P-Value Calculator (from Z)
Convert a z-score to a one- or two-tailed p-value.
Results update as you type.
About this calculator
This calculator converts a z-score (standard-normal test statistic) into a p-value — the probability of observing a result at least as extreme as yours if the null hypothesis is true. Choose a left-tailed, right-tailed or two-tailed test. The p-value comes from the standard-normal cumulative distribution, so it applies to z-tests and large-sample approximations.
Frequently asked questions
What is a p-value?
The p-value is the probability, assuming the null hypothesis is true, of getting a test statistic at least as extreme as the one observed. A small p-value means your data would be unlikely under the null hypothesis, which is evidence against it.
How does a two-tailed p-value differ from one-tailed?
A two-tailed test counts extreme results in both directions, so its p-value is double the one-tailed value for the same |z|. Use two-tailed when you only care that there is a difference, and one-tailed when you predicted the direction in advance.
Is p < 0.05 always significant?
0.05 is only a common convention, not a law. It means a 5% chance of a false positive. Depending on the field and the cost of being wrong, stricter thresholds like 0.01 are often more appropriate.
Results are estimates for general guidance only, not financial, medical or tax advice.