Mega Backdoor Roth 401k 2026 Conversion Calculator
A mega backdoor Roth 401k 2026 uses the after-tax bucket inside a workplace 401k to stash up to $46,500 extra in Roth dollars beyond the $23,500 elective deferral. Total IRC §415(c) annual addition limit is $70,000 (or $77,500 with age-50 catch-up, $81,250 ages 60-63).
| Employee deferral (pre-tax + Roth) | — |
| Catch-up (age 50+ / 60-63) | — |
| Employer match | — |
| Employer profit share | — |
| After-tax (non-Roth) contribution | — |
| Total annual addition | — |
| Max in-plan Roth conversion | — |
A mega backdoor Roth 401k 2026 uses the after-tax bucket inside a workplace 401k to stash up to $46,500 extra in Roth dollars beyond the $23,500 elective deferral. Total IRC §415(c) annual addition limit is $70,000 (or $77,500 with age-50 catch-up, $81,250 ages 60-63). Two plan features are required: after-tax (non-Roth) contributions allowed, and in-plan Roth conversion or in-service rollover permitted.
The Three-Step Mega Backdoor Roth Mechanic
Step 1: Max your employee deferral at $23,500 (pre-tax or Roth, your choice). Step 2: Receive your employer match and profit share per the plan's formula. Step 3: Contribute after-tax non-Roth dollars to fill the gap up to $70,000 total. Then immediately execute an in-plan Roth conversion (or roll out to a Roth IRA via in-service rollover) so the after-tax basis becomes Roth — earnings are minimal and tax bill is near-zero if converted quickly.
Why Speed Matters
After-tax contributions earn taxable returns until converted. If you wait six months and the bucket grew by $2,000, that $2,000 becomes ordinary income on the conversion. Best practice: schedule the after-tax deposit and conversion as a recurring sequence — many plans offer automatic in-plan Roth conversion that converts the after-tax money the same day. If your plan lacks that, request manual conversions monthly.
Confirming Your Plan Supports It
Ask HR or download the plan's Summary Plan Description (SPD). Look for two phrases: "after-tax contributions" (not the same as Roth contributions) and "in-plan Roth conversion" or "in-service rollover." Approximately 50% of large-employer 401k plans now offer mega backdoor capability (Plan Sponsor Council of America 2024 survey). Some plans cap after-tax contributions below the IRS maximum — push HR to remove or raise the cap.
Common Mega Backdoor Roth Mistakes
(1) Assuming "Roth 401k" and "after-tax" are the same — they are different sub-accounts with different IRS treatment. (2) Forgetting to convert promptly — earnings on after-tax money become taxable. (3) Hitting the 415(c) $70,000 cap and not knowing employer profit-share counts toward it. (4) Not coordinating with a second employer — the $70,000 limit is per-employer but the $23,500 deferral is aggregate. (5) Failing the ACP non-discrimination test — after-tax contributions can fail if highly-compensated employees over-contribute.
Last updated May 2026. Sources: IRC §415(c), IRS Notice 2024-80 (2026 inflation adjustments), IRS Notice 2014-54 (separate basis treatment).