P Value Calculator

This calculator uses the Student t distribution. For large degrees of freedom (above 30), the t distribution closely approximates the standard normal, so results will be nearly identical to a z-based p-value. For exact normal p-values, use a z-score based tool.

Use two-tailed when your alternative hypothesis is non-directional (you are testing for any difference). Use one-tailed only when your hypothesis specifies a direction (e.g., the mean is greater than, not just different from, the null value) and you decided this before collecting data.

The conventional threshold is 0.05 (5%). A p-value below 0.05 is typically considered statistically significant. However, some fields use 0.01 or 0.001 for stricter evidence standards, and the threshold should be set before data collection.

Lower degrees of freedom produce a t distribution with heavier tails, meaning you need a larger t statistic to achieve the same p-value. As df increases, the distribution approaches the normal distribution and p-values converge toward z-test values.

In theory, p-values are continuous and never exactly zero. In practice, very large t statistics with reasonable df produce p-values so small that computers round them to zero. The calculator shows these as less than 0.0001.

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