Convert a z-statistic into one-sided and two-sided p-values under the standard normal curve.
| Two-sided p | – |
| One-sided p | – |
Use this calculator after a z-test when you already have the test statistic z and want the matching p-values. The one-sided value is the upper tail beyond |z|; the two-sided value doubles that tail for a symmetric alternative.
The formulas are: one-sided p = 1 − Φ(|z|) and two-sided p = 2 (1 − Φ(|z|)), where Φ is the standard normal CDF. A z of 1.96 gives a two-sided p ≈ 0.05 and a one-sided p ≈ 0.025. The sign of z is ignored because the tail is taken from |z|.
This is a normal approximation, not a t-test. It does not apply a continuity correction or adjust for multiple tests. Interpret p against your chosen significance level; it is not the probability that the null is true.
z: Test statistic from a z-test. Sign is ignored.
Two-sided p: Probability of a result at least as extreme as |z| in either tail.
One-sided p: Upper-tail probability beyond |z|.
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