# 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 10
^{308}(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…

Please see my example below:

*We’ll continue our A to Z of Excel Functions soon. Keep checking back – there’s a new blog post every business day.*

*A full page of the function articles can be found here. *