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Excel FACTDOUBLE Function

Excel FACTDOUBLE function
Summary 

The Excel FACTDOUBLE function returns the double factorial of a number. A double factorial is symbolized by two exclamation marks (!!).

Purpose 
Get double factorial of a number
Return value 
Calculated doublefactorial
Syntax 
=FACTDOUBLE (number)
Arguments 
  • number - A number greater than or equal to -1.
Usage notes 

The FACTDOUBLE function returns the double factorial of a number. A double factorial is calculated differently for even and odd numbers. For an even number, n, it's the product of all even integers less than or equal to n and greater than or equal to 2. For an odd number, the double factorial is the product of all odd integers less than or equal to n and greater than or equal to 1. The double factorial for both zero and -1 are defined as 1. For numbers less than -1, a double factorial is not defined.

For example, the double factorial of 8 is:

=FACTDOUBLE(8)
=8*6*4*2
=384

For example, the double factorial of 7 is:

=FACTDOUBLE(7)
=7*5*3*1
=105

Notes:

  • The number cannot be negative or FACTDOUBLE will return the #NUM error.
  • If the number is not an integer it will be truncated to an integer, then solved.
Related functions 
Excel FACT function
Excel FACT Function

The Excel FACT function returns the factorial of a given number. For example, =FACT(3) returns 6, equivalent to 3 x 2 x 1.

Good links 
Microsoft FACTDOUBLE function documentation

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