Buried in the fine print of your retirement plan documents is a feature that lets a small group of workers funnel tens of thousands of extra dollars into tax-free growth every single year.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the IRA contribution most people already know about.
The IRS caps total 401(k) contributions — your money plus your employer's match — at $69,000 in 2024.
The standard employee deferral limit is only $23,000 (or $30,500 if you're 50 or older).
If your plan allows after-tax contributions, you can fill that gap, then convert the money to a Roth, where it grows tax-free and comes out tax-free in retirement.
The catch is that your plan has to permit it.
According to retirement researchers, only a minority of 401(k) plans offer after-tax contributions, and fewer still allow the in-plan conversions that make the strategy clean.
Employees at large tech firms, hospitals, and some financial companies are the most likely to have access.
Even when it's available, the mechanics trip people up.
You contribute after-tax dollars, then either roll them into a Roth IRA or convert them inside the plan.
Any earnings that pile up before the conversion are taxable, so speed matters.
Some plans let you convert automatically after every paycheck, which keeps the tax bill near zero.
Someone with a high salary who maxes out the strategy could move roughly $46,000 beyond the normal limit in 2024, assuming no employer match eats into the space.
Do that for a decade and the Roth balance can dwarf a regular 401(k) — with no required minimum distributions and no tax on qualified withdrawals.
The strategy mostly benefits higher earners who can afford to save that much.
Lower-income workers rarely have the cash flow, and plans at small employers often lack the feature entirely.
It also doesn't make sense if you're carrying high-interest debt or don't have an emergency fund.
If you want to check, pull your plan's summary description or call your HR benefits line and ask two questions: Does the plan allow after-tax contributions, and does it allow in-plan Roth conversions?
Our take: this is one of the few legitimate tax breaks left that rewards people who read their paperwork.
It won't help everyone, and it's not worth chasing if it stretches your budget thin.
Final Thoughts
But if you have the income and the plan supports it, ignoring it is leaving free tax-free growth on the table.