Problem Formatting double numbers.

Hi all,

I have problem formatting double number. When double number is stored in computer(binary representation), it is not exact as decimal representation. when I try to format the double number with more decimal points(ex- 12) then as number gets bigger, I get more and more noise which makes number look different than the one entered as decimal number. How can I find the way to format the number so I do not see noise? or How can I find out how many decimal points I can safely show without getting noise for any double number? thanks...