Mega Backdoor Roth
Routing large after-tax 401(k) contributions into a Roth — far beyond the normal IRA limit — where the plan allows conversions or in-service rollovers.
If your 401(k) plan allows after-tax contributions and in-plan Roth conversions (or in-service rollouts), you can move tens of thousands of additional dollars into Roth each year — well above the standard contribution caps. It's one of the most powerful tax-free-growth levers available to high earners, but it depends entirely on your specific plan's features. Worth checking before year-end.
The bigger version, inside your 401(k)
The mega backdoor moves far more than the IRA backdoor — potentially tens of thousands a year — into Roth via your workplace plan. You make after-tax (not Roth, not pre-tax) 401(k) contributions up to the overall plan limit, then move them to Roth through an in-plan conversion or a rollout to a Roth IRA. It only works if your specific plan allows both pieces.
Two plan features make or break it
Your 401(k) must permit after-tax contributions beyond the normal employee deferral, and it must allow either in-plan Roth conversions or in-service distributions of those after-tax dollars. Many plans offer one and not the other. Converting promptly also matters: any earnings on the after-tax money before conversion are taxable when moved.
How Formation handles it
Formation surfaces the headroom between what you've contributed and the overall plan limit, so the mega-backdoor opportunity is visible rather than left on the table — then you confirm your plan's features and execute with your advisor.
A worked example
Your employee deferral and the employer match together use part of the overall plan limit, leaving room before the total cap. If your plan allows after-tax contributions, you fill that remaining room with after-tax dollars and convert them to Roth — moving money to tax-free growth that the IRA backdoor alone could never reach.
Frequently asked
How is the mega backdoor different from the regular backdoor Roth?
The regular backdoor moves a small IRA contribution; the mega backdoor uses after-tax 401(k) contributions to move far more — but only if your employer's plan supports after-tax contributions plus in-plan conversion or in-service distributions.
Does my 401(k) support it?
Not all do. You need two features — after-tax contributions above the normal deferral, and a way to convert them to Roth. Your plan's summary description or administrator will say.
In Formation
Roth headroom tracker
Related terms
See this in your own numbers.
Formation organizes your whole household by entity and cites every figure to its source — the context that makes terms like this actionable.
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