What Is a Mega Backdoor Roth?
A Mega Backdoor Roth is a strategy that lets high earners funnel far more than the standard $24,500 annual 401(k) deferral limit into Roth savings — potentially tens of thousands of dollars more per year. It works by making after-tax contributions (a third type of 401(k) contribution, separate from pretax and Roth) up to the overall IRC 415(c) limit, then converting those after-tax dollars to Roth — either through an in-plan Roth conversion or an in-service withdrawal rolled into a Roth IRA. This only works if your employer's 401(k) plan specifically allows both after-tax contributions and in-plan conversions or in-service withdrawals — not all plans do.
2026 Limits That Make This Work
| Limit | 2026 Amount |
|---|---|
| Employee elective deferral (pretax + Roth) | $24,500 |
| Catch-up contribution (age 50-59 or 64+) | +$8,000 |
| Enhanced "super" catch-up (age 60-63) | +$11,250 (instead of $8,000) |
| Overall limit — employee + employer + after-tax (IRC 415(c)) | $72,000 |
Catch-up contributions are additional and are not counted against the $72,000 overall limit — they're added on top. Your after-tax contribution room is whatever's left of the $72,000 after your elective deferral and any employer contributions are subtracted.
Worked Example
Elective deferral: $24,500 (maxed out)
Employer match: $8,000
Catch-up: $0 (under 50)
After-tax room: $72,000 − $24,500 − $8,000 = $39,500 that can potentially go in as after-tax contributions and later be converted to Roth — on top of the $24,500 already maxed through normal deferrals.
How the Conversion Step Works
Making the after-tax contribution is only half the strategy — you also need to move that money into Roth space before it accumulates much taxable growth. There are two common ways plans allow this:
- In-plan Roth conversion: Your plan converts the after-tax balance to a Roth 401(k) sub-account automatically or on request, often same-day or on a regular schedule.
- In-service withdrawal: You roll the after-tax contributions (and only the after-tax contributions, not pretax money) out to a Roth IRA while still employed.
Any investment growth on your after-tax contributions before conversion is taxable when converted (or withdrawn). The strategy works best when your plan allows frequent or automatic conversions, minimizing the time your after-tax dollars sit around accumulating taxable earnings. Check your plan's Summary Plan Description, or ask HR, before assuming this is available to you.
Frequently Asked Questions
No. Your plan must specifically allow after-tax (non-Roth) contributions beyond the standard deferral limit, and it must also allow in-plan Roth conversions or in-service withdrawals. Many employer plans — especially at smaller companies — don't offer these features. Check your Summary Plan Description or ask your plan administrator.
No. A regular Backdoor Roth IRA involves contributing to a nondeductible traditional IRA (limited to $7,000-$8,000 depending on age) and converting it to a Roth IRA, typically used by high earners who exceed the Roth IRA income limits. The Mega Backdoor Roth uses after-tax 401(k) contributions instead, allowing for a much larger dollar amount — tens of thousands more.
No. Catch-up contributions (for those 50 and older) are added on top of the $72,000 overall limit, not counted within it. This calculator reflects that — your after-tax room is based on the $72,000 figure alone, with any catch-up shown separately.
Left unconverted, after-tax 401(k) contributions still grow tax-deferred, but the growth (unlike a true Roth) will be taxed as ordinary income on withdrawal. Converting to Roth as soon as possible is what makes the growth tax-free going forward — that conversion step is the entire point of the "Mega Backdoor Roth" strategy.