The Excel COMBIN function calculates the number of combinations (in any order) of a given number objects, from a set. (Note that combinations are different from permutations in that for permutations, the order of the selected objects is counted).
The format of the COMBIN function is :
Where the arguments are as follows :
number    The number of objects in the set 
number_chosen    The number of objects to be chosen from the set 
Note that if any of the arguments are input as decimal values, they will be truncated to integers.
For any 6 objects (eg. a, b, c, d, e, f), there are 15 different combinations of 2 objects. These are:
ab  ac  ad  ae  af  bc  bd  be  bf  cd  ce  cf  de  df  ef 
This is calculated using the Excel Combin function in cell A2 of the following spreadsheet. The spreadsheet also shows the Combin function used to calculate the number of combinations for other numbers of objects taken from a set of 6.
Formulas:

Results:

Further examples of the Excel Combin function can be viewed on the Microsoft Office website.
If you get an error from the Excel Combin function this is likely to be one of the following :
#NUM!   
Occurs if either :

#VALUE!    Occurs if either of the supplied arguments are nonnumeric 