Mega Backdoor Roth Calculator
Calculate the extra after-tax 401(k) contributions you can make and convert to Roth, on top of your normal deferral and employer match.
Want to understand the concept, not just the number? Read The Mega Backdoor Roth Guide and more below.
How it's calculated
Your after-tax capacity is the section 415(c) total-additions limit minus your elective deferral minus your employer match. Whatever room is left can be filled with after-tax dollars and then converted to Roth.
For 2026 the total-additions limit is $72,000. Take someone earning $200,000 who defers the full $24,500 and gets a 4% match worth $8,000. That uses $32,500, which leaves $39,500 of after-tax room to convert to Roth, far beyond the $7,500 IRA limit.
This only works if your plan allows both after-tax, non-Roth contributions and in-plan Roth conversions or in-service withdrawals. Without both, the strategy is not available, so confirm with your plan administrator before you rely on it.
Assumptions
- Uses the 2026 total defined-contribution limit of $72,000, known as the section 415(c) limit. — IRS Rev. Proc. 2025-32
- Only usable if your plan allows both after-tax contributions and in-plan Roth conversion. Confirm with your plan administrator.
Last updated: 2026-08-07 · Tax year 2026
These assumptions follow our general methodology.
Frequently asked questions
How do I know if my plan supports this?
Ask your plan administrator two things. Does the plan allow after-tax, non-Roth contributions, and does it allow in-plan Roth conversions or in-service withdrawals. You need both of these for the strategy to work.
Is this the same as a backdoor Roth IRA?
No. The mega backdoor uses your 401(k) and much higher limits. The backdoor Roth IRA uses an IRA and the smaller $7,500 limit.
How much can the mega backdoor Roth add in 2026?
Up to the gap between the $72,000 total-additions limit and everything else going into your 401(k), which is your own deferral plus the employer match. For many high earners that is tens of thousands of dollars a year on top of the normal limits.
Do catch-up contributions change the limit?
Yes. If you are 50 or older the ceiling rises by the catch-up amount, which is $8,000 in 2026, or $11,250 at ages 60 to 63. This calculator applies it when you enter your age.