BETAINV function
Returns the inverse of the cumulative beta probability density function for a specified beta distribution. That is, if probability = BETADIST(x,...), then BETAINV(probability,...) = x. The beta distribution can be used in project planning to model probable completion times given an expected completion time and variability.
For more information about the new function, see BETA.INV function.
Syntax
BETAINV(probability,alpha,beta,[A],[B])
The BETAINV function syntax has the following arguments:

Probability Required. A probability associated with the beta distribution.

Alpha Required. A parameter of the distribution.

Beta Required. A parameter the distribution.

A Optional. A lower bound to the interval of x.

B Optional. An upper bound to the interval of x.
Remarks

If any argument is nonnumeric, BETAINV returns the #VALUE! error value.

If alpha ≤ 0 or beta ≤ 0, BETAINV returns the #NUM! error value.

If probability ≤ 0 or probability > 1, BETAINV returns the #NUM! error value.

If you omit values for A and B, BETAINV uses the standard cumulative beta distribution, so that A = 0 and B = 1.
Given a value for probability, BETAINV seeks that value x such that BETADIST(x, alpha, beta, A, B) = probability. Thus, precision of BETAINV depends on precision of BETADIST.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Data 
Description 

0.685470581 
Probability associated with the beta distribution 

8 
Parameter of the distribution 

10 
Parameter of the distribution 

1 
Lower bound 

3 
Upper bound 

Formula 
Description 
Result 
=BETAINV(A2,A3,A4,A5,A6) 
Inverse of the cumulative beta probability density function for the parameters above 
2 