In Excel 2010, the Finv function has been replaced by the F.Inv.Rt function, which has improved accuracy.
Although it has been replaced, the Finv function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
However, the Finv function may not be available in future versions of Excel, so it is advised that you use the F.Inv.Rt function, if possible.
The Excel FINV function calculates the inverse of the (righttailed) F Probability Distribution for a supplied probability.
The syntax of the function is:
Where the function arguments are:
probability    The probability (between 0 and 1) at which to evaluate the inverse F Probability Distribution. 
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.
Note that the Excel Finv function is the inverse of the Excel FDIST function.
I.e. ifThe above chart on the right shows the inverse of 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 calculate the value of this function for a probability equal to 0.2, this can be done using the Finv function, as follows:
This gives the result 3.555555556.
For further details and examples of the Excel Finv function, see the Microsoft Office website.
If you get an error from the Excel Finv function this is likely to be one of the following:
#NUM!   
Occurs if either:

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