If I haven't fild and did not pay before april 15 2019 what do I put on the box full blance due

[BillM223]

I take it you are doing your 2018 return and filing it now?

I do not know what box you are referring to "full blance due", but I would enter whatever tax was due according to your return.

After you file the return and pay the tax, if you owe penalties and interest for filing late, the IRS will calculate that for you and send you a letter asking for the penalties and interest.

Don't hold up your filing because you are afraid of putting the wrong number - pay what you think you owe (according to the tax return), then let the IRS come back to you if they want anything else.