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 enter 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.
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