In Plan 401k to Roth 401k Basis

[Edge10]

This tax year 2022 I did a partial in-plan 401k to Roth 401k conversion. Only pre-tax money was used for the partial conversion, but I do have after-tax money in the same 401k account.  I is there any cost basis that should be applied that will reduce my tax and if so, where is this entered in TT interview or forms ?

[Anonymous_]

I'll page @dmertz 

[dmertz · Accepted solution]

It's odd that only an In-plan Roth Rollover would be performed on only the pre-tax sub-account in the 401(k), but I'll tax your word for it.  The amount shown in box 2a of the Form 1099-R is the taxable amount.  With no after-tax basis included in the IRR (nothing in box 5 of the Form 1099-R), the entire amount is taxable.