Student t-Value Calculator. In order to calculate the Student T Value for any degrees of freedom and given probability. The calculator will return Student T Values for one tail (right) and two tailed probabilities. Please input degrees of freedom and probability level and then click "CALCULATE" The function gives the critical value of t for the one-tailed test. If you want the critical value of t for a two-tailed test, divide the significance level by two. Example: Calculating the critical value of t in R To calculate the critical value of t for a two-tailed test with df = 29 and α = .05: To use this T critical value (t table) calculator, enter values in input box. The t table value calculator will instantly give you the t value for one tailed probability as well as two tailed probability. This table contains critical values of the Student's t distribution computed using the cumulative distribution function . The t distribution is symmetric so that t1-α,ν = -tα,ν . The t table can be used for both one-sided (lower and upper) and two-sided tests using the appropriate value of α . bZvM.