## How to use the FACTDOUBLE function

**What is the FACTDOUBLE function?**

The FACTDOUBLE returns the double factorial of a number.

**How is the FACTDOUBLE function calculated?**

The factorial double is the product of all positive integers up to n that have the same parityÂ (odd or even) asÂ n, in other words, it skips every other number.

The double factorial is calculates like this:

**For even numbers:** the double factorial of a number n is the product of all positive even integers less than or equal to n, and is represented by n!! Note the double exclamation marks.

n!! = n*(n-2)*(n-4)...(4)(2)

**For odd numbers:** the double factorial of a number n is the product of all positive odd integers less than or equal to n.

n!! = n*(n-2)*(n-4)...(3)(1)

Some examples:

1!! = 1

4!! = 4 * 2 * 1 = 8

5!! = 5 * 3 * 1 = 15

**What is parity?**

Parity is whether an integer is even or odd.

**What is an even number?**

The parity of a number determines if it is divisible by 2 or not without remainders. Even numbers alternate with odd numbers numerically.

**What is an odd number?**

Odd numbers are integers that are not divisible by 2. Some examples of odd numbers: 1, 3, 5, 7, 9, 11, 13, etc.

An number that is not divisible by two returns a remainder of one, in other words, it is not cleanly divided.

**What is a remainder?**

For example, if we divide 5 by 2:

5 / 2 = 2 with a remainder of 1.

**What is the difference between the FACT function and the FACTDOUBLE function?**

It may seem that the FACTDOUBLE returns a much larger number than the FACT function but that is not the case.

The factorial of a number n is the product of all positive integers less than or equal to n, whereas the factorial double only uses every other number.

Example of factorials:

5! = 5 * 4 * 3 * 2 * 1 = 120

6! = 6*5*4*3*2*1 = 720

Example of double factorials:

7!! = 7 * 5 * 3 * 1 = 105

6!! = 6 * 4 * 2 = 48

Formula in cell C3:

### Excel Function Syntax

FACTDOUBLE(*number*)

### Arguments

number |
Required. The number you want to calculate the double factorial of. |

### Comments

The double factorial of 6 is 48, 6*4*2 = 48.

The double factorial of 9 is 945, 9*7*5*3 = 945.

### Functions in 'Math and trigonometry' category

The FACTDOUBLE function function is one of many functions in the 'Math and trigonometry' category.

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