About this tool
The P-value Calculator is a free, in-browser tool that converts a test statistic — z, Student's t, chi-square (χ²) or F — into its p-value. It reports the left-tailed, right-tailed and two-tailed probabilities and compares your chosen tail against a significance level α to give an instant significant / not significant verdict.
The normal CDF is evaluated through the regularized incomplete gamma function, and the t, χ² and F distributions use the standard regularized incomplete beta and gamma functions, so results match statistical tables to the displayed precision. Enter the degrees of freedom where required (one value for t and χ², numerator and denominator for F) and everything recomputes live in your browser — no data leaves your device.
Frequently asked questions
What is a p-value?
It is the probability, assuming the null hypothesis is true, of observing a test statistic at least as extreme as yours. A p-value below your significance level α (commonly 0.05) is conventionally called statistically significant.
Should I use a one-tailed or two-tailed p-value?
Use two-tailed when your alternative hypothesis is a difference in either direction (the usual default for z and t tests), and one-tailed only when the direction was specified in advance. Chi-square and F tests are conventionally right-tailed.
What is the two-tailed p-value for chi-square and F?
These distributions are not symmetric, so the tool uses the standard doubling rule: twice the smaller of the left- and right-tail probabilities, capped at 1. For most goodness-of-fit and variance-ratio tests you should still read the right-tailed value.
How accurate are the results?
All four distributions — normal, t, χ² and F — are evaluated with regularized incomplete beta/gamma series iterated to about 14 digits, so p-values agree with standard software at the six decimals displayed; values below 0.000001 are reported as < 0.000001.