Buried inside many workplace retirement plans sits a feature with a name that sounds like a tax loophole and behaves like one.
It's called the mega backdoor Roth, and it lets certain high earners funnel tens of thousands of extra dollars a year into tax-free growth.
The catch: your employer's plan has to allow it, and most don't.
The standard 401(k) limit for 2025 is $23,500, plus a catch-up of $7,500 if you're 50 or older.
But the true ceiling on all contributions to a single plan — employee plus employer match plus after-tax dollars — is $70,000, or $77,500 with catch-up.
If your plan permits after-tax contributions, you can stash money beyond the normal limit, then convert those dollars into a Roth account.
A Roth 401(k) or an in-plan Roth conversion, or a rollover to a Roth IRA, all can work.
Once converted, the money grows tax-free and comes out tax-free in retirement, assuming you follow the rules.
Roth accounts sidestep required minimum distributions and future tax hikes.
For someone maxing out a traditional 401(k) and still sitting on extra cash, this is one of the few remaining tax-advantaged buckets wide enough to matter.
Financial planners often call it the single most underused tool in the retirement code.
Plans must offer after-tax contributions and either in-service conversions or a Roth rollover option.
Many employers skip this because it requires extra recordkeeping and testing.
Even when offered, human resources departments rarely advertise it, so workers have to ask directly.
Converting after-tax dollars can trigger taxes on any earnings that piled up before the conversion.
Do it quickly and the earnings stay small.
Some plans let you convert automatically each pay period, which keeps the taxable piece near zero.
The $70,000 ceiling counts employer matches, so a generous match eats into your room.
Highly compensated employees may face restrictions if the plan fails nondiscrimination testing.
And if you leave the job, you'll want to move the Roth money carefully to avoid a taxable event.
For most workers, the simpler move is funding a Roth IRA, capped at $7,000, or $8,000 if you're 50-plus.
The mega backdoor is for the narrower slice already maxing every other account and hunting for more shelter.
If you think your plan might allow it, request the summary plan description from HR and search for "after-tax" and "in-plan conversion." If the terms aren't there, you have your answer.
If they are, a fee-only advisor or CPA can run the numbers on whether the conversion taxes are worth the long-term payoff.
None of this is a pitch to chase a trendy acronym.
It's a reminder that retirement rules reward people who read the fine print.
Final Thoughts
The money is sitting there either way — the only question is who gets to keep it.