Most Americans know the standard 401(k) limit: $23,500 in 2025, plus a $7,500 catch-up if you're 50 or older.
But a lesser-known maneuver inside many workplace plans lets savers pour in more than $40,000 on top of that — legally, and without paying taxes on the growth.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the regular backdoor Roth IRA that high earners use.
This one runs entirely through your employer's 401(k) plan, and it hinges on a number most people never notice on their benefits paperwork.
The IRS caps total contributions to a 401(k) — you, your employer match, and any after-tax dollars — at $70,000 for 2025, or $77,500 if you're 50 or over.
Your own pre-tax deferral is only a slice of that.
If your plan allows after-tax contributions, you can fill the gap between your deferral plus match and that $70,000 ceiling.
Say you earn $150,000, contribute $23,500 pre-tax, and your employer kicks in a $7,500 match.
That leaves roughly $39,000 of room for after-tax money.
The catch: after-tax contributions grow tax-deferred, but the earnings get taxed when you withdraw.
If your plan permits it, you convert those after-tax dollars to a Roth account — either inside the 401(k) if your plan offers a Roth option, or by rolling them into a Roth IRA.
The conversion itself is typically tax-free because you already paid tax on the contributions.
From then on, the money grows and comes out tax-free in retirement.
Fidelity, Schwab, and Vanguard-administered plans often do, but smaller employers may not.
You'll need to check your summary plan description or call your HR benefits line and ask two questions: Do you allow after-tax contributions, and do you allow in-plan Roth conversions or in-service withdrawals?
If you let after-tax money sit for years before converting, the earnings pile up and become taxable at conversion.
Many people convert immediately or every pay period to keep that tax bill near zero.
The strategy is most useful for high earners who've maxed out every other tax-advantaged account and still have cash to invest.
If you're still building an emergency fund or carrying credit card debt at 22% APR, this isn't your move — paying down that debt is a guaranteed return no Roth can match.
One more wrinkle: the IRS applies a pro-rata rule if you hold a traditional IRA with pre-tax money and try to convert after-tax 401(k) dollars to a Roth IRA.
Rolling the after-tax money into a Roth 401(k) inside your plan sidesteps that headache entirely, which is why in-plan conversions are often the cleaner path.
The mega backdoor Roth isn't advertised, and your plan may not offer it.
But for disciplined savers with the right employer, it's one of the few remaining ways to shelter a serious amount of money from future taxes.
Final Thoughts
Ask the question — the worst answer is a simple no.