In Excel 2010, the Chiinv function has been replaced by the Chisq.Inv.Rt function, which has improved accuracy.
Although it has been replaced, the Chiinv function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
The Excel Chiinv function calculates the inverse of the righttailed probability of the ChiSquare Distribution.
The syntax of the function is:
Where the function arguments are:
probability    The probability of the chisquare distribution, for which you want to find the value of x. 
degrees_freedom    The number of degrees of freedom (must be a positive integer). (If the supplied degrees_freedom is a decimal, this is truncated to an integer). 
The Chiinv function is the inverse of the Excel Chidist function.
I.e. for a supplied probability, the Chiinv function uses an iterative method to find the value of x such that:
The spreadsheet below shows 5 examples of the Excel Chiinv function, used to calculate the inverse righttailed chisquare distribution.
Formulas:
 Results:

Further examples of the Excel Chiinv function are provided on the Microsoft Office website.
If you get an error from the Excel Chiinv function this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 