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For the backdoor (IRA -> Roth IRA, generally up to $5500 per year) conversion, we need to file Form 8606.

Are there any equivalent forms that must be filed for mega-backdoor (After-tax 401k -> Roth IRA, generally up to $53k minus Pre-tax/Roth 401k contributions per year) conversions?

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    You should consider making a post on how the Mega Backdoor Roth should be reported on a 1099-R, 5498 and 1040 forms. I’m sure I’m not the only one wondering if everything is being reported correctly and if I’m reporting it correctly. Feb 9 '19 at 23:50
  • This question, which is answered, covers two out of three IRA Backdoor Conversion, Tax Filing 1040, 8606 and 1099R Jun 9 '19 at 8:07
  • @EllieKesselman: The fact that the forms are the same doesn't mean that how to fill them out in an entirely different scenario is covered (and yes, it is an entirely different scenario -- there's no IRA contribution, there is a rollover into an IRA, etc)
    – Ben Voigt
    Jul 21 '19 at 3:46
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The "Mega-Backdoor Roth" contribution option moves funds from a Traditional 401(k) -> Roth IRA while the "Backdoor Roth" contribution option moves funds from a IRA -> Roth IRA.

The Plan Administrator is not required to file anything for contributions to the Traditional 401(k). Filings for contributions to an IRA are reported on IRS Form 8606.

When the distribution or rollover occurs which moves the money from the Traditional 401(k) to the Roth IRA, the Plan Administrator will issue a form to the IRS on 1099-R. You should receive a copy of this form, which you will file with your taxes. If the contribution was performed and recorded correctly, the taxable amount on this form should be 0 (or "not calculated"). To see a copy of this form, visit PDF Form. See Instructions.

If you are the Plan Administrator, and the total assets in your 401(k) are worth more than $250,000, IRS Form 5500 needs to be filed. The Plan Sponsor may need to file additional forms.

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  • Nondeductible contributions to trad IRA (as for plain backdoor) go on 8606, but not deductible contributions to trad or (normal) contributions to Roth. (All contributions plus rollovers/conversions-in to either IRA are on 5498 filed and provided by the trustee/guardian.) 2 days ago

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