In Excel 2010, the TDIST function has been replaced by two functions, the T.DIST.RT function (which calculates the righttailed Student's TDistribution), and the T.DIST.2T function (calculates the twotailed Student's TDistribution).
The Excel 2010 T.DIST.RT has an advantage over the TDIST function, in that it can accept negative values of x. Therefore, although the TDIST function has been kept in Excel 2010 (for compatibility with earlier versions of Excel), if you have Excel 2010, the T.DIST.RT or the T.DIST.2T should be used rather than the TDIST function.
The Excel TDIST function calculates the 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:
x    The value at which you want to evaluate the Student's T Distribution.  
degrees_freedom    The number of degrees of freedom (must be ≥ 1).  
tails    The number of tails for the distribution. This must be either:

Note that the Excel Tdist Function doesn't allow the argument x to be < 0. Therefore, if you want to calculate the Student's T Distribution for values of x that are < 0, you should use the relationships:
TDIST( x, df, 1 ) = 1  TDIST( x, df, 1 ) and TDIST( x, df, 2 ) = TDIST( x, df, 2 ) 
The above chart on the right shows the 1tailed 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 Tdist function, as follows:
This gives the result 0.170446566, or 17.04%
If you want to calculate the value of the function, at x = 1, this must be done using the relationship TDIST( x, df, 1 ) = 1TDIST( x, df, 1 ). Therefore, the formula to calculate the function at x = 1 is:
This gives the result 0.829553434, or 82.96%.
The above chart on the right shows the twotailed 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 Tdist function, as follows:
This gives the result 0.340893132, or 34.09%.
If you want to calculate the value of the function, at x = 1, this must be done using the relationship TDIST( x, df, 2 ) = TDIST( x, df, 2 ). Therefore, the formula to calculate the function at x = 1 is simply:
which, as shown above, gives the result 0.340893132, or 34.09%.
Further information and examples of the Excel Tdist function can be found on the Microsoft Office website.
If you get an error from the Excel Tdist function this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 