You contribute up to the annual limit, invest, and your money grows tax-free.
But there's a lesser-known strategy that lets high earners stash far more into Roth accounts each year — and it has nothing to do with the $7,000 limit most people recognize.
It's called the mega backdoor Roth, and it's built into a surprising number of workplace 401(k) plans.
Instead of being capped at a few thousand dollars, some savers are moving $40,000 or more annually into tax-free growth territory.
The IRS caps total 401(k) contributions — your money plus your employer's match — at $69,000 for 2024 (it rises to $70,000 in 2025).
If your employer matches generously and you're under 50, you might only need a fraction of that to hit your own pre-tax limit.
The leftover space can often be filled with after-tax dollars.
Many plans let you convert those after-tax contributions into a Roth account — either inside the 401(k) or by rolling them into a Roth IRA.
Once converted, future earnings grow tax-free, and withdrawals in retirement can come out tax-free too.
You need two features: after-tax contributions and either in-plan Roth conversions or in-service withdrawals.
Roughly a third of 401(k) plans offer one or both, according to retirement research firm Willis Towers Watson, though that share has been climbing as employers compete for talent.
If your plan qualifies, the mechanics matter.
You'll want to convert quickly after each contribution, because any investment gains before conversion are taxable.
Some plans handle this automatically; others require a phone call or a form every pay period, which gets tedious fast.
After-tax contributions and conversions often trigger a separate tax form (Form 1099-R), and getting the basis right can confuse even experienced tax preparers.
If you go this route, keep meticulous records or work with someone who has done it before.
High earners who've maxed out traditional 401(k) and IRA options but still want more tax-advantaged space.
People who expect higher tax rates later, or who want to leave tax-free money to heirs, tend to find it especially appealing.
If you're in a low tax bracket now, the math looks different.
If you're carrying high-interest credit card debt or don't have an emergency fund, locking money into a retirement account is a mistake.
And if your plan doesn't support it, there's no workaround — you can't replicate this in a regular brokerage account.
Start by pulling your plan's summary description or calling your HR benefits line.
Ask two questions: Do we allow after-tax contributions, and can I convert them to Roth?
If the answer to both is yes, you may have just found thousands of dollars in extra tax-free savings room sitting unused.
The mega backdoor Roth is one of the few remaining legal ways for higher earners to shelter serious money from future taxes.
It rewards people who read their plan documents — and quietly punishes those who assume the standard limits are the ceiling.
Final Thoughts
If your employer offers it, it's worth a hard look before the year closes.