What is a mega backdoor Roth?
In one paragraph
The mega backdoor Roth is a strategy that allows certain 401(k) plan participants to contribute up to roughly $46,000 in after-tax dollars beyond the standard pre-tax limit, then convert those funds to a Roth account — potentially adding tens of thousands of additional tax-free retirement dollars annually. It requires a plan that permits after-tax contributions and in-service withdrawals or in-plan Roth conversions.
What this actually means
Standard 401(k) contribution limits cap employee deferrals at $23,500 in 2025 (plus $7,500 for those 50 and older). The mega backdoor Roth exploits a higher ceiling: the total annual addition limit under IRC Section 415, which covers all sources — employee deferrals, employer match, and after-tax contributions — up to $70,000. The gap between the employee deferral limit and the Section 415 limit is the space the mega backdoor Roth uses.
The mechanics require two features in the 401(k) plan. First, the plan must allow after-tax (non-Roth) contributions beyond the standard deferral limit. Second, the plan must allow either in-service withdrawals (taking money out while still employed) or in-plan Roth conversions. Not all plans offer both; many plans offer neither, which makes this strategy unavailable to workers at those employers.
When both features are present, the process works as follows. The participant contributes after-tax dollars up to the Section 415 ceiling (minus pre-tax contributions and employer match). Those dollars are then either rolled into a Roth IRA via an in-service withdrawal or converted to a Roth 401(k) within the plan. The conversion is immediately taxable only on any earnings that accrued in the brief window between contribution and conversion — ideally a very small amount if the conversion happens promptly.
The result is a Roth account that grows and distributes tax-free in retirement, with no income limit on eligibility. High-income earners who exceed the direct Roth IRA contribution phase-out — $161,000 for single filers in 2025 — find this particularly valuable since it provides a backdoor to Roth treatment at scale.
The strategy has real risk: Congress has periodically proposed eliminating or restricting Roth conversion strategies. Participants using the mega backdoor Roth should monitor legislative changes and plan accordingly. Tax professionals and financial planners familiar with retirement accounts should be consulted before implementation, as errors in execution can create unexpected tax liabilities.