The Excel T.INV function calculates the lefttailed inverse of the Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The T.INV function is new in Excel 2010, and so is not available in earlier versions of Excel.
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 lefttailed 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.
For further information and examples of the Excel T.Inv function, see the Microsoft Office website.
If you get an error from the Excel T.Inv 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. 