P-Value Calculator accuracy and testing

Formula source

• The P-value is the area under the standard normal distribution curve beyond the given Z-score. * **One-Tailed:** Uses the tail implied by the sign of the Z-score. Positive Z uses the right tail, while negative Z uses the left tail. * **Two-Tailed:** Calculates area in both directions (extreme positive OR extreme negative), typically doubling the smaller one-tailed area.

Test cases

Known limitations

• This page documents the checked-in calculator logic and does not certify results for every real-world edge case.
• Use the result as a planning estimate unless the calculator page states a stricter purpose.

Assumptions

• This calculator expects a z-score from a z-test or normal-approximation workflow.
• Choose one-tailed only when your hypothesis was directional before you saw the data.
• The p-value is evidence against the null hypothesis, not the probability that the null or alternative is true.

Unit handling

• Z-Score accepts numeric values.
• Test Type accepts one of the configured options.

Rounding rules

• Number displays use up to 2 decimal places in the calculator UI.

Version history

Change log