The Excel Beta.Dist function calculates the cumulative beta distribution function or the probability density function of the Beta distribution, for a supplied set of parameters.
The function was first introduced in Excel 2010 and so is not available in earlier versions of Excel. However, the Beta.Dist function is an updated version of the Betadist function, which is available in earlier versions of Excel.
The syntax of the Beta.Dist function is:
Where the function arguments are:
x    The value at which the function is to be calculated (must be between [A] and [B]).  
alpha    A parameter of the distribution (must be > 0).  
beta    A parameter of the distribution (must be > 0).  
cumulative   
A logical argument that specifies the type of distribution to be used. This can be either:


[A]   
An optional argument which gives the lower bound of the interval of x. (If omitted, [A] takes the default value 0). 

[B]   
An optional argument which gives the upper bound of the interval of x. (If omitted, [B] takes the default value 1). 
If the arguments [A] and [B] are set to 0 and 1 respectively, this is the Standard Beta Distribution.
The above chart on the right shows the Cumulative Beta Distribution with the parameter alpha set to 4 and the parameter beta set to 5.
The interval of x is set to [0, 1], which makes this the Standard Cumulative Beta Distribution.
If you want to use Excel to calculate the value of this function at x = 0.4, this can be done with the Beta.Dist function, as follows:
This gives the result 0.4059136.
The above chart on the right shows the Probability Density Function of the Beta distribution with the parameters alpha and beta set to 4 and 5 respectively.
Again, the interval of x is set to [0, 1], which makes this the Standard Beta Probability Density Function.
If you want to use Excel to calculate the value of this function at x = 0.6, this can be done using the Excel Beta.Dist function, as follows:
This gives the result 1.548288.
Note: although the arguments [A] and [B] have been supplied to both of the above two example functions, these arguments could have been omitted, as they would have then used the default values of 0 and 1.
For further information and examples of the Excel Beta.Dist function, see the Microsoft Office website.
If you get an error from the Excel Beta.Dist function this is likely to be one of the following:
#NUM!   
Occurs if either:

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