Mega Backdoor Roth Calculator
Calculate how much extra you can contribute to Roth via the Mega Backdoor — after-tax 401k contributions + in-plan conversion.
How Mega Backdoor Roth Works
The Mega Backdoor Roth uses three IRS rules together: (1) The 2026 total 401k contribution limit of $70,000 ($77,500 if 50+) — combining employee deferral, employer match, AND after-tax contributions. (2) After-tax 401k contributions allowed at some plans, fills the gap between regular contributions + match and the $70K total cap. (3) In-plan Roth conversion (or in-service distribution to Roth IRA) — converts after-tax contributions to Roth, locking in tax-free growth forever.
Example: Employee contributes $23,500 (pre-tax or Roth elective), employer matches $8,000, leaving $38,500 of after-tax headroom. The employee contributes $38,500 after-tax, then immediately converts to Roth. Total Roth contribution: $38,500/year ON TOP of regular IRA limits — a massive boost over Backdoor Roth's $7K. Source: IRS Section 415, Notice 2014-54. Last updated: May 2026.
Plan Requirements
Mega Backdoor requires BOTH: (1) The 401k plan allows after-tax contributions beyond the regular $23,500 elective deferral. (2) The plan allows immediate in-plan Roth conversion OR in-service distribution to Roth IRA. Approximately 30-40% of large-company 401k plans support both as of 2026. Some companies (Microsoft, Google, Amazon, Meta) explicitly enable it; many smaller employers don't. Verify with your plan administrator.
Why Convert Immediately
Critical: convert after-tax dollars to Roth BEFORE they earn meaningful gains. Earnings on after-tax contributions BEFORE conversion are taxable upon conversion. If you contribute $38,500 after-tax and let it grow to $42,000 before converting, you owe ordinary income tax on the $3,500 of gains. Convert weekly or monthly via auto-conversion to minimize this.
Mega Backdoor Roth IRA vs In-Plan Conversion
Two methods: (1) In-plan Roth conversion — keeps money in 401k but in Roth account. Subject to 401k withdrawal rules. (2) In-service distribution — moves after-tax dollars to your personal Roth IRA. More flexible — earlier withdrawals, more investment options. Most knowledgeable employees prefer in-service distribution to Roth IRA when their plan allows it.