P-Value Calculator (Z-Test)
Calculate the p-value for a z-score in a one-tailed or two-tailed test, and check it against common significance levels.
P-value
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Significant at α=0.05? —
Significant at α=0.01? —
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How it works
One-tailed p = 1 − Φ(|z|); two-tailed p = 2 × (1 − Φ(|z|))Frequently Asked Questions
How do I calculate a p-value from a z-score?
Find the area under the standard normal curve beyond your z-score. A z-score of 2.0 gives a one-tailed p-value of about 0.0228 and a two-tailed p-value of about 0.0456 — both below the common 0.05 significance threshold.
One-tailed or two-tailed — which should I use?
Two-tailed tests whether an effect exists in either direction (most common, more conservative); one-tailed tests a specific direction only, and should be chosen before seeing the data, not after, to avoid inflating significance.
What does 'statistically significant' mean?
Conventionally, a p-value below your chosen threshold (often 0.05) means the observed result would be unlikely under the null hypothesis. It does not measure the size or practical importance of an effect — only how surprising the data would be if there were truly no effect.