In Excel 2010, the Chidist function has been replaced by the Chisq.Dist.Rt function.
Although it has been replaced, the Chidist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
However, the Chidist function may not be available in future versions of Excel, so it is advised that you use the Chisq.Dist.Rt function if possible.
The Excel Chidist function calculates the righttailed probability of the chisquare distribution.
The syntax of the function is:
Where the function arguments are:
x    The value at which the chisquare distribution is to be evaluated (must be ≥ 0). 
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 above chart on the right shows the righttailed chisquare distribution, with 1, 2 and 3 degrees of freedom.
The value of any of these functions, at a specified value of x can be calculated using the Excel Chidist function. Examples are shown in the spreadsheet below:
Formulas:

Results:

For further information and examples of the Excel Chidist function, see the Microsoft Office website.
If you get an error from the Excel Chidist function this is likely to be one of the following:
#NUM!   
Occurs if either:

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