BETADIST function
Returns the cumulative beta probability density function. The beta distribution is commonly used to study variation in the percentage of something across samples, such as the fraction of the day people spend watching television.
For more information about the new function, see BETA.DIST function.
Syntax
BETADIST(x,alpha,beta,[A],[B])
The BETADIST function syntax has the following arguments:

X Required. The value between A and B at which to evaluate the function.

Alpha Required. A parameter of the distribution.

Beta Required. A parameter of 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, BETADIST returns the #VALUE! error value.

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

If x < A, x > B, or A = B, BETADIST returns the #NUM! error value.

If you omit values for A and B, BETADIST uses the standard cumulative beta distribution, so that A = 0 and B = 1.
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 

2 
Value at which to evaluate the function 

8 
Parameter of the distribution 

10 
Parameter of the distribution 

1 
Lower bound 

3 
Upper bound 

Formula 
Description 
Result 
=BETADIST(A2,A3,A4,A5,A6) 
Cumulative beta probability density function, for the above parameters 
0.6854706 