P-Value Calculator
Choose the distribution of your test statistic, the type of test and enter the statistic with its degrees of freedom. The calculator returns the p-value, the critical value or values for your significance level, and whether the result is significant. It covers the normal (z), Student's t, chi-square and F distributions. Everything runs in your browser.
The p-value is the chance of a test statistic at least as extreme as yours if the null hypothesis is true. It is not the chance that the null hypothesis is true, and a small p-value does not show that an effect is large or important. For a two-sided z or t test it is twice the smaller tail. The chi-square and F distributions are not symmetric, so the two-sided value here is twice the smaller tail, capped at 1, which is the common convention. Most chi-square and F tests use only the right tail. Everything runs in your browser.
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A p-value is the chance of getting a test statistic at least as extreme as yours if the null hypothesis is true. A small p-value means the data would be surprising under the null hypothesis, and you reject it when the p-value is below your significance level, usually 0.05. For a z score of 1.96 the two-sided p-value is 0.04999579, which is why 1.96 is the usual cutoff at 5%. A t value of 2.1 with 15 degrees of freedom gives a two-sided p-value of 0.05306, just above 0.05, and the critical value at that level is 2.1314. A chi-square value of 7.8 with 3 degrees of freedom has a right-tail p-value of 0.05033, against a critical value of 7.8147. An F value of 3.5 with 2 and 27 degrees of freedom gives 0.04451, against a critical value of 3.3541. The tails are computed from the incomplete gamma and beta functions, so even tiny p-values are accurate. The chi-square and F tests normally use only the right tail. The p-value is not the probability that the null hypothesis is true, and it says nothing about how large an effect is.
What is a p-value?▾
It is the chance of a test statistic at least as extreme as the one you observed, assuming the null hypothesis is true. It is not the chance that the null hypothesis is true.
What p-value is significant?▾
The usual cutoff is 0.05, so a p-value below 0.05 is called statistically significant. Some fields use 0.01 or 0.001, and the level should be chosen before looking at the data.
What is the difference between one-tailed and two-tailed?▾
A two-tailed test looks for a difference in either direction and doubles the smaller tail. A one-tailed test looks for a difference in one direction only.
Which tail do chi-square and F tests use?▾
Almost always the right tail, because large values signal a poor fit or large differences between groups. The two-sided option is the smaller tail doubled, capped at 1.
Is my data uploaded?▾
No. The calculation runs in your browser and nothing you enter is sent anywhere.