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.
Frequently Asked Questions
What is the 2026 Mega Backdoor Roth limit?
Up to $46,500 in additional after-tax contributions per year if your employer match is zero. Most employees with match have $30,000-$40,000 of Mega Backdoor headroom. Capped by the $70,000 total 401k limit ($77,500 if 50+).
Does my 401k plan support Mega Backdoor Roth?
Only about 30-40% of 401k plans do. You need TWO features: (1) after-tax contributions allowed beyond the elective deferral, and (2) in-plan Roth conversion or in-service distribution. Ask your HR/plan administrator \u2014 if your plan lacks either feature, the Mega Backdoor isn't available.
Is the Mega Backdoor Roth legal?
Yes. IRS Notice 2014-54 explicitly authorized the after-tax to Roth conversion path. Congressional proposals have considered restricting it but none have passed. The strategy is legitimate and well-established.
How quickly should I convert after-tax to Roth?
As quickly as the plan permits \u2014 ideally same-day or weekly. Earnings on after-tax contributions BEFORE conversion are taxable. Many top plans (Microsoft, Google) auto-convert every paycheck.
Can I also do regular Backdoor Roth IRA?
Yes \u2014 they're separate strategies that stack. Regular Backdoor Roth uses Traditional IRA ($7,000/$8,000 limit 2026). Mega Backdoor uses 401k after-tax ($30K+ depending on plan). High earners can do both for $40K+ of Roth contributions per year.
What if I leave the company mid-year?
Your year-to-date Mega Backdoor contributions stay in your account \u2014 roll them to a new employer's 401k (if it accepts rollovers) or to a Roth IRA. You can't do additional Mega Backdoor at a new employer until you're enrolled in their plan and they offer it.