How to use the P-Value Calculator
- 1. Pick the testChoose z, t, chi-square or F, and enter the statistic.
- 2. Add degrees of freedomEnter df for t and χ², or df1 and df2 for F, and choose the tail.
- 3. Get the p-valueSee the p-value, both tail areas and the decision at your α.
What the p-value means
The p-value is the probability of getting a test statistic at least as extreme as yours if the null hypothesis were true. Small p-values (below your chosen α, often 0.05) count as evidence against the null hypothesis. For a two-tailed z or t test, the p-value is twice the smaller tail area, because an extreme result in either direction counts. χ² and F tests are almost always right-tailed.
The tool computes tail areas from the exact distributions using the regularised incomplete gamma and beta functions.
Worked example
A paired t-test on 15 students gives t = 2.31 with 14 degrees of freedom. The right-tail area is 0.0183, so the two-tailed p-value is 0.0366, below 0.05: reject the null hypothesis. For a χ² goodness-of-fit test with χ² = 7.81 and 3 degrees of freedom, the right-tailed p-value is 0.0501, just above 0.05, so you don't reject at the 5% level. 7.81 is almost exactly the textbook critical value.
Accuracy and caution
- Results match SciPy's distribution functions to 9 significant digits, checked on 400 random cases.
- A p-value isn't the probability that the null hypothesis is true, and it says nothing about how big or important an effect is.
- Decide α before looking at the data, and report the effect size alongside p.
Frequently asked questions
One-tailed or two-tailed?
Use one-tailed only if you decided in advance that only one direction matters. Otherwise use two-tailed.
Where do I get the test statistic?
From your test's formula or software output; our ANOVA calculator gives the F statistic for comparing group means.
Is the p-value calculator free?
Yes. You can open the p-value calculator and enter everything without signing up. When you calculate a p-value for the first time, we ask once for your name and WhatsApp number; after that it, and every other Uncodemy tool, opens straight away.