Most people know the standard 401(k) limit.
For 2025, you can defer up to $23,500 of your salary, or $31,000 if you're 50 or older.
That number shows up on every retirement checklist and payroll portal in America.
But there's a second limit almost nobody talks about.
It's called the 415(c) limit, and this year it caps total contributions—your money plus your employer's match—at $70,000.
That gap between the two numbers is where the "mega backdoor Roth" lives.
If your employer's match is modest and you have spare cash, you may be able to contribute after-tax dollars above the $23,500 deferral cap, filling the space up to $70,000.
Then you convert those after-tax dollars into a Roth account, where growth and withdrawals can be tax-free in retirement.
Some plans allow the conversion automatically; others require a phone call or a form.
The catch is that this isn't for everyone.
It only works if your 401(k) plan permits after-tax contributions—and plenty don't.
You also need the disposable income to max out $70,000 in a single year, which is a long way from the reality of most households.
With grocery bills still running high and rent eating a third of many paychecks, setting aside that kind of money is out of reach for the majority of workers.
Still, for high earners who've been locked out of regular Roth IRAs by income limits, this is one of the few remaining doors.
A backdoor Roth IRA only lets you move $7,000 per year.
The mega version can move tens of thousands, depending on your plan and your employer's match.
After-tax contributions sitting in the plan can generate earnings before you convert, and those earnings may be taxable at conversion time.
If your plan doesn't allow in-service withdrawals, you might be stuck waiting until you leave the job.
And if you already hold a traditional IRA, the pro-rata rule can muddy the tax math on any conversion.
The first step costs nothing: call your plan administrator and ask two questions.
Does the plan allow after-tax contributions?
And does it allow in-plan Roth conversions or in-service withdrawals?
The answers determine whether this strategy is even available to you.
If the answer is yes, run the numbers with a tax professional before you commit.
The strategy rewards people with steady cash flow, low existing IRA balances, and a plan that plays along.
Everyone else should focus on the basics first—capturing the full employer match and building an emergency fund, because those moves pay off regardless of what the tax code does next. **The takeaway:** The mega backdoor Roth is a genuine tool, not a myth, but it's built for a narrow slice of savers with the right plan and the right budget.
For most Americans, the real win is simpler—grab the match, avoid credit card interest, and keep more of each paycheck.
Final Thoughts
Check your plan documents this week; the answer takes ten minutes and could shape the next thirty years.