Significant digits of a number are the digits which contribute to its accuracy  generally these are all digits except leading and trailing zeros. For a more precise definition please see WolframMathworld.
An Excel © worksheet function solution to calculate n significant digits of d (I found this in an Excel forum but I could not identify the original author): A1: d A2: n Result: =TEXT(A1,"."&REPT("0",A2)&"E+0")
Example: If you want to show pi with 3 significant digits:
