The Excel BETA.INV function calculates the inverse of the cumulative beta probability density function for a supplied probability.
The Beta.Inv function is new to Excel 2010 and so is not available in earlier versions of Excel. However, the function is simply an updated version of the Betainv function, which is available in earlier versions of Excel.
The syntax of the Beta.Inv function is:
Where the function arguments are:
probability    The probability of the Beta distribution, for which you want to find the value of x. 
alpha    A parameter of the distribution (must be > 0). 
beta    A parameter of the distribution (must be > 0). 
[A]   
An optional numerical argument which gives the lower bound of the interval of x. (If omitted, [A] takes the default value 0). 
[B]   
An optional numerical argument which gives the upper bound of the interval of x. (If omitted, [B] takes the default value 1). 
Note that, if the arguments [A] and [B] are set to 0 and 1 respectively, this defines the Inverse Standard Cumulative Beta Distribution.
The charts on the left, below, shows the Cumulative Beta Distribution with the parameters alpha and beta set to 4 and 5, respectively. The interval of x is set to [0, 1], which makes this the Standard Cumulative Beta Distribution.
The chart on the right shows the Inverse of the Cumulative Beta Distribution, as calculated by the Beta.Inv function.
Standard Cumulative Beta Distribution Function with α = 4 and β = 5 
Inverse Standard Cumulative Beta Distribution with α = 4 and β = 5 
If you want to calculate the value of the Inverse Cumulative Beta Distribution for any probability (for example, the probability 0.2), this can be done by the Excel Beta.Inv function, as follows:
This gives the result 0.303225845.
Note that, in the above example, the arguments [A] and [B] have been supplied as 0 and 1 respectively. However, these values could have been omitted from the function, as their default values are 0 and 1.
For further details and examples of the Excel Beta.Inv function, see the Microsoft Office website.
If you get an error from the Excel Beta.Inv function this is likely to be one of the following:
#NUM!   
Occurs if either:

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