The Chisq.Inv.Rt function is the inverse of the Excel Chisq.Dist.Rt function.
I.e., for a supplied probability, the Chisq.Inv.Rt function finds the value of x such that:
The Excel Chisq.Inv.Rt function calculates the inverse of the righttailed probability of the chisquare distribution.
The function is new in Excel 2010 and so is not available in earlier versions of Excel. However, the Chisq.Inv.Rt function is simply a new version of the Chiinv function that is available in earlier versions of Excel.
The syntax of the Chisq.Inv.Rt function is :
Where the function arguments are:
probability    The probability of the righttailed chisquare distribution, for which you want to find the value of x 
degrees_freedom   
The number of degrees of freedom (must be an integer between 1 and 10^{10}) (If the supplied degrees_freedom is a decimal, this is truncated to an integer) 
The spreadsheets below show examples of the Excel Chisq.Inv.Rt function, used to calculate the inverse of the righttailed chisquare distribution for different probability values, and different degrees of freedom.
The spreadsheet on the left shows the formulas and the spreadsheet on the right shows the results.
Formulas:

Results:

Further examples of the Excel Chisq.Inv.Rt function can be found on the Microsoft Office website.
If you get an error from the Excel Chisq.Inv.Rt function this is likely to be one of the following :
#NUM!   
Occurs if either:

#VALUE!    Occurs if either of the supplied arguments are not recognised as numeric values 