P-value calculator
The p-value of a z, t, chi-square or F test statistic, one- or two-tailed.
About this calculator
This p-value calculator turns a test statistic into a p-value. It works with z-scores, t-scores, chi-square and F statistics.
Choose a one- or two-tailed test and your significance level, and it tells you whether the result is statistically significant.
How it works
The p-value is the probability of a result at least as extreme as yours if the null hypothesis is true: the area under the normal, t, chi-square or F curve beyond your statistic, worked out with the incomplete gamma and beta functions.
Last reviewed:
Worked example
- Test statistic: Z-score (normal)
- Value of the statistic: 2.1
- Test: Two-tailed
- Significance level: 5%
P-value: 0.035729 (significant at this level: reject the null hypothesis)
Common questions
What does a p-value of 0.05 mean?
If the null hypothesis were true, you would see a result at least this extreme 5% of the time. It doesn't mean there is a 95% chance your hypothesis is right.
Should I use a one-tailed or two-tailed test?
Use two-tailed when a difference in either direction matters, which is the usual choice. Use one-tailed only if you decided beforehand to test one direction.
What is a significant p-value?
One below your chosen significance level, often 0.05. The result is then called statistically significant, and the null hypothesis is rejected.
Sources and further reading
The story behind it
Where the idea came from: Ronald Fisher popularised the 5% significance level in his 1925 book Statistical Methods for Research Workers.
Fisher meant it as a rough guide, not a fixed rule. In 2016 the American Statistical Association warned that a p-value alone does not measure the size of an effect or the importance of a result.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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