About the P-Value Calculator
A p-value tells you how surprising your test result would be if the null hypothesis were true. This p-value calculator turns a test statistic you already have into a p-value. It supports the four distributions used most in statistics courses and research: the standard normal (z), Student's t, chi-square and F. You choose a left-tailed, right-tailed or two-tailed test, enter the degrees of freedom where they apply and a significance level, and the tool shows the p-value and whether the result is significant at that level.
Updated October 2026
How to use the P-Value Calculator
- 1Choose the type of test statistic: z, t, chi-square or F.
- 2Enter the value of your statistic.
- 3Enter the degrees of freedom, if the test needs them.
- 4Choose a two-tailed, right-tailed or left-tailed test.
- 5Enter your significance level, usually 0.05, and read the p-value and the verdict.
Checking against known values
A good way to trust a p-value tool is to test it on familiar numbers. A z-score of 1.96 should give a two-tailed p of 0.05. A t-score of 2.228 with 10 degrees of freedom should also give 0.05. A chi-square of 3.841 with 1 degree of freedom and an F of 4.103 with 2 and 10 degrees of freedom each give a right-tailed p of 0.05. This calculator matches all of them.
Common mistakes to avoid
- Treating p as the chance that the null hypothesis is true.
- Choosing a one-tailed test after seeing which direction the data went.
- Running many tests and quoting only the significant ones.
- Confusing statistical significance with practical importance.
- Forgetting to use the right degrees of freedom.
Related statistics tools
The Z-Score Calculator gives you the z statistic from a value, mean and standard deviation. The Confidence Interval Calculator provides an interval estimate, the Standard Deviation Calculator and Average Calculator summarise data, and the Sample Size Calculator helps you plan a study. Use these results for education and analysis, and consult a statistician for important research decisions.
Features
- z, t, chi-square and F distributions
- Left, right and two-tailed p-values
- Significance verdict at your chosen alpha
- Cumulative probability shown too
- Accurate to several decimal places
- Small values shown in scientific form
- Runs in your browser
Frequently asked questions
What is a p-value?
A p-value is the probability of seeing a result at least as extreme as yours if the null hypothesis were true. A small p-value, usually below 0.05, suggests the data do not fit the null hypothesis well. It is not the probability that the null hypothesis is true, and it does not measure the size or importance of an effect.
How do I find the p-value from a z-score?
For a right-tailed test, the p-value is the area to the right of the z-score under the normal curve; for a left-tailed test, the area to the left; for a two-tailed test, double the smaller tail. A z-score of 1.96 gives a two-tailed p-value of about 0.05.
Should I use a one-tailed or two-tailed test?
Use a two-tailed test when you are looking for any difference, higher or lower. Use a one-tailed test only when you decided before seeing the data that you care about one direction. Chi-square and F tests are normally right-tailed because they measure how large the statistic is.
What are degrees of freedom?
Degrees of freedom describe how many values in a calculation are free to vary. For a one-sample t-test they equal the sample size minus 1. For a chi-square goodness-of-fit test they equal the number of categories minus 1. An F test has two: one for the numerator and one for the denominator.
What does it mean if p is below 0.05?
It means the result is statistically significant at the 5 percent level, so you would reject the null hypothesis if you chose 0.05 as your threshold. The cutoff is a convention, not a law, and results close to it deserve caution. Also consider the size of the effect and the quality of the study.
Does this calculator run the test on raw data?
No. It takes the test statistic as input. To get the statistic you need to compute it from your data first, for example a z-score with the Z-Score Calculator. This tool then converts that statistic into a probability.