Most Americans max out their 401(k), pat themselves on the back, and call it a day.
But a smaller group of savers is quietly pushing nearly $70,000 into retirement accounts annually—and the tax code allows it.
The strategy has a clunky nickname: the mega backdoor Roth.
For 2024, the IRS caps total 401(k) contributions—you plus your employer combined—at $69,000, or $76,500 if you're 50 or older.
The elective deferral limit most people know, the one that gets all the headlines, is just $23,000.
That leaves a huge runway of unused space for anyone whose plan permits after-tax contributions.
You contribute after-tax dollars beyond your pre-tax or Roth limit.
Then you either convert that money to a Roth account inside the plan, if your employer allows it, or roll it into a Roth IRA.
Because the contributions were already taxed, the conversion bill is usually tiny.
The earnings grow tax-free from there, and qualified withdrawals in retirement cost you nothing.
Because the Roth IRA itself caps you at $7,000 a year.
The mega backdoor route can multiply that by five or more.
For high earners who are locked out of deducting traditional IRA contributions and phased out of direct Roth IRA contributions, it's one of the few legal doors still open.
Roughly a third of 401(k) plans offer after-tax contributions, and fewer still allow in-plan Roth conversions or automatic spillover.
If your plan doesn't support it, you're out of luck—this isn't something you can do in a taxable brokerage account.
Call your HR benefits line and ask three questions: Does the plan allow after-tax contributions?
Is there an in-service withdrawal option?
If the answer to all three is no, the door is shut.
If you let after-tax money sit and grow for years before converting, the earnings portion becomes taxable at conversion.
The cleaner move is converting immediately or setting up automatic same-day conversions.
Some plans do this; others make you log in and click the buttons yourself every pay period.
One more wrinkle: the 2024 total cap includes employer match.
If your boss kicks in $10,000, your personal ceiling drops to $59,000.
Run the math before you commit, because overcontributing triggers corrections and paperwork that nobody enjoys.
This strategy won't move the needle for the median household, and it isn't a magic trick—it's just the tax code doing exactly what it was written to do.
But if you're a six-figure earner with a supportive plan, ignoring it means leaving serious tax-free growth on the table.
The people using it aren't smarter than you.
They just read the fine print. **The takeaway:** Check your plan documents this week, not next April.
Final Thoughts
A fifteen-minute call to HR could be worth six figures over a career—and that's the kind of return no market can promise.