GAMMADIST function
Returns the gamma distribution. You can use this function to study variables that may have a skewed distribution. The gamma distribution is commonly used in queuing analysis.
Important This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. Although this function is still available for backward compatibility, you should consider using the new functions from now on, because this function may not be available in future versions of Excel.
For more information about the new function, see GAMMA.DIST function.
Syntax
GAMMADIST(x,alpha,beta,cumulative)
The GAMMADIST function syntax has the following arguments:

X Required. The value at which you want to evaluate the distribution.

Alpha Required. A parameter to the distribution.

Beta Required. A parameter to the distribution. If beta = 1, GAMMADIST returns the standard gamma distribution.

Cumulative Required. A logical value that determines the form of the function. If cumulative is TRUE, GAMMADIST returns the cumulative distribution function; if FALSE, it returns the probability density function.
Remarks

If x, alpha, or beta is nonnumeric, GAMMADIST returns the #VALUE! error value.

If x < 0, GAMMADIST returns the #NUM! error value.

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

The equation for the gamma probability density function is:
The standard gamma probability density function is:

When alpha = 1, GAMMADIST returns the exponential distribution with:

For a positive integer n, when alpha = n/2, beta = 2, and cumulative = TRUE, GAMMADIST returns (1  CHIDIST(x)) with n degrees of freedom.

When alpha is a positive integer, GAMMADIST is also known as the Erlang distribution.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article. If you are copying the example in Excel Web App, copy and paste one cell at a time.Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

Create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Web App, repeat copying and pasting for each cell in the example.

Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

After you copy the example to a blank worksheet, you can adapt it to suit your needs.

