Buried inside many 401(k) plans is a feature that lets high earners stash tens of thousands more each year into tax-free growth.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the backdoor Roth IRA you may have read about.
The catch: your employer's plan has to allow it, and most don't.
In 2025, you can contribute up to $70,000 total to a 401(k) across employee deferrals and employer matches.
If you're under 50 and already maxing your own $23,500 deferral, that leaves a big gap — money your plan may let you add as after-tax contributions.
Then you convert those after-tax dollars into a Roth account, where future growth comes out tax-free in retirement.
First, after-tax 401(k) contributions are separate from the $23,500 limit, so they don't compete with your regular deferrals.
Second, a Roth conversion on after-tax money typically triggers little or no tax bill, since you already paid income tax on that money going in.
The growth before you convert is the part that can be taxable, which is why speed matters.
Some plans allow an "in-plan conversion," meaning you can flip the after-tax money to a Roth 401(k) with a few clicks.
Others let you roll it to a Roth IRA while still employed.
A few make you wait until you leave the job, which is far less useful.
Call your HR benefits line or log into your plan's website and search for "after-tax contributions" and "in-service distributions." Not everyone should bother.
If you're not already maxing your regular 401(k) and, ideally, an IRA or HSA, this is the wrong priority.
The mega backdoor is a tool for people who have already filled the standard tax-advantaged buckets and still have money left to invest.
It also makes the most sense if you expect higher taxes later or want a bigger tax-free bucket in retirement.
Some plans limit after-tax contributions to a percentage of pay, and highly paid employees can run into nondiscrimination testing that caps what they're allowed to put in.
Investment options inside the plan may also be limited or carry higher fees than you'd find in an IRA.
Run the numbers before assuming the extra complexity pays off.
If your plan doesn't offer after-tax contributions, you're in good company — a large share of employers still don't.
You can ask benefits to add it, but don't hold your breath.
In the meantime, a taxable brokerage account isn't a terrible consolation prize; you just lose the tax-free wrapper.
The mega backdoor Roth isn't a loophole so much as a feature Congress left in the code, and it rewards people who read their plan documents.
A 20-minute call to your 401(k) administrator could tell you whether tens of thousands in extra Roth space is sitting there unused.
Final Thoughts
That's a better use of an afternoon than most financial advice you'll get this year.