Most workers know the standard 401(k) limit: $23,000 in employee contributions for 2025, or $30,500 if you're 50 or older.
But a lesser-known strategy lets high earners stash away as much as $69,000 in a single year—and it's gaining traction among people who suddenly find themselves with more income than tax-advantaged space to put it in.
It's called the mega backdoor Roth, and it has nothing to do with the $7,000 IRA contribution most people are familiar with.
The mechanics work entirely through your workplace plan, which is why so many eligible savers either don't know it exists or assume they can't use it.
Your employer's 401(k) has an overall contribution cap—$69,000 for 2025, including your own deferrals and any company match.
If your plan allows after-tax contributions, you can fill the gap between the $23,000 limit and that $69,000 ceiling with after-tax dollars.
Then you convert that money into a Roth account, either inside the plan or by rolling it to a Roth IRA.
Roth money grows tax-free and comes out tax-free in retirement.
For someone already maxing out a traditional 401(k) and a backdoor Roth IRA, this is the next available bucket—and often the last one left.
Because incomes rose during the inflation years while tax brackets and contribution limits shifted too.
A household earning $200,000 that feels squeezed by everyday costs may still have the cash flow to save aggressively—if they know where to put it.
The after-tax contribution feature is optional for employers, and many mid-size companies simply don't offer it.
In-plan Roth conversions add another layer of plan-specific rules.
That means step one is reading your summary plan description or calling your HR benefits line.
If you convert after-tax money and it has already grown, that growth is taxable in the year you convert.
Many plans let you convert immediately or even automatically, which keeps the taxable piece near zero.
Doing it manually once a year is riskier than doing it every pay period.
High earners also need to watch the pro-rata rule if they hold a traditional IRA.
That rule can complicate backdoor Roth IRA conversions, though the mega version runs through the 401(k) itself, which changes the math.
For employers, the feature is becoming a recruiting tool.
Tech firms and large professional services companies often include it precisely because it matters to senior staff.
Smaller employers are slower to adopt, largely due to administrative cost and testing requirements.
Check whether your plan allows after-tax contributions.
If it does, ask whether conversions are automatic.
Then calculate the real gap between what you already contribute and the $69,000 ceiling.
That number is your opportunity—or your confirmation that the door is closed for now.
Putting an extra $10,000 a year into Roth space from age 40 to 65, at a hypothetical 6% return, changes the retirement picture meaningfully.
Results vary, and taxes depend on your situation, so a quick conversation with a tax professional before converting is worth the fee.
The mega backdoor Roth isn't for everyone, and it isn't a magic fix for stretched budgets.
Final Thoughts
But for savers who have already maxed the obvious accounts, it may be the most underused line item in their financial life—quietly available, rarely explained, and entirely legal.