📈 p-value Calculator
Enter your z-score or t-statistic (and degrees of freedom for a t-test) to get the matching one-tailed and two-tailed p-values.
What a p-value tells you
A p-value is the probability of observing a test statistic at least as extreme as the one calculated from your sample, assuming the null hypothesis is true. Smaller p-values indicate stronger evidence against the null hypothesis. This calculator converts a z-score or t-statistic directly into its corresponding p-value.
The method used
For a z-score, the p-value is derived from the standard normal distribution’s cumulative distribution function using a well-known polynomial approximation of the error function. For a t-statistic, the p-value is computed from the Student’s t-distribution using the regularized incomplete beta function, the standard numerical method for t-distribution tail probabilities, combined with your specified degrees of freedom. A two-tailed p-value simply doubles the one-tailed tail probability.
- Degrees of freedom only matter for the t-statistic option; they are ignored for z-scores.
- A common (but arbitrary) significance threshold is 0.05 — always interpret p-values alongside effect size and study design, not in isolation.
This calculator is for general educational and statistical-planning purposes only.