Microsoft Excel supports a maximum of 15 significant digits at all times.
This limit applies to a value that is calculated by a formula. Because of
this limitation, if at any time a formula calculates a value that exceeds
15 digits in length, digits beyond the fifteenth significant digit are
changed to zeroes. This may cause the formula to return an incorrect
result.
For example, when you use the STDEV function to calculate the standard
deviation of a set of numbers, part of the function sums the numbers and
then squares the result. If this number exceeds 15 digits in length,
digits beyond the fifteenth digit are changed to zeroes. This affects the
final result delivered by the formula. To see an example of this behavior,
enter the following data in a worksheet:
A1: 999999 B1: 99999999
A2: 1000000 B2: 100000000
A3: 1000001 B3: 100000001
A4: =STDEV(A1:A3) B4: =STDEV(B1:B3)
Although both formulas return the same standard deviation, 1, the second
formula returns a zero. Because the square of the sum of the three values
in B1:B3 is greater than 15 digits in length, digits beyond the fifteenth
digit are changed to zeroes. This causes the formula to return an
incorrect result.
This behavior may occur when you use any of the statistical functions
listed in this article and you work with very large values. This is true
because these functions all use squared values, which makes it probable
that the limit of 15 significant digits is exceeded.
The exact formulas used by functions in Microsoft Excel are listed in the
Help topic for each function in Microsoft Excel Help.