A growing number of six-figure earners have quietly discovered that their workplace 401(k) can do something most people assume only an IRA can: shovel massive amounts of after-tax money into a Roth, where it grows and eventually comes out tax-free in retirement.
The maneuver is nicknamed the "mega backdoor Roth," and it has almost nothing to do with the regular backdoor Roth most finance-savvy savers already know.
Instead of being capped by the $7,000 IRA limit, it rides on your employer's 401(k) plan — where the total contribution ceiling for 2025 sits at $70,000 for workers under 50, or $77,500 including catch-up contributions for those 50 and older.
You max out your pre-tax or Roth 401(k) contributions, grab every dollar of employer match, and then — if your plan allows it — keep contributing after-tax dollars on top.
Those after-tax dollars can then be converted into Roth money, either inside the plan or by rolling them into a Roth IRA.
The result: tens of thousands of dollars a year moving into tax-free growth territory, far beyond what a normal IRA allows.
The catch is that your plan has to permit it.
Some allow after-tax contributions but block the conversion step.
Others cap after-tax deposits at a low percentage of pay.
Before you get excited, call your HR department or log into your 401(k) portal and look for language about "after-tax contributions" and "in-plan Roth conversions" or "Roth rollovers." If those terms don't appear, the door is likely shut.
You'll owe income tax on any earnings that pile up before you convert.
The fix most professionals recommend is converting fast — ideally the same day or within days — so the taxable growth stays close to zero.
Some plans automate this, which is the cleanest setup.
If yours doesn't, you may need to call the recordkeeper and request the conversion manually each pay period.
First, the infamous pro-rata rule can muddy a regular backdoor Roth if you hold a traditional IRA balance, but it generally doesn't apply to after-tax 401(k) money converted inside your workplace plan.
Second, if your employer offers a Roth 401(k) match, that match still lands in pre-tax dollars unless your plan says otherwise, adding another layer to track.
High earners already maxing out every other tax-advantaged account, people who expect higher tax rates later, and anyone with decades of growth ahead.
If you're in a low bracket now, the math may not favor you, and a financial planner can run your specific numbers.
For everyone else, the takeaway is simpler: check whether your plan offers this feature before year-end.
The window resets annually, and unused space doesn't roll over.
This isn't a secret trick so much as a benefit many plans offer and few employees bother to use.
Final Thoughts
If your plan qualifies, it may be one of the most valuable line items in your entire compensation package — and worth a phone call this week.