P-Value Calculator

Enter a z-score and choose a test type to calculate the p-value.

How to Use the P-value Calculator

A z-score and a test type, one-tailed or two-tailed, are the two inputs — picking the wrong tail for the hypothesis being tested is the most common way to misread this one.

Left-tailed:  p = P(Z ≤ z)
Right-tailed: p = P(Z ≥ z) = 1 − P(Z ≤ z)
Two-tailed:   p = 2 × P(Z ≥ |z|)

A p-value is the probability of seeing a result at least this extreme if nothing real were actually going on. This P-Value Calculator converts a z-score into that probability, for whichever test type applies — two-tailed doubles the one-sided area because it's watching for extremes in either direction, not just one.

Two-tailed is the right default almost always. Reach for a one-tailed test only when your hypothesis specifically predicted a direction before you saw any data — picking the tail after peeking at results is exactly the kind of thing that makes statisticians twitch. As for what counts as "significant": 0.05 is the number everyone's heard, but it's a convention, not a law of nature, and the right bar shifts with the field and what's actually at stake if you're wrong.