The Excel F.INV.RT function calculates the inverse of the (righttailed) F Probability Distribution for a specified probability.
The function is new to Excel 2010 and so is not available in earlier versions of Excel. However the F.Inv.Rt function is simply a new version of the Finv function, which is available in earlier versions of Excel.
The format 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 
Note that, if either deg_freedom1 or deg_freedom2 are decimal numbers, these are truncated to integers by Excel.
The Excel F.Inv.Rt function is the inverse of the Excel F.Dist.Rt function.
I.e. if  
probability = F.DIST.RT( x, deg_freedom1, deg_freedom2 )  
then  
x = F.INV.RT( probability, deg_freedom1, deg_freedom2 ) 
F.Inv.Rt Function ExampleThe 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 value of 0.2, this can be done using the Excel F.Inv.Rt function, as follows: =F.INV.RT( 0.2, 1, 2 ) This gives the result 3.555555556. 
Inverse F Prob. Dist. with deg_freedom1 = 1 & deg_freedom2 = 2

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


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