P-value Calculator

A p-value calculator is a statistical tool used to measure how likely an observed result would be if the null hypothesis were true. In hypothesis testing, the p-value helps decide whether the evidence against the null hypothesis is weak or strong.

This online p-value calculator lets you calculate p-values from a z-score directly or estimate a p-value from a t-score using a normal approximation. It is useful for statistics students, researchers, analysts, and anyone working with hypothesis tests.

To calculate a p-value from a z-score, enter the z-score and choose whether your test is left-tailed, right-tailed, or two-tailed. The calculator then uses the standard normal distribution to return the probability.

To calculate an approximate p-value from a t-score, enter the t-score and the degrees of freedom. This tool uses a normal approximation, which becomes more accurate as the degrees of freedom increase.

A smaller p-value means the observed result would be less likely if the null hypothesis were true. For example, if your p-value is below 0.05, many analyses would treat the result as statistically significant.

A larger p-value means the data are more consistent with the null hypothesis. That does not prove the null hypothesis is true, but it does mean the evidence against it is weaker.

Interpreting a p-value always depends on context, study design, chosen significance level, and whether the test is one-tailed or two-tailed.

A left-tailed test is used when you want to check whether the result is significantly lower than expected. A right-tailed test is used when you want to check whether the result is significantly higher than expected.

A two-tailed test is used when either a lower or higher result would matter. In many real-world hypothesis tests, a two-tailed p-value is the default because it checks for differences in both directions.