This article describes the formula syntax and usage of the GAMMAINV function in Microsoft Excel.
Returns the inverse of the gamma cumulative distribution. If p = GAMMADIST(x,...), then GAMMAINV(p,...) = x. You can use this function to study a variable whose distribution may be skewed.
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.INV function.
Syntax
GAMMAINV(probability,alpha,beta)
The GAMMAINV function syntax has the following arguments:

Probability Required. The probability associated with the gamma distribution.

Alpha Required. A parameter to the distribution.

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

If any argument is text, GAMMAINV returns the #VALUE! error value.

If probability < 0 or probability > 1, GAMMAINV returns the #NUM! error value.

If alpha ≤ 0 or if beta ≤ 0, GAMMAINV returns the #NUM! error value.
Given a value for probability, GAMMAINV seeks that value x such that GAMMADIST(x, alpha, beta, TRUE) = probability. Thus, precision of GAMMAINV depends on precision of GAMMADIST. GAMMAINV uses an iterative search technique. If the search has not converged after 100 iterations, the function returns the #N/A error value.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Data 
Description 

0.068094 
Probability associated with the gamma distribution 

9 
Alpha parameter to the distribution 

2 
Beta parameter to the distribution 

Formula 
Description 
Result 
=GAMMAINV(A2,A3,A4) 
Inverse of the gamma cumulative distribution for the probability, alpha, and beta arguments in A2, A3, and A4. 
10.0000112 