In Excel 2010, the Betadist function was replaced by the Beta.Dist function, which has improved accuracy.
Although it has been replaced, the Betadist function is still available in current versions of Excel (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
However, the Betadist function may not be available in future versions of Excel, so it is advised that you use the Beta.Dist function if possible.
The Excel BETADIST function calculates the cumulative beta probability density function for a supplied set of parameters.
The syntax of the 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). 
[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, the function calculates the Standard Cumulative Beta Distribution.
The above chart on the right shows the Standard Cumulative Beta Distribution with the parameter alpha set to 4 and the parameter beta set to 5.
If you want to use Excel to calculate the value of this function at x = 0.4, this can be done with the Betadist function, as follows:
This gives the result 0.4059136.
Note that, in the above function call, the arguments [A] and [B] have been omitted, and so take on the default values 0 and 1 respectively. This is the Standard Cumulative Beta Distribution.
For further information and examples of the Excel Betadist function, see the Microsoft Office website.
If you get an error from the Excel Betadist function this is likely to be one of the following:
#NUM!   
Occurs if either:

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