TDIST function
Returns the Percentage Points (probability) for the Student tdistribution where a numeric value (x) is a calculated value of t for which the Percentage Points are to be computed. The tdistribution is used in the hypothesis testing of small sample data sets. Use this function in place of a table of critical values for the tdistribution.
Important This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. Although this function is still available for backward compatibility, you should consider using the new functions from now on, because this function may not be available in future versions of Excel.
For more information about the new functions, see T.DIST.2T function and T.DIST.RT function.
Syntax
TDIST(x,deg_freedom,tails)
The TDIST function syntax has the following arguments:

X Required. The numeric value at which to evaluate the distribution.

Deg_freedom Required. An integer indicating the number of degrees of freedom.

Tails Required. Specifies the number of distribution tails to return. If tails = 1, TDIST returns the onetailed distribution. If tails = 2, TDIST returns the twotailed distribution.
Remarks

If any argument is nonnumeric, TDIST returns the #VALUE! error value.

If deg_freedom < 1, TDIST returns the #NUM! error value.

The deg_freedom and tails arguments are truncated to integers.

If tails is any value other than 1 or 2, TDIST returns the #NUM! error value.

If x < 0, then TDIST returns the #NUM! error value.

If tails = 1, TDIST is calculated as TDIST = P( X>x ), where X is a random variable that follows the tdistribution. If tails = 2, TDIST is calculated as TDIST = P(X > x) = P(X > x or X < x).

Since x < 0 is not allowed, to use TDIST when x < 0, note that TDIST(x,df,1) = 1 – TDIST(x,df,1) = P(X > x) and TDIST(x,df,2) = TDIST(x,df,2) = P(X > x).
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article. If you are copying the example in Excel Web App, copy and paste one cell at a time.
Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

Create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Web App, repeat copying and pasting for each cell in the example.
Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.


Note To view the result as a percentage, select the cell, and then on the Home tab, in the Number group, click the arrow next to Number Format, and click Percentage.