The Excel GAMMA.INV Function

Related Function:
GAMMA.DIST
GAMMA.DIST and GAMMA.INV

The Gamma.Inv function is the inverse of the Excel Gamma.Dist function (when used to calculate the cumulative distribution function).

I.e. for a supplied probability, P, the Gamma.Inv function returns the value of x such that:

P = Gamma.Dist( x, alpha, beta, TRUE )

Function Description

The Excel Gamma.Inv function returns the inverse of the Gamma Cumulative Distribution.

The function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the Gamma.Inv function is simply a new version of the Gammainv function, which is available in earlier versions of Excel.

The syntax of the Gamma.Inv function is:

GAMMA.INV( probability, alpha, beta )

where the function arguments are:

probability-The probability value (between 0 and 1), for which you want to calculate the inverse of the Gamma Cumulative Distribution Function.
alpha-A parameter of the distribution.
beta-

A parameter of the distribution.

(Note: if beta=0, this specifies the inverse of the Standard Gamma Distribution).


Gamma.Inv Function Example

The charts below show the Gamma Cumulative Distribution and the Inverse Gamma Cumulative Distribution, with the parameters alpha = 3 and beta = 2.

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

The Excel Gamma.Inv function can be used to calculate the value of the Inverse Gamma Cumulative Distribution function for a given probability. For example for the probability 0.5, the function is:

=GAMMA.INV( 0.5, 3, 2 )

which gives the result 5.348120627.


For further examples of the Excel Gamma.Inv function, see the Microsoft Office website.


Gamma.Inv Function Errors

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

Common Errors
#NUM!-

Occurs if either:

  • the supplied probability value is < 0 or ≥ 1;
  • the supplied alpha ≤ 0 or the supplied beta ≤ 0.
#N/A-Occurs if Excel fails to converge to a solution after 64 iterations.
#VALUE!-Occurs if any of the supplied arguments is non-numeric.