How is the "Federal Tax Due" amount calculated?

[twentw]

I'm talking about the running number in the upper left of the screen that shows your tax due or refund at any particular time while you're doing your taxes.  My specific problem is this: I'm unable to enter mortgage deductions yet since the form is not ready.  For purposes of the "Federal Tax Due" number, is Turbotax using my 2017 number (which for some reason is wildly inaccurate), or the 2018 number (which for some reason has a number that I never entered)?

[Carl]

The number you see in the upper left is based solely on the data the program has to work with *at that specific point in time*. That number should hold no weight and have absolutely no meaning or relevance to you, until "after" you have completed your tax returns in their entirety. Until you finish, that number only tells you one thing each time that number changes - The program *did something* and that's pretty much it.

As it stands now, there are quite a lot of forms the IRS has not released in their electronic format yet - not just the 1098. So the program itself won't be and can't be complete until that happens. With the ongoing shutdown, there's no telling how long that could be. But in the past, all forms have been released and the program updated by the end of January. But again, with the shutdown that may not happen before then, this year.