Let's say you have six bells, each with a different tone, and you want to find the number of unique sequences in which each bell can be rung once. In this example, you are calculating the factorial of six. In general, use a factorial to count the number of ways in which a group of distinct items can be arranged (also called permutations). To calculate the factorial of a number, use the FACT function.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example
 Create a blank workbook or worksheet.
 Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 In the worksheet, select cell A1, and press CTRL+V.
 To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

A 
Data 
6 
Formula 
Description (Result) 
=FACT(A2) 
Factorial of 6, or 6*5*4*3*2*1 (720) 
=FACT(3) 
Factorial of 3, or 3*2*1 (6) 
=FACT(A2+4) 
Factorial of 10 (3628800) 

Function details
FACT