirement strategy that lets high earners stuff up to tens of thousands of extra dollars into tax-free growth each year is moving from the fringes of financial planning into everyday workplace benefit packages.
It's nicknamed the mega backdoor Roth, and a growing number of employers now allow it through their 401(k) plans.
For 2025, the total amount you can put into a 401(k) from all sources — your paycheck, your employer match, and any after-tax contributions — caps out at $70,000, or $77,500 if you're 50 or older.
The standard elective deferral most people know about is just $23,500 of that.
The gap between those two numbers is where the strategy lives.
After you've maxed out your regular pre-tax or Roth contributions and collected your full employer match, some plans let you keep contributing on an after-tax basis until you hit that overall ceiling.
Then you convert those after-tax dollars into a Roth account, either inside the plan or by rolling them into a Roth IRA.
Because the money was already taxed going in, the conversion generally carries little or no extra tax bill on the contributions themselves.
Only a slice of 401(k) plans offer the two features this requires: after-tax contributions and either in-plan Roth conversions or the ability to roll after-tax money out while you're still working.
If your plan doesn't allow it, you're out of luck for now — you can't do this through an IRA on your own.
For those whose plans do allow it, the payoff can be substantial.
Someone who contributes the full after-tax amount for a decade could shift six figures into tax-free growth, where qualified withdrawals in retirement come out owing nothing.
That's a meaningful difference from a taxable brokerage account, where dividends, gains and sales all can trigger taxes.
Even if you can't max it out, partial contributions still help.
Putting in $500 or $1,000 a month on an after-tax basis and converting it steadily can build a meaningful Roth balance over time.
Your plan might limit how often you can convert, and some charge fees for in-plan rollovers.
If your after-tax money earns investment gains before you convert, those gains are taxable at conversion time — so moving the money quickly, sometimes automatically, keeps the tax bill small.
And because this is designed for people already hitting high contribution levels, it usually makes the most sense after you're capturing any employer match and paying down high-interest debt.
The first move is simple: dig out your plan's summary description or log into your account and search for the words "after-tax contributions" and "in-plan Roth conversion." A quick call to your HR or plan administrator can confirm whether the door is open.
Our take: this is one of the few legitimate tax advantages still sitting inside ordinary workplace plans, and too many people never bother to check if they qualify.
Final Thoughts
It won't fit everyone's budget, but if you're already maxing your regular contributions, finding out costs you nothing but a phone call.