The test statistic for a Student **t-test** hypothesis test is called a **t-statistic**. The t-statistic is used to measure how extreme an estimate is. The formula used to calculate t-statistic is the following: $$ t = \frac{x - \mu}{s / \sqrt{n}} $$ Where \(x\) is the mean, \(\mu\) is the hypothesized mean (in many situations, the hypothesized value is 0), \(s\) is the standard deviation and \(n\) is the sample size. **Degrees of freedom** determines how much a final calculation of a statistic is free to vary. The formula used to calculate it is the following: $$ df = n - 1 $$ Use the calculator below to solve your t-statistic problems.