P-Value Calculator

How to use

1. 1Pick the test family: z-test, t-test, chi-square, or F-test. The form fields update for the degrees of freedom that test needs.
2. 2Type the test statistic you already have (for example 1.96 for a z-test, 2.04 for a t-test, 3.84 for chi-square, 4.26 for an F-test).
3. 3Enter the degrees of freedom: ν for t, k for chi-square, or d1 and d2 (numerator and denominator) for F. The z-test does not need any.
4. 4Choose the tail: two-tailed for symmetric z and t tests, upper (right) tail for the usual chi-square and F report, or left tail for textbook problems.
5. 5Read the p-value, the percent of the distribution in the chosen tail, the reject or fail-to-reject decision at alpha 0.10, 0.05, and 0.01, and the plain-English interpretation.
6. 6Press Copy summary to grab a one-line result, or Reset to start over.

About this tool

P-Value Calculator returns the exact one-tailed or two-tailed p-value for the four test statistics that come up in almost every introductory and applied statistics workflow: the standard normal z, Student's t with degrees of freedom (ν), the chi-square statistic with k degrees of freedom, and the F-ratio with numerator (d1) and denominator (d2) degrees of freedom. Pick the test family, paste the test statistic and any required degrees of freedom, choose the tail direction (two-tailed for symmetric z and t tests, upper-tail for chi-square and F by default), and the page returns the p-value to six significant figures, a percent-of-distribution reading, and a reject-or-fail-to-reject decision at the conventional alphas 0.10, 0.05, and 0.01. The math is closed-form: the standard normal CDF uses the Abramowitz and Stegun erf approximation, the t and F CDFs use the regularized incomplete beta function from the Numerical Recipes continued fraction, and chi-square uses the regularized lower incomplete gamma function (series for x below a+1, continued fraction otherwise), with the Lanczos log-gamma approximation underneath. Everything runs locally in your browser; the test statistic and degrees of freedom you paste never leave your device. Useful for grading a t-test result from a research paper, sanity-checking an ANOVA F-ratio from SPSS, R, or Python, converting a z-statistic from a proportions test or an A/B test into a p-value, computing a chi-square goodness-of-fit p-value during exam prep, or any time you have a test statistic in hand and need the p-value without re-running the full analysis. Pair this tool with the Z-Score Calculator when you need to first convert a raw value to a z, with the Confidence Interval Calculator when you want the matching interval estimate, or with the A/B Test Significance Calculator when you are starting from conversions and visitors instead of a statistic.

Free to use. Works in your browser. No signup, no login.