Mega Backdoor Roth Calculator
Calculate max after-tax 401(k) contribution for mega backdoor Roth. 2026 total limit: $72,000.
By Konstantin Iakovlev · Updated September 2026 · Source: IRS Notice 2025-67 — 2026 plan limits (§415(c) $72,000)
Max After-Tax (Mega Backdoor)
$37,500.00
Total Retirement Savings
$72,000.00
Savings Rate
36.00%
415(c) Limit Breakdown
| 2026 Total 415(c) Limit | $72,000.00 |
| Employee Pre-Tax/Roth | - $24,500.00 |
| Employer Match | - $10,000.00 |
| Available for After-Tax | $37,500.00 |
| Your After-Tax Contribution | $37,500.00 |
Mega Backdoor Roth Benefit
| Annual Roth Conversion | $37,500.00 |
| Tax-Free Growth (10yr @ 7%) | $36,268.18 |
| Total Retirement Savings | $72,000.00 |
Use the Mega Backdoor Roth Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.
Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.
How It Works
The mega backdoor Roth strategy lets high savers push after-tax dollars into a 401(k) well past the usual elective limits, then move that money into Roth territory where it grows tax-free. The headroom available to you in 2026 hinges on the total contribution limit of $72,000 and how much of it your other contributions already consume.
Your maximum after-tax 401(k) contribution comes from one subtraction: take the total IRS 401(k) contribution limit for 2026 ($72,000), then remove your own pre-tax or Roth 401(k) deferrals and any employer contributions. Whatever is left is the after-tax amount you are eligible to put in.
Not every 401(k) plan permits after-tax contributions or the in-service distributions needed for Roth conversions, so the math here only works if your plan does. Check the specific rules with your HR department or plan administrator before you commit to a contribution figure.
Example: Maximizing Your Roth Conversion
- 1 Input: a $200,000 salary, the full 2026 employee deferral of $24,500 and a $10,000 employer match (the calculator's defaults), in a plan that allows after-tax contributions and in-plan Roth conversions.
- 2 The calculator subtracts both from the 2026 total limit of $72,000: $72,000 − $24,500 − $10,000 = $37,500.
- 3 Your maximum after-tax 401(k) contribution for a mega backdoor Roth is $37,500, which brings total contributions to $72,000, or 36% of salary.
- 4 Converted to Roth, the $37,500 grows tax-free. At the calculator's 7% assumption it adds $36,268 of growth in 10 years ($37,500 × (1.07^10 − 1)), all of it tax-free if the Roth distribution rules are met.
Source: IRS Notice 2025-67 — 2026 plan limits (§415(c) $72,000) · Last updated: September 2026
Frequently Asked Questions
What is the mega backdoor Roth?
Does my employer 401(k) allow the mega backdoor Roth?
How much can I contribute through the mega backdoor Roth in 2026?
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