P-value Calculator
Calculate the p-value from a z-score or t-statistic for hypothesis testing. Compares one- and two-tailed tests.
Inputs
Two-tailed p-value
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One-tailed p-value
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Significant at p < 0.05?
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Significant at p < 0.01?
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Significance code
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About P-value Calculator
What p-value means
The probability of seeing a test statistic at least as extreme as yours, *assuming the null hypothesis is true*. Small p-values cast doubt on the null. p < 0.05 is conventional but arbitrary.
One-tailed vs two-tailed
Two-tailed: tests "is there any effect?". One-tailed: tests "is the effect in this specific direction?". One-tailed has more power but requires a strong prior reason for the direction.
P-hacking
Running many tests and reporting the smallest p-value inflates false positives. Modern best practice: pre-register hypotheses, use Bonferroni or FDR correction for multiple tests, and emphasize effect sizes over significance.