T.INV function
This article describes the formula syntax and usage of the T.INV function in Microsoft Excel.
Returns the lefttailed inverse of the Student's tdistribution.
Syntax
T.INV(probability,deg_freedom)
The T.INV function syntax has the following arguments:

Probability Required. The probability associated with the Student's tdistribution.

Deg_freedom Required. The number of degrees of freedom with which to characterize the distribution.
Remarks

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

If probability <= 0 or if probability > 1, T.INV returns the #NUM! error value.

If deg_freedom is not an integer, it is truncated.

If deg_freedom < 1, T.INV returns the #NUM! 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.
Formula 
Description 
Result 
=T.INV(0.75,2) 
The lefttailed inverse of the Student's tdistribution with a probability of 75% and 2 degrees of freedom. 
0.8164966 