If you max out a 401(k) every year and still have money left to invest, there's a maneuver that can push tens of thousands more into tax-free growth.
It's nicknamed the mega backdoor Roth, and it has nothing to do with the backdoor Roth IRA you've probably heard about.
The catch: your employer's plan has to allow it, and most don't.
According to retirement plan data tracked by Vanguard, only about a quarter of 401(k) plans offer the features this strategy requires.
So before you get excited, you need to find out which side of that line your plan sits on.
The IRS caps total 401(k) contributions in 2025 at $70,000 for workers under 50, including your own deferrals and any employer match.
If you contribute the standard $23,500 elective limit and your employer kicks in, say, $6,000, that leaves roughly $40,500 of unused room.
The mega backdoor Roth lets you fill that gap with after-tax dollars, then convert the money to Roth so future growth comes out tax-free.
Two plan features make it possible: after-tax contributions (different from Roth 401(k) contributions) and either in-plan Roth conversions or the ability to roll after-tax money out to a Roth IRA.
Miss either one and the strategy is off the table.
Because that leftover space is enormous compared to a regular IRA, which caps at $7,000 in 2025.
Someone with a generous plan could move four or five times that amount into Roth treatment annually.
Over a decade, that's a meaningful pile of tax-free income in retirement.
The tax bill is the part people underestimate.
When you convert after-tax dollars, you generally owe tax only on the earnings that piled up before the conversion, not the contributions themselves.
If you convert quickly, those earnings stay small.
If you wait years, you could get stuck owing tax on a big gain, which defeats some of the appeal.
There's also the question of whether you should do this at all.
Locking money into a Roth means giving up the upfront deduction you'd get from a traditional contribution.
If you're in a high tax bracket now and expect to be in a lower one later, the math may not favor Roth treatment.
This strategy is mostly available to higher earners at larger employers with well-designed plans, which is exactly the group that least needs the help.
Workers at small businesses and many mid-size companies simply can't access it.
If you want to check your own plan, log into your 401(k) account and look for "after-tax contributions" in the deferral options.
If you don't see it, call your plan administrator and ask directly.
Plan documents spell out whether conversions are allowed and whether there's a limit on how often you can do them.
One warning: the rules here are fiddly, and mistakes can trigger unexpected taxes or penalty headaches.
A fee-only financial planner or a tax professional who knows your plan is worth the cost if you're moving serious money.
My take: this is a legitimate tool, not a gimmick, but it's designed for a narrow slice of savers.
If your plan doesn't support it, don't twist yourself into knots trying to force it.
Final Thoughts
Maxing a regular 401(k), funding an IRA, and using a taxable brokerage account will get most people where they need to go.