The number of permutations for a given number of objects is the number of combinations in each possible order.
Note that permutations differ from combinations in that, for a permutation, the order of the objects matters, but in a combination, the order does not matter.
The number of possible permutations is given by the formula:
where k is the number of objects chosen and n is the number of possible objects.
For further information, see the Wikipedia Permutation page
The Excel Permut function calculates the number of permutations of a specified number of objects from a set of objects.
The syntax of the function is:
Where the function arguments are:
number    The total number of objects available 
number_chosen    The number of objects in each permutation (i.e. the number of objects selected from the set) 
Note that, if any of the arguments are supplied as decimal values, they are truncated to integers by the Permut function.
In the following spreadsheet, the Excel Permut function is used to calculate the number of permutations of six objects, selected from different sized sets:
Formulas:

Results:

For further examples of the Excel Permut function, see the Microsoft Office website.
If you get an error from the Excel Permut Function, this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if one or both of the supplied arguments are nonnumeric. 