In Excel 2010, the TINV function has been replaced by the T.INV.2T function.
Although the Tinv function has been replaced, it is still available in Excel 2010, (stored within the list of compatibility functions), to allow compatibility with earlier versions of Excel.
However, the Tinv function may not be available in future versions of Excel, so it is advised that you use the T.INV.2T function if possible.
The Excel TINV function calculates the inverse of the twotailed Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The syntax of the function is:
Where the function arguments are:
probability    The probability (between 0 and 1) for which you want to evaluate the inverse of the twotailed Student's T Distribution. 
degrees_freedom    The number of degrees of freedom (must be ≥ 1). 
Note that:
The Excel Tinv Function calculates the inverse of the twotailed Student's T Distribution. If you want to calculate the inverse of the onetailed tdistribution you can simply double the probability.
I.e. The onetailed inverse Student's T Distribution = TINV( 2*probability, degrees_freedom ).The above chart on the right shows the 2tailed Inverse Student's T Distribution with 10 degrees of freedom.
If you want to calculate the value of this function for a probability of 0.25, this can be done using the Excel Tinv function, as follows:
This gives the result 1.221255395.
If you want to calculate the value of the onetailed inverse Student's TDistribution for a probability of 0.25, this can be done by doubling the probability, as follows:
This gives the result 0.699812061.
For further information and examples of the Excel Tinv function, see the Microsoft Office website.
If you get an error from the Excel Tinv function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if one or both of the supplied arguments are nonnumeric. 