To get more than 15 significant digits you need to make the cell text by
either formattting the cell as text and re-entering the number or preceed the
number with an apostrophy '.
15 digits is a limit of your PC and not XL specifically so there is no easy
way around it. There are some addins that will allow you to use more the 15
digits but there is a bunch of overhead associated with doing that...
