The T.INV function calculates the onetailed inverse Student's TDistribution. This function is new to Excel 2010, and so is not availiable in earlier versions of Excel.
The T.INV function is not to be confused with the TINV function, which is available in earlier versions of Excel. The TINV function calculates the twotailed inverse Student's TDistribution and is replaced in Excel 2010 by the T.INV.2T function.
The Excel T.INV function calculates the twotailed inverse Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The format 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) 
The above chart on the right shows the lefttailed 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 T.Inv function, as follows:
This gives the result 0.699812061.
Further information and examples of the Excel T.Inv function can be found on the Microsoft Office website.
If you get an error from your Excel T.Inv function this is likely to be one of the following :
#NUM!   
Occurs if either:

#VALUE!    Occurs if either of the supplied arguments are nonnumeric 