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NSig

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:

20091118_PB_01_NSig

The corresponding VBA solution:

Function nsig(d As Double, n As Long) As Double
'Returns double with n most significant digits of d.
nsig = Format(d, "." & String(n, "0") & "E+0")
End Function

Klaus K. offeres another worksheet formula for this on the web site www.Excelformeln.de:

=ROUND(A1,A2-1-INT(LOG(ABS(A1))))

A related function is sbNum2Str which returns a non-scientific string representation of a given number with all significant digits and all leading and trailing zeros.

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