The mega backdoor Roth is an advanced retirement strategy that lets you contribute after-tax dollars to a 401k and convert them to Roth — potentially adding tens of thousands in tax-free retirement savings beyond the standard IRA and pre-tax 401k limits. This calculator determines your maximum mega backdoor Roth contribution given your salary, 401k elections, and employer match.
Your 401k Situation
2025 limit: $23,500 (under 50) / $31,000 (50+)
Annual employer match in dollars
Contribution Breakdown
Roth vs Taxable Account Comparison
For educational purposes only. IRS limits and plan rules change annually. Consult a financial advisor or tax professional. This is not financial or tax advice.
How to Use the Mega Backdoor Roth Calculator
The mega backdoor Roth is one of the most powerful retirement savings strategies for high earners, allowing up to $46,500 in additional Roth contributions annually (in 2025) beyond the standard pre-tax limit. But it requires specific 401k plan features and understanding the IRS Section 415(c) limit.
Step 1: Enter Your 401k Contributions
Enter your annual pre-tax and/or designated Roth 401k contributions. For 2025, the employee contribution limit is $23,500 (under age 50) or $31,000 (age 50+, including $7,500 catch-up). Enter the full amount you plan to contribute, regardless of whether it's pre-tax or Roth 401k.
Step 2: Enter Your Employer Match
Enter the total annual employer match in dollars. This counts toward the 415(c) total limit. For example, if your employer matches 50% of contributions up to 6% of salary, and you earn $150,000, the maximum match is $4,500 (3% of salary).
Step 3: Understanding the 415(c) Limit
The IRS Section 415(c) limit is the total cap on all contributions to a defined contribution plan — combining your pre-tax contributions, employer match, and any after-tax contributions. For 2025, this is $70,000 ($77,500 if age 50+). Your mega backdoor Roth space equals: 415(c) Limit − Pre-Tax Contributions − Employer Match.
Step 4: The Roth vs Taxable Comparison
The calculator projects how much your mega backdoor Roth contributions could grow over your time horizon, compared to the same amount in a taxable brokerage account. The Roth advantage comes from eliminating long-term capital gains taxes on all future growth — a significant edge compounded over decades. The longer your time horizon, the more powerful this advantage becomes.
Frequently Asked Questions
Is this mega backdoor Roth calculator free?
Yes, completely free with no signup required. All calculations run in your browser — your financial data is never sent to any server.
Is my financial data private?
Absolutely. Everything runs entirely in your browser using client-side JavaScript. No data is transmitted or stored anywhere.
What is the mega backdoor Roth strategy?
The mega backdoor Roth allows you to contribute after-tax dollars to a 401k plan (above the standard pre-tax limit) and then convert those dollars to Roth — either through an in-service withdrawal to a Roth IRA or a Roth in-plan conversion. This can add up to tens of thousands in Roth contributions annually beyond the standard IRA limit.
What is the 415(c) limit and how does it apply?
Section 415(c) of the IRS code sets the total annual additions limit for defined contribution plans — the combined ceiling for all contributions including pre-tax, after-tax, and employer match. For 2025, this limit is $70,000 (or $77,500 for those 50+). Your mega backdoor Roth space equals the 415(c) limit minus your pre-tax contributions and employer match.
Does my 401k plan have to allow this strategy?
Yes. The mega backdoor Roth requires two plan features: (1) the ability to make after-tax (non-Roth) contributions, and (2) either in-service withdrawals or in-plan Roth conversions. Many plans, especially at larger employers, support this, but not all do. Check your Summary Plan Description (SPD) or HR department.
How does the mega backdoor Roth compare to a taxable brokerage account?
The mega backdoor Roth wins for long-term retirement savings because all growth and qualified withdrawals are tax-free. A taxable account subjects dividends, interest, and realized gains to annual taxes, significantly reducing compounding. The comparison is most powerful the longer your time horizon and the higher your expected return.
What are the tax implications of converting after-tax 401k to Roth?
Since after-tax contributions are made with money you've already paid income tax on, converting the principal to Roth is typically tax-free. However, any earnings on those after-tax contributions are pre-tax and will be taxable upon conversion. Converting promptly — ideally the same paycheck cycle — minimizes taxable earnings.