Depends on what you want to do.
If you just need to display, sort, or use as an ID, then enter as text
(prefix with a single quote).
If you want to calculate, do you really need 16 figure accuracy? Most
general purpose computer software uses binary IEEE double precision,
which is limited to 15 digit accuracy. Excel chooses to zero digits
after the 15th, other programs may display the approximation they use to
17 digits, but all have the same underlying accuracy. 11111111111111.55
happens to be be accurate to about 17 figures, so those that display
more figures will display the 2nd 5, but if you enter 11111111111111.54
into those other programs, they will display 11111111111111.539. Which
is a better display convention is a matter of taste, but Excel's is
probably easier for a novice to understand.
I don't know of any spreadsheet that does not use IEEE double precision.
If you are VBA savvy, you can use the Decimal data type, which gives
28 figure accuracy but does not support exponents. The Windows
calculator claims to be accurate to 32 digits, and stores rational
numbers as fractions, to avoid losing precision with numbers such as
1/3. You can download a 64 digit calculator from
http://www.crbond.com/applications.htm
but if you routinely need more than double precision for complicated
calculations, you might do better to look at Maple, Mathematica,
MathCad, or some a similar program that can do symbolic calculations.
Jerry
