P-Value Calculator
What it is, and pointedly what it is not.
Work out P-Value. What it is, and pointedly what it is not. Shows the working, not just the answer.
Two-tailed p-value
0.05
Moderate evidence against the null hypothesis
A p-value is the probability of a test statistic at least this extreme if the null hypothesis were true and the model's assumptions held. It is not the probability that the null hypothesis is true, and it is not the probability your result was a fluke — those are different quantities and the confusion between them is the single most consequential error in applied statistics. 0.05 is a convention from the 1920s, not a law of nature, and nothing distinguishes 0.049 from 0.051 in evidential terms. The strength wording above deliberately describes a gradient rather than a verdict. A p-value alone says nothing about how large an effect is. With a big enough sample, a difference too small to care about will clear any threshold you like — which is why the effect size and the confidence interval matter more than this number does.
How the P-Value Calculator works
p-values from z, t, chi-square and F statistics, computed rather than looked up so the answers are continuous instead of snapping to a printed table. The strength of evidence is described as a gradient, because 0.049 and 0.051 differ by nothing.
Also known as: p value from z score · what does p 0.05 mean · is my result significant · convert t statistic to p value
Frequently asked questions
What is a p-value?
The probability of a test statistic at least this extreme if the null hypothesis were true and the model's assumptions held. It is a statement about data under an assumed model, not about the hypothesis.
Is a p-value the probability the null hypothesis is true?
No, and this is the most consequential misunderstanding in applied statistics. It is also not the probability your result was a fluke. Those are different quantities, and getting them from a p-value requires a prior you have not supplied.
What is the difference between one-tailed and two-tailed?
A one-tailed test asks whether the effect goes in a specified direction; two-tailed asks whether it differs in either. Two-tailed is the default, and switching to one-tailed after seeing the data doubles your effective false positive rate.
Why is 0.05 the threshold?
Convention, dating to Fisher in the 1920s, and he treated it as a rough guide rather than a rule. Nothing evidential distinguishes 0.049 from 0.051, which is why the strength wording here is a gradient.
Can a significant result be unimportant?
Routinely. With a large enough sample, an effect too small to care about will clear any threshold. That is why the effect size and confidence interval matter more than the p-value does.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/p-value-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="P-Value Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/p-value-calculator" style="color:#64748b">P-Value Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.