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.
Syntax
TDIST(x,degrees_freedom,tails)
X is the numeric value at which to evaluate the distribution.
Degrees_freedom is an integer indicating the number of degrees of freedom.
Tails 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 degrees_freedom < 1, TDIST returns the #NUM! error value.

The degrees_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 to copy an example

Create a blank workbook or worksheet.

Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In the worksheet, select cell A1, and press CTRL+V.

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.


Note To view the number as a percentage, select the cell, and then on the Sheet tab, in the Number group, click Percent Style .