When an underpayment penalty is calculated on a tax return, this penalty is automatically added to the amount you owe on Form 1040 U.S. Individual Income Tax Return, Line 23. If you are entering information on IRS Form 1040-X Amended U.S. Individual Income Tax Return, Line 16, keep in mind that the underpayment penalty may have already been included with the amount you originally owed/paid.
Make sure to subtract Form 1040, Line 38 from Line 37 before entering the amount on Form 1040-X.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.