The Excel GAMMADIST Function

Related Function:
GAMMAINV
GAMMADIST and GAMMA.DIST Functions

In Excel 2010, the Gammadist function has been replaced by the Gamma.Dist function, which has improved accuracy.

Although it has been replaced, the Gammadist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.

Basic Description

The Excel Gammadist function returns the Gamma Distribution, which is frequently used to provide probabilities for values that may have a skewed distribution, such as queuing analysis.

The user can specify whether the cumulative distribution or the probability density function should be calculated.

The syntax of the Gammadist function is:

GAMMADIST( x, alpha, beta, cumulative )

where the function arguments are listed in the following table:

x-The value at which you want to evaluate the function (must be ≥ 0)
alpha-A parameter of the distribution
beta-

A parameter of the distribution.

(Note: if beta=0, then the Standard Gamma distribution is calculated).
cumulative-

A logical argument that specifies the type of gamma distribution to be calculated. This can have the value TRUE or FALSE, meaning:

TRUE-Calculate the cumulative distribution function;
FALSE-Calculate the probability density function.


Gammadist Function Examples

Example 1 - Probability Density Function

Plot of the Probability Density Gamma Distribution with α = 3 and β = 2
Gamma Probability Density Distribution with α=3 & β=2

The above chart on the right shows the probability density function for the gamma distribution with α = 3 and β = 2.

If you want to calculate the value of this function at the value x = 6, this can be done using the Excel Gammadist function, as follows:

=GAMMADIST( 6, 3, 2, FALSE )

This returns the result 0.112020904.


Example 2 - Cumulative Distribution Function

Plot of the Cumulative Gamma Distribution Function with α = 3 and β = 2
Cumulative Gamma Distribution Function with α=3 & β=2

The above chart on the right shows the Cumulative Gamma Distribution with the parameter α = 3 and the parameter β = 2.

If you want to calculate the value of this function at the value x = 6, this can be done using the Excel Gammadist function, as follows:

=GAMMADIST( 6, 3, 2, TRUE )

In this case, the function returns the result 0.576809919.


Further examples of the Excel Gammadist function can be found on the Microsoft Office website.


Gammadist Function Errors

If you get an error from the Excel Gammadist function this is likely to be one of the following:

Common Errors
#NUM!-

Occurs if either:

  • the supplied value of x is < 0;
  • the supplied value of alpha is ≤ 0 or the supplied value of beta is ≤ 0.
#VALUE!-

Occurs if either:

  • Any of the supplied arguments, x, alpha or beta are not recognised as numeric values;
  • The supplied value of cumulative is not recognised as a logical or a numeric value.