Mega Backdoor Roth Contribution Room Calculator
Example: Annual income: 150000 $ · Current pre-tax 401(k) contribution: 24500 $ · Employer match / profit-sharing contribution: 8000 $
| After-tax 401(k) contribution room (mega backdoor Roth potential) | $39,500 |
| Amount you could convert to Roth this year | $39,500 |
| Estimated annual tax advantage vs taxable investing | $608 |
| 30-year lifetime value of this extra Roth room | $106,045 |
Worked example
Maxing pre-tax at $24,500 and receiving an $8,000 employer match: total = $32,500. The 2026 §415(c) cap of $72,000 leaves $39,500 in after-tax contribution room. At a 22% marginal rate and a 7% return, that one year's $39,500 compounds to about $300,700 in a Roth over 30 years. The same money in a taxable account, where the 7% is taxed each year and so compounds at 5.46%, reaches about $194,600. The difference is roughly $106,000 in additional wealth from that single year's contribution — because Roth growth and withdrawals are both tax-free.
Frequently asked questions
How do I know if my 401(k) plan allows mega backdoor Roth?
Your plan must allow: (1) after-tax (non-Roth) contributions above the standard deferral limit, AND (2) either in-service withdrawals or in-plan Roth conversions. Review your plan's Summary Plan Description (SPD) or ask your HR/benefits department. Many large-employer plans support this; many small-employer plans do not.
Is the mega backdoor Roth the same as a regular Roth 401(k) contribution?
No. A Roth 401(k) counts toward the $24,500 employee deferral limit for 2026. The mega backdoor uses after-tax (non-Roth) contributions within the $72,000 §415(c) all-sources limit — a separate bucket above the deferral limit. Once converted to Roth (via in-plan conversion or rollover to Roth IRA), the money grows tax-free with no income limits.
What happens if I leave my employer before converting?
After separation, you can roll after-tax 401(k) contributions directly into a Roth IRA — the earnings go to a traditional IRA, contributions go to Roth, and you pay tax only on the earnings portion. This is one of the cleanest ways to access the mega backdoor Roth even if your current plan does not allow in-service conversions.