What is a Mega Backdoor Roth? The Mega Backdoor Roth is a US tax strategy that lets high earners contribute tens of thousands of after-tax dollars per year to a Roth account by exploiting the after-tax contribution bucket in a 401(k) that allows in-plan Roth conversions or in-service rollovers. It massively accelerates tax-free growth for FIRE aspirants whose income exceeds the direct Roth IRA limits.
Worked example: 2025 total 401(k) limit is $70,000 across employee + employer + after-tax contributions. You contribute the $23,500 employee limit pretax, your employer matches $10,000, and you add $36,500 in after-tax dollars, which you convert to Roth immediately via the plan’s in-plan Roth or Roth IRA rollover. Over 10 years, that is $365,000 in Roth contributions (plus growth) that would otherwise have gone to taxable accounts.
Requirements: your 401(k) plan must (1) allow after-tax contributions beyond the $23,500 limit and (2) allow in-plan Roth conversions or in-service rollovers. Ask HR or check the Summary Plan Description. Not all plans support it, but if yours does, it is one of the highest-leverage FIRE moves available.