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Tax simulator
Mega backdoor Roth capacity calculator
If your 401(k) allows after-tax contributions and conversions, the mega backdoor Roth can let you put far more into a Roth than the usual limits suggest. Enter your own salary deferrals and your employer's contributions for the year, and this calculates the after-tax room you have left under the total 401(k) limit. It also explains why catch-up contributions do not eat into that room.
Background reading: Advanced contribution strategies
After-tax space available to convert
$38,500
How the capacity is reached
415(c) total-additions limit$72,000
Less: your elective deferrals-$23,500
Less: employer contribution-$10,000
Remaining after-tax space$38,500
Where this goes next
This shows the after-tax space your plan limits allow; it does not tell you how much to contribute. That decision is a conversation with a CFP.
Assumptions and simplifications
- After-tax capacity is the IRC 415(c) total-additions limit minus your elective deferrals and employer contributions.
- Catch-up contributions sit on top of the 415(c) limit and are shown for reference; they do not reduce the after-tax capacity.
- The result is only usable if your plan supports after-tax contributions with in-plan conversion or in-service withdrawal; without that feature there is no mega backdoor.