When entering your 1098 for Mortgage Interest, Box 2 asks for your Outstanding Mortgage Principal as of 01/01/2018. This information should be provided to you on your 1098.
As you click through the screens, it will then ask you for your balance on 01/01/2019. This information can be found on your monthly mortgage statements or on your lender website if you have account access.
TurboTax will need the balance at both the beginning and the end of 2018.
This information can be found on either a mortgage statement or by contacting your lender.
There is a more in-depth explanation of the 2018 tax reform changes and how it affects your mortgage interest deduction below:
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