Deductions & credits
On the screen where you enter the Form 1098 for each mortgage, you should have entered the outstanding mortgage principal (i.e. the mortgage balance) in box 2. See the screen shot below. Then TurboTax will compute the limitation. Unfortunately, if you omit the outstanding mortgage principal TurboTax does not flag it as an error. But it does ask about it by providing the box for the entry.
