Buried in the fine print of many workplace 401(k) plans is a feature most employees have never heard of, and it has nothing to do with the usual $23,500 contribution limit for 2025.
It is nicknamed the "mega backdoor Roth," and for a certain slice of workers, it can move tens of thousands of extra dollars into tax-free retirement accounts every year.
One is your own pre-tax or Roth contributions, capped at $23,500 this year, or $31,000 if you are 50 or older.
A third bucket — the one almost nobody uses — is called after-tax contributions, and it can push your total combined additions up to $70,000 in 2025.
Subtract your contributions, your match, and any profit-sharing, and whatever room is left can often be filled with after-tax dollars.
Then, if your plan allows it, you convert that money into a Roth account — either inside the plan or by rolling it to a Roth IRA.
Investment gains from that point forward can grow tax-free, and qualified withdrawals in retirement generally come out tax-free too.
The catch is that most plans do not offer this.
You need three things lined up: a plan that accepts after-tax contributions, a plan that permits in-service conversions or rollovers, and enough cash flow to save well past the normal limit.
Human resources can tell you in one email whether your plan checks those boxes.
The plan document, usually available on your benefits portal, spells out the rules in writing.
Because the annual Roth IRA limit is just $7,000, or $8,000 for those 50 and up.
For a household already maxing out every other tax-advantaged account, this is one of the few remaining legal ways to shelter a large sum.
Someone who fills the full $70,000 ceiling could tuck away more than $40,000 beyond the standard limit.
If any of the converted money has earnings before you move it, you may owe income tax on that slice.
Ask about "after-tax" versus "pre-tax" balances before you click anything, and consider talking to a tax professional.
Many plans only allow conversions once or twice a year, so gains can pile up in between.
Some plans now offer automatic same-day conversions, which keeps the taxable portion close to zero.
If you roll after-tax money to a Roth IRA, keep it separate from traditional IRA money.
Mixing pre-tax and after-tax dollars can trigger the IRS pro-rata rule and create a tax bill you did not plan for.
Label the accounts clearly and check with a tax pro the first time you do this.
For most households, this strategy is not the next step.
Funding an emergency account, grabbing the full employer match, and paying down high-interest credit card debt come first — a 20% card rate is a guaranteed loss that no Roth conversion can beat.
But for dual-income professionals who already max out a 401(k) and a Roth IRA, ignoring this feature can mean leaving real money on the table year after year.
Our take: this is not a loophole, it is a feature Congress wrote into the tax code, and it is sitting unused in a lot of plans.
Spend ten minutes reading your plan document or emailing HR.
Worst case, you learn your plan does not allow it and you move on.
Final Thoughts
Best case, you find four figures of tax-free retirement space you have been walking past for years.