P-Value Calculator
A p-value converts a test statistic into the probability your hypothesis test actually reports. Enter a z-score or a t-statistic (with degrees of freedom), pick the tail that matches your alternative hypothesis, and get the p-value with a plain-English reading.
Results
Formula
How to use this calculator
- Choose the z or t distribution based on your test.
- Enter the test statistic (and degrees of freedom for a t test).
- Select the tail that matches your alternative hypothesis, then Calculate.
Worked example
A two-tailed z test gives z = 1.96.
- The standard normal has 2.5% of its area beyond +1.96 and 2.5% below −1.96.
- Two-tailed p = 0.025 + 0.025 = 0.05.
Result: p ≈ 0.0500 — exactly at the conventional 0.05 boundary.
Frequently asked questions
What does a p-value of 0.03 mean?
If the null hypothesis were true, results at least this extreme would occur about 3% of the time. It does not mean there is a 3% chance the null is true, or a 97% chance the effect is real.
One-tailed or two-tailed — which is right?
Match your alternative hypothesis, decided before looking at the data. Most reported tests are two-tailed because effects in either direction would matter.
Why does the t test need degrees of freedom?
The t distribution’s shape depends on sample size — heavier tails for small samples. Degrees of freedom (usually n − 1) pin down which t curve to use.
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