Hi @patrick cornish !
TurboTax will offer to calculate your underpayment penalty, not interest, based on the information you enter, but the penatly and interest will be determined by the IRS at the exact time they actually process your return. This will still generate a letter from them advising you of the amount owed. If you "opt out" of having your penalty calculated within TurboTax, it won't change what your penalty or interest actually is.
Hope this helps!
Cindy
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