Buried inside many workplace retirement plans is a feature so obscure that even HR departments sometimes forget it exists.
It's nicknamed the "mega backdoor Roth," and it lets some savers shovel tens of thousands of extra dollars into tax-free growth each year.
The catch: it only works if your specific plan allows it, and most don't.
The IRS caps your own 401(k) salary deferrals at $23,000 in 2024, or $30,500 if you're 50 or older.
But that's just the limit on what comes out of your paycheck.
The total cap on all contributions to a single plan, including employer matches and after-tax dollars, is $69,000 this year, or $76,500 with the catch-up.
That gap between the two numbers is where the strategy lives.
The mechanics are simple once you see them.
You contribute after-tax money beyond your normal pre-tax or Roth deferrals, up to the total limit.
Then you either convert that after-tax bucket to a Roth account inside the plan, or roll it into a Roth IRA.
Any earnings that piled up before the conversion are typically taxable, which is why speed matters.
The longer the money sits, the bigger the tax bill on the conversion.
Roth dollars grow tax-free and come out tax-free in retirement, with no required minimum distributions for the original owner.
If you're already maxing out your traditional 401(k) and a Roth IRA, and you still have cash to save, this is often the next best bucket.
High earners who are locked out of normal Roth IRA contributions because of income limits can use this route inside a workplace plan, since the income caps that apply to IRAs don't apply here.
First, your employer's plan has to permit after-tax contributions, and many simply don't.
Second, the plan has to allow in-service conversions or withdrawals while you're still employed, which is another common blocker.
Third, if you're converting after-tax money to a Roth IRA outside the plan, watch the pro-rata rule, which looks at all your traditional IRA balances and can make part of your conversion taxable.
Keeping the conversion inside the 401(k) sidesteps that headache.
This strategy is built for people who already have a comfortable emergency fund and no high-interest debt.
Carrying a 22% credit card balance while chasing tax-free growth is a losing trade.
And if your marginal tax rate now is high and you expect it to be lower later, front-loading Roth conversions may not be the win it looks like on paper.
Call your plan administrator and ask two questions: do you allow after-tax contributions, and do you allow in-service Roth conversions?
If the answer is yes to both, talk to a tax professional before you pull the trigger.
If the answer is no, you've lost nothing but a phone call.
Most Americans will never touch this strategy, and that's fine.
It's a tool for a narrow slice of savers with high income, maxed-out accounts, and a plan that happens to allow it.
But for that slice, it's one of the few remaining legal ways to move serious money into tax-free territory.
Final Thoughts
Check your plan documents before you assume you're out of options.