CHIINV function
Returns the inverse of the righttailed probability of the chisquared distribution. If probability = CHIDIST(x,...), then CHIINV(probability,...) = x. Use this function to compare observed results with expected ones in order to decide whether your original hypothesis is valid.
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 functions, see CHISQ.INV function and CHISQ.INV.RT function.
Syntax
CHIINV(probability,deg_freedom)
The CHIINV function syntax has the following arguments:

Probability Required. A probability associated with the chisquared distribution.

Deg_freedom Required. The number of degrees of freedom.
Remarks

If either argument is nonnumeric, CHIINV returns the #VALUE! error value.

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

If deg_freedom is not an integer, it is truncated.

If deg_freedom < 1, CHIINV returns the #NUM! error value.
Given a value for probability, CHIINV seeks that value x such that CHIDIST(x, deg_freedom) = probability. Thus, precision of CHIINV depends on precision of CHIDIST. CHIINV 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 

0.050001 
Probability associated with the chisquared distribution 

10 
Degrees of freedom 

Formula 
Description 
Result 
=CHIINV(A2,A3) 
Inverse of the onetailed probability of the chisquared distribution. 
18.306973 