There's a retirement maneuver hiding inside many workplace 401(k) plans, and it has nothing to do with the $23,500 most people are told is their annual limit.
It's called the mega backdoor Roth, and for households with the cash flow to use it, the ceiling is dramatically higher.
The strategy works like this: after you max out your regular 401(k) contributions, some employers let you keep saving on an after-tax basis, up to a combined limit that hit $70,000 in 2025 for workers under 50.
If your plan allows it, you can then convert that after-tax money into a Roth account, where it grows tax-free and comes out tax-free in retirement.
The reason it exists traces back to a 2014 IRS ruling that clarified after-tax 401(k) dollars could be rolled into a Roth IRA.
Lawmakers later tried to kill it in 2022, but the provision was removed before the final bill passed.
Nearly a decade on, adoption has been slow and lopsided.
Only about 22% of employers offered the feature as of 2024, according to plan administrator data, and even fewer let workers move money automatically.
That matters, because the manual version has a trap: if your after-tax savings sit in the plan and earn interest before you convert, those gains are taxed as ordinary income at conversion time.
The fix is converting quickly, ideally every pay period, which many plans now support.
Some plans charge for each conversion, which can chip away at the benefit for workers only adding a few hundred dollars at a time.
And the whole play is irrelevant if you can't already afford to max out traditional contributions and cover your regular bills — most financial planners put the priority order as: emergency fund, employer match, high-interest debt, then the advanced stuff.
The math gets more appealing the higher your tax bracket climbs, since Roth growth is most valuable to people who expect to be in a similar or higher bracket later.
For someone in their 40s with 20 years of compounding ahead, the difference between $23,500 and $70,000 in annual tax-advantaged space is not a rounding error.
The catch is that none of this is a sure thing.
Employers can drop the feature, change administrators, or restrict in-service conversions — and some plans that offer after-tax contributions don't allow conversions at all.
Read your summary plan description or call your provider, and if the answer is no, that's common.
Our take: this is a legitimate tool, not a loophole for billionaires, but it's built for people who have already handled the basics.
If a $7,000 conversion fee or a surprise tax bill would strain your budget, the simpler path wins.
Final Thoughts
Do the boring stuff first, then ask HR the awkward questions.