If your employer's 401(k) lets you save more than $23,000 a year, you might be leaving a sizable tax advantage on the table.
It's called the mega backdoor Roth, and it has nothing to do with the backdoor Roth IRA you may have heard about.
The strategy lets certain high earners funnel tens of thousands of extra dollars into a Roth account each year.
Instead of being capped at the standard employee contribution limit, you can potentially move five figures beyond that into tax-free growth territory.
Your plan needs to allow after-tax contributions, not just traditional pre-tax or Roth deferrals.
You contribute after-tax money above your normal limit, then convert those dollars into your Roth 401(k) or roll them into a Roth IRA.
The catch is that most 401(k) plans don't offer this.
Fidelity, Vanguard, and other big recordkeepers report that only a minority of employer plans permit after-tax contributions at all.
So step one is logging into your account or calling HR to ask a simple question: does our plan allow after-tax contributions and in-service conversions?
If the answer is yes, the math can get interesting.
For 2024, the total limit on all 401(k) contributions — you plus your employer — is $69,000, or $76,500 if you're 50 or older.
Subtract your pre-tax or Roth deferrals and any company match, and whatever room remains can potentially be filled with after-tax dollars and converted.
Say you earn $150,000 and your employer matches 4 percent.
You max out your $23,000 deferral, your company kicks in $6,000, and you've got roughly $40,000 of space left.
That's money that could grow tax-free for decades.
Roth dollars come out tax-free in retirement, and there are no required minimum distributions on Roth IRAs.
For someone who expects higher taxes later or wants to leave a tax-free inheritance, that's real value.
After-tax money needs to be converted promptly, or earnings on it become taxable at your ordinary rate.
Some plans limit how often you can convert.
And if you leave the job, you'll want to roll that Roth money carefully.
If you're in a low tax bracket now and expect lower taxes later, pre-tax saving may make more sense.
And none of this matters if you're carrying high-interest credit card debt; paying that down is usually the better return.
Still, for a household already maxing out every other retirement bucket, the mega backdoor Roth is one of the few remaining ways to shelter a serious amount of money from taxes.
The main barrier isn't the IRS — it's whether your plan's rules allow it.
Our take: this is worth a 10-minute phone call to your benefits department.
If your plan supports it, you've found a rare combination of legal, boring, and genuinely lucrative.
Final Thoughts
If it doesn't, ask your HR team to add it — plan features do change when enough employees ask.