Most people know the standard retirement playbook: max out your 401(k), maybe fund an IRA, call it a day.
But there's a lesser-known move hiding inside many workplace plans that lets high earners shovel tens of thousands of extra dollars into tax-free growth each year.
It's nicknamed the "mega backdoor Roth," and it has nothing to do with the regular backdoor Roth IRA you've probably heard about.
This one runs through your 401(k), and the totals are far bigger.
In 2025, the total amount you and your employer can pour into a 401(k) — including employer match — is capped at $70,000, or $77,500 if you're 50 or older.
Your own elective deferrals max out at $23,500.
If your plan allows after-tax contributions, you can fill that gap with post-tax money, then convert it to Roth.
The payoff: that converted money grows tax-free forever, and qualified withdrawals in retirement come out tax-free too.
Do it for a decade and you're talking about hundreds of thousands of dollars shielded from future taxes.
You need two things in your plan documents: the ability to make after-tax contributions, and either in-plan Roth conversions or the option to roll after-tax money into a Roth IRA.
Roughly a fifth of 401(k) plans offer after-tax contributions, according to retirement industry surveys, and even fewer make the conversion part easy.
The catch that trips people up is the pro-rata rule and timing.
If your after-tax money sits in the account and earns investment gains before you convert, those gains are taxable at conversion.
Many plans now offer automatic same-day conversions to sidestep that.
If yours doesn't, you may need to convert frequently — some people do it every pay period.
If you have a traditional IRA with pre-tax dollars, rolling after-tax 401(k) money into a Roth IRA can trigger taxes on a portion of the conversion under IRS aggregation rules.
Clearing out or rolling over existing IRAs first can help.
If you're already maxing your 401(k) and still have cash left to save, this is one of the last big tax shelters standing.
If you're nowhere near the $23,500 deferral limit, skip it — fund the basics first, then look at a Roth IRA or HSA.
The move isn't right for everyone, and tax situations vary widely.
A fee-only financial planner or CPA can tell you whether your specific plan allows it and what the conversion would cost you.
Don't assume your HR department will volunteer the details — plenty of workers only discover this option after reading their plan's summary document. **The bottom line:** If you've got surplus savings and a plan that supports it, the mega backdoor Roth is one of the few remaining ways to shelter serious money from taxes.
Final Thoughts
But it lives or dies on plan rules and conversion timing, so read the fine print before you move a dollar.