Excel's number precision is limited to 15 digits, so the last digit is
simply rounded to accommodate that. You could try formatting the cell as
text, or keeping it general format and placing spaces or hyphens between
the groups of 4 digits.

wrote:
This is driving me nuts...what am I doing wrong??!! I created a
spreadsheet to keep track of personal budgeting info. When I type a
16-digit credit card account number into a cell and then hit "Enter",
the last digit of my number automatically changes to a zero (0). Any
advice?? Thnx!