The Factorial of a number, N, is given by the product:
N * (N1) * ... * 2 * 1
More information can be found on the Wikipedia Factorial page
The Excel FACT function returns the factorial of a supplied number
The format of the function is :
where the number argument is the positive integer that you want to calculate the factorial of.
Cells A1A3 of the following spreadsheet shows examples of the Excel Fact function being used to calculate the factorial of different integers. The format of the functions is shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Further examples of the Excel Fact function can be found on the Microsoft Office website.
If you get an error from the Excel Fact function this is likely to be one of the following :
#NUM!    Occurs if the supplied number argument is < 0 
#VALUE!    Occurs if the supplied number argument is not recognised as a numeric value 