The Excel F.DIST.RT function calculates the (righttailed) F Probability Distribution, which measures the degree of diversity between two data sets.
The function is new to Excel 2010 and so is not available in earlier versions of Excel. However the F.Dist.Rt function is simply an updated version of the Fdist function, which is available in earlier versions of Excel.
The syntax of the F.Dist.Rt 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 supplied as decimal numbers, these are truncated to integers by Excel.
The above chart on the right shows the righttailed F Probability Distribution with deg_freedom1 (the numerator degrees of freedom) set equal to 1 and deg_freedom2 (the denominator degrees of freedom) set equal to 2.
If you want to calculate the value of this function at x = 5, this can be done using the Excel F.Dist.Rt function, as follows:
This gives the probability 0.154845745.
For further details and examples of the Excel F.Dist.Rt function, see the Microsoft Office website.
If you get an error from the Excel F.Dist.Rt function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if any of the supplied arguments is nonnumeric. 