Buried in the plan documents of roughly one in four large US employers is a feature that lets high earners shove up to $46,500 extra into tax-free retirement accounts this year — and a surprising number of eligible workers have no idea it exists.
It's called the mega backdoor Roth, and it has nothing to do with the backdoor Roth IRA your accountant may have mentioned.
This one runs through your workplace 401(k), and the mechanics are almost embarrassingly simple once you see them.
In 2025, the total amount that can flow into a 401(k) — you plus your employer — is $70,000, or $77,500 if you're 50 or older.
The standard employee deferral caps out at $23,500.
Most people assume the gap belongs to their company's match, then stop thinking about it.
But if your plan allows after-tax contributions, you can fill that gap yourself, dollar for dollar, up to the $70,000 ceiling.
From there, many plans let you convert that after-tax money into a Roth account — either inside the plan or by rolling it to a Roth IRA.
The earnings on those dollars then grow tax-free for decades.
Because the provision that created the after-tax loophole, a 2014 IRS notice, sits in a tax code that's about to get a lot of attention.
Lawmakers on both sides have floated retirement account caps in recent budget debates, and once a feature this generous draws a spotlight, it tends not to survive untouched.
Your plan has to permit after-tax contributions, and separately, it has to allow either in-plan Roth conversions or in-service withdrawals.
Fidelity, Vanguard, and Schwab all administer plans that offer it, but the feature is switched on employer by employer — so two coworkers at the same company get the same answer, while your neighbor at the firm down the street may have nothing.
Log into your 401(k) portal, pull the summary plan description, and search for the phrase "after-tax." If it's there, look next for "in-plan Roth conversion" or "in-service distribution." Both present means you're in business.
One mechanics note that trips people up: the $7,000 IRA contribution limit does not apply here.
This runs entirely through the workplace plan, which is why the ceiling is so much higher.
Say you're 45, earning $180,000, and your employer matches $9,000 a year.
Max out your regular deferral, add the after-tax money, convert it, and you could be moving roughly $60,000 annually into Roth territory.
Do that for fifteen years and the tax-free balance can dwarf what most households accumulate in a lifetime.
Your money is locked in the plan's investment menu, which may have limited funds and higher fees than a bare-bones IRA.
And if you convert after-tax dollars that have already earned gains, you'll owe tax on those gains — so converting early and often keeps the bill near zero.
Plan rules change, tax law shifts, and employers quietly drop features during administrative overhauls.
The mega backdoor Roth is one of the few remaining legal maneuvers that rewards ordinary planning over exotic strategies.
Final Thoughts
If your plan offers it, ignoring it is a choice — and a costly one.