If you've already maxed out your 401(k) and your IRA, you might think you're out of tax-advantaged options for the year.
But there's a lesser-known move called the mega backdoor Roth that can let some workers stash tens of thousands more into tax-free growth — and it stays under the radar for most Americans.
It only works if your employer's plan allows it.
You need two specific features: the ability to make after-tax contributions beyond the normal deferral limit, and the option to convert those contributions to Roth, either in-plan or by rolling them into a Roth IRA.
Not every 401(k) offers this, so the first step is a phone call to your plan administrator.
For 2025, the total amount that can go into a 401(k) from all sources — you and your employer — is capped at $70,000, or $77,500 if you're 50 or older.
Regular pre-tax deferrals top out at $23,500.
If your employer kicks in a match and you still have room under that $70,000 ceiling, the leftover space is where after-tax contributions come in.
Say you earn $150,000, contribute the full $23,500 pre-tax, and your employer adds $7,500 in matching funds.
You could potentially add up to $39,000 in after-tax money and convert it to Roth — turning a chunk of your paycheck into decades of tax-free growth.
After-tax contributions grow tax-deferred, but any earnings on them are taxable when you convert.
That's why many people convert immediately or set up automatic same-day conversions, so the money doesn't have time to generate a tax bill.
Roth dollars come out tax-free in retirement, and unlike a regular Roth IRA, there's no income limit blocking high earners from this route.
It's a legal strategy written into the tax code, not a loophole that's likely to vanish tomorrow — though rules can change, so it's worth checking current guidance.
Your money gets locked into retirement accounts, so you lose flexibility.
If you need cash for a house or emergency, it's harder to reach.
And if your plan charges fees for each conversion, doing it frequently can add up.
The other hurdle is that many employers simply don't offer after-tax contributions.
A survey from the Plan Sponsor Council of America has found only about a fifth of 401(k) plans include the feature.
If yours doesn't, you can't do this — and asking HR to add it rarely moves the needle for a single employee.
If you're self-employed, a solo 401(k) can be designed to allow this, which gives business owners more room.
For everyone else, the practical move is to log into your 401(k) account, search the summary plan description for "after-tax," and call your administrator if you're unsure.
A ten-minute call could be worth thousands over a career.
The bottom line: this isn't for everyone, and it won't replace the basics like an emergency fund or paying down high-interest debt.
But for disciplined savers who've run out of other tax shelters, it's one of the few remaining ways to move serious money into tax-free territory.
Our take: if your plan supports it and your budget can handle the cash flow, it's worth a serious look before the year closes.
Final Thoughts
Just verify the details with a tax professional first, because the rules around conversions aren't forgiving of mistakes.