Frequently Asked Questions
Everything you need to know about the P-Value Calculator.
What is a p-value?
The probability of observing a result as extreme as (or more extreme than) yours, assuming the null hypothesis is true.
Which statistic type should I choose?
Match it to the test you ran: z for a z-test, t for a t-test, chi-square for a goodness-of-fit/independence test, F for ANOVA or a variance-ratio test.
What's the difference between one-tailed and two-tailed?
A one-tailed test only considers extremity in one direction; two-tailed considers both, and generally gives a larger p-value for the same statistic.
Do chi-square and F tests use tails differently?
Yes, they're typically evaluated as right-tailed tests since their statistics are always non-negative; the calculator reflects this.
What degrees of freedom do I enter?
For t and chi-square, one df value; for F, two (numerator and denominator degrees of freedom).
What does a small p-value mean?
Conventionally, p < 0.05 is considered statistically significant, meaning the observed result is unlikely under the null hypothesis โ though significance thresholds depend on your field and study design.
Does it show a graph?
Yes, a shaded curve of the relevant distribution highlights the area corresponding to your p-value.
How accurate is the calculation?
It uses precise CDF approximations for each distribution, matching standard statistical tables to several decimal places.
Is my data stored?
No, everything is calculated client-side in your browser.
Is it free?
Yes, completely free with no sign-up or limits.