## Summary

The Excel FACTDOUBLE function is used to return the double factorial of a number. The double factorial is symbolized using double exclamation marks. For example, 8!!.

See the FACT function if you need the factorial of a number.

## Syntax

=FACTDOUBLE (number)

## Syntax Breakdown

Number
Required. The number for which you want to find the double factorial. If it is not an integer, it will be truncated.

## Usage Notes

The FACTDOUBLE function returns the double factorial of a given non-negative number. The function returns the #NUM! error if a negative number is entered.

If number is non-numeric the function will return the #VALUE! error.

Calculation
The double factorial is calculated differently for even and odd numbers. If the number, n, is even, then the double factorial is the product of all even integers less than or equal to n and greater than or equal to 2. For example, the double factorial of 6 would be found by 6 * 4 * 2 = 48.

If the number, n, is odd, then the double factorial is the product of all odd integers less than or equal to n and greater than or equal to 1. For example, the double factorial of 9 would be found by 9 * 7 * 5 * 3 * 1 = 945.

A double factorial is not defined for numbers less than -1. For 0 (zero) and 1, the double factorial is 1.