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P-value Calculator

P-values from a z, t, chi-square or F statistic.

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Significance

ThresholdTwo-tailedOne-tailed

Two things a p-value is not

It is not the probability that the null hypothesis is true. It is the probability of data at least this extreme given that the null is true, which is a different conditional and not interchangeable with it.

It is not a measure of effect size. With a large enough sample, a difference too small to care about will produce a very small p-value. Always report an effect size and a confidence interval alongside it.

Two-tailed p-value

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Details

One-tailed (right)—
One-tailed (left)—
Distribution—
Statistic—

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At the conventional 0.05 threshold

A p-value is the probability of a result at least as extreme as yours if the null hypothesis were true. The 0.05 threshold is a convention, not a law of nature. Choose one- or two-tailed before seeing the data — switching afterwards to reach significance is a well-known misuse.

Quick Tool Overview

The P-value Calculator converts a test statistic into a p-value for the normal, Student's t, chi-square and F distributions, giving one- and two-tailed results with significance flagged at the usual thresholds.

What it does
Converts a test statistic into a p-value
You give it
A statistic, a distribution and degrees of freedom
You get back
One- and two-tailed p-values with a significance verdict
Best for
Hypothesis testing and interpreting statistical output
Calculators Free, unlimited use No sign-up Runs in your browser

About the P-value Calculator

Turn a test statistic into a p-value for the four distributions that cover most introductory and applied statistics: normal, Student's t, chi-square and F. One-tailed and two-tailed results are given, along with the verdict at the usual significance thresholds.

A p-value is the probability of seeing a result at least as extreme as yours if the null hypothesis were true. It is not the probability that the null hypothesis is true, and it is not a measure of effect size — a tiny, meaningless difference will produce a very small p-value given a large enough sample. Those two misreadings cause most of the trouble p-values get blamed for, so the page states both alongside the result rather than in a footnote.

Key features of the P-value Calculator

Four distributions

Normal, t, chi-square and F, covering most introductory and applied statistics.

One- and two-tailed

Both are shown, with a note that switching to one-tailed after seeing the data is a known misuse.

Thresholds flagged

Significance at 0.05, 0.01 and 0.001 marked so you do not have to compare by eye.

The two misreadings addressed

A p-value is not the probability the null is true, and it is not an effect size — both stated next to the result.

Accurate tail areas

Computed numerically rather than interpolated from a printed table.

How to use the P-value Calculator

  1. 1

    Choose the distribution

    Normal (z), Student's t, chi-square or F, depending on your test.

  2. 2

    Enter the test statistic

    The value your test produced.

  3. 3

    Add the degrees of freedom

    Needed for t, chi-square and F. The z distribution does not use them.

  4. 4

    Read the p-value

    One-tailed and two-tailed, with significance flagged at 0.05, 0.01 and 0.001.

Frequently asked questions

The probability of observing a result at least as extreme as yours, assuming the null hypothesis is true. A p-value of 0.03 means such a result would occur 3 percent of the time by chance alone.

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