The Excel T.DIST function calculates the onetailed Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The T.DIST function is new to Excel 2010 and so is not available in earlier versions of Excel.
The format of the function is:
where the function arguments are:
x    The value at which you want to evaluate the distribution.  
degrees_freedom    The number of degrees of freedom (must be ≥ 1).  
cumulative    A logical argument which denotes the type of distribution to be used:  

The above chart on the right shows the onetailed cumulative Student's T Distribution with 10 degrees of freedom.
If you want to calculate the value of this function at x = 1, this can be done using the Excel T.Dist function, as follows:
This gives the result 0.829553434, or 82.96%.
The above chart on the right shows the onetailed probability density function for the Student's T Distribution, with 10 degrees of freedom.
If you want to calculate the value of this function at x = 1, this can be done using the Excel T.Dist function, as follows:
This gives the result 0.230361989, or 23.04%.
Further information and examples of the Excel T.Dist function can be found on the Microsoft Office website.
If you get an error from the Excel T.Dist function this is likely to be one of the following:
#NUM!    Occurs if either:

#DIV/0!    Occurs if cumulative is FALSE and degrees_freedom = 0. 
#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 