In Excel 2010, the Fdist function has been replaced by the F.Dist.Rt function, which has improved accuracy.
Although it has been replaced, the Fdist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
The Excel FDIST function calculates the (righttailed) F Probability Distribution, which measures the degree of diversity between two data sets.
The format of the function is :
Where the function arguments are:
x    The (positive) numeric value at which the function is to be evaluated 
deg_freedom1    An integer specifying the numerator degrees of freedom 
deg_freedom2    An integer specifying the denominator degrees of freedom 
If either deg_freedom1 or deg_freedom2 are decimal numbers, these are truncated to integers by Excel.
The above chart on the right shows the righttailed F Probability Distribution with the numerator degrees of freedom equal to 1 and the denominator degrees of freedom equal to 2.
If you want to use Excel to calculate the value of this function at x = 5, this can be done by the Fdist function, as follows:
This gives the result 0.154845745.
Further information and examples of the Excel Fdist function can be found on the Microsoft Office website.
If you get an error from the Excel Fdist function this is likely to be one of the following :
#NUM!   
Occurs if either:

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