If you've maxed out your 401(k) and still have money sitting in a savings account earning 4%, there's a legal maneuver that lets high earners shove tens of thousands more into tax-free retirement accounts every year.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the regular backdoor Roth conversion most people know.
The name sounds sketchy, but it's built entirely into the tax code.
The catch: your employer's 401(k) plan has to allow it, and most don't.
For 2024, the total amount that can go into a 401(k) — your contributions plus your employer's match — is capped at $69,000, or $76,500 if you're 50 or older.
Your own elective deferrals max out at $23,000.
That leaves a gap of roughly $46,000 that can potentially be filled with after-tax contributions.
That after-tax money is the secret sauce.
Unlike traditional or Roth 401(k) dollars, after-tax contributions don't get tax breaks going in, but they can be converted to Roth dollars — often right inside the plan or via a rollover to a Roth IRA.
Once converted, the growth is tax-free forever, assuming you follow the withdrawal rules.
Because a Roth IRA's annual limit is just $7,000.
The mega backdoor lets savers turbocharge that by six or seven times, all through payroll.
The plan has to offer after-tax contributions, allow in-service conversions or rollovers, and your employer match typically eats into the $69,000 ceiling.
Many plans also limit how much of your paycheck can go to after-tax savings, and some cap it at a percentage that makes hitting $46,000 impossible.
You'll owe income tax on any earnings that pile up before you convert.
Convert quickly and that number stays tiny.
Financial planners say this strategy tends to make the most sense for people already maxing out every other tax-advantaged account, carrying little high-interest debt, and sitting in a high tax bracket today.
If you expect lower taxes in retirement, a traditional 401(k) may still win.
The first move is boring: call your HR department or log into your plan's website and search the summary plan description for "after-tax" and "in-service." If those words don't appear, you're out of luck until your employer changes the plan — and plenty of workers have successfully lobbied HR by pointing out competitors offer it.
For those who qualify, the paperwork usually takes one phone call or a few clicks.
The payoff compounds quietly for decades. **Our take:** The mega backdoor Roth is a legitimate, IRS-blessed tool, not a gray-area trick — but it's designed for people who've already handled the basics.
If you're still carrying credit card balances or haven't maxed a regular 401(k), close those gaps first.
Final Thoughts
This loophole rewards the already-prepared, and it's worth a five-minute check of your plan documents to see if you're leaving free tax shelter on the table.