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# A to Z of Excel Functions: the FACTDOUBLE Function

6 May 2019

Welcome back to our regular A to Z of Excel Functions blog.  Today we look at the FACTDOUBLE function.

The FACTDOUBLE function

Once voted the most useless function ever in Excel, chances are you have never used the FACTDOUBLE function – and you are exceedingly unlikely to do so in the future too.  This function, denoted n!! for FACTDOUBLE(n), returns the double factorial of a number.

The FACTDOUBLE function employs the following syntax to operate:

FACTDOUBLE(number)

The FACTDOUBLE function has the following argument:

• number: this is required and is a non-negative number for which you require the factorial.

It should be further noted that:

• if number is not an integer, it is truncated
• the factorial of a negative number less than -1 returns a #NUM! error (not sure why FACTDOUBLE (-1) equals 1)
• if number is even, n!! = n x (n – 2) x (n – 4) x … x 4 x 2
• if number is odd, n!! = n x (n – 2) x (n – 4) x … x 3 x 1
• the largest allowed positive number (which has to be calculated rather than input) is 1.7976931348623158 x 10308 (also denoted as 1.79769E+308).  301!! is larger than this upper bound and hence 301!! or any higher value will return a #NUM! error.

We understand it may be useful for working out the odds of winning in poker…