Excel FACTDOUBLE Function

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.