David,

Use N[]

N[Pi,5]

returns

3.1416

while

N[100*Pi,5] returns

314.16

Duplicate. Please delete

David,

I read your post too fast and did not realize what you wanted. Sander is correct, for printing you need to use NumberForm (or its cousin, PaddedForm). However, what you want to do is a bit more complicated -- NumberForm will give you places after the decimal and you want 5 digits, period. I posted this answer a while back here for someone else-- you need to make a function that uses NumberForm:

The idea is that you can specify digits after the decimal for NumberForm but this varies depending on the order of magnitude of the number so you need a function to compute it.

This other post will also help you if you want more complicated formatting-- see my replies.

Notice the differences beween the following methods:

c = 1/1.7;

Round[%, 0.00001]
FullForm[%]

N[c, 5]
FullForm[%]

NumberForm[c, {\[Infinity], 5}]
FullForm[%]

Round will round the number, but because numbers are stored in binary, the least significant digits might cause a slight deviation.

N will change a MachinePrecision number to arbitrary precision number, this number will now 'act' differently, because you set the precision of a Machine precision number to 2.

N[Sqrt[2], 2]
%^2

will not return 1.96, but rather 2. because of the limited precision it was given/told.

NumberForm will only display a certain number of digits but does not change the number, i.e. excellent for printing.

... Also, I strongly suggest that you avoid using Capital letters to begin your variable names -- these are reserved for internal Mathematica variables and functions. It works fine until you accidentally stumble on an internal name and then it is really hard to figure out why something suddenly does not work. For example C1 and C2, etc. are often returned as arbitrary constants from equation solving.