Type the confidence level into your calculator, click the divide button, type "100" and then click the equals button. As an example, use a confidence level of 95%. Dividing 95 by 100 gives you .95. This number is called the "cumulative probability."
Type "1" into your calculator, click the subtract button, type the result from Step 1 and then click equals to calculate alpha. In the example, you would subtract 0.95 from 1 and get 0.05. Alpha is 0.05.
Enter the alpha number into your calculator, click the divide button, click "2" and then click the equals button. In the example, dividing 0.05 by 2 gives you 0.025.
Type "1" into your calculator, click the subtract button, type the result from Step 3 and click the equals button. In the example, subtracting 0.025 from 1 gives you 0.975.
Find the degrees of freedom by typing the sample size into the calculator, clicking the subtract button, typing "1" and then clicking the equals button. As an example, the sample size is 500. Subtracting 1 from 500 gives you 499.
Navigate to an online t distribution calculator, select "t score," enter the degrees of freedom and the cumulative probability as a decimal. Click "Calculate." The critical value will be displayed in the box beside "t score." Using the example, when you enter 499 degrees of freedom and a cumulative probability of 0.95 into the calculator, you get a critical value of 1.648.