P-Value Calculator
The p-value calculator computes the p-value from a test statistic (z-score or t-score) and test direction (one-tailed or two-tailed). The p-value is the probability of observing a test statistic as extreme as the one obtained, assuming the null hypothesis is true. A smaller p-value suggests stronger evidence against the null hypothesis. For a z-score, enter the score and select one-tailed or two-tailed. For a t-score, enter the score, degrees of freedom, and test direction. The calculator uses the normal distribution for z-scores and an approximation for t-distributions.
Interpretation
If p-value < 0.05: Typically considered statistically significant; reject the null hypothesis.
If p-value >= 0.05: Typically considered not significant; fail to reject the null hypothesis.
Common significance levels are 0.05 (5%), 0.01 (1%), and 0.001 (0.1%).
P-value calculator: frequently asked questions
What is a p-value?
A p-value is the probability of observing data as extreme or more extreme than what was actually observed, assuming the null hypothesis is true. Small p-values (typically less than 0.05) suggest the observed data is unlikely under the null hypothesis.
What is the difference between one-tailed and two-tailed?
A one-tailed test checks in one direction (e.g., is X greater than expected?). A two-tailed test checks both directions (e.g., is X different from expected?). The two-tailed p-value is approximately twice the one-tailed p-value (for symmetric distributions).
What is a z-score?
A z-score measures how many standard deviations a value is from the mean. In hypothesis testing, the z-score comes from (sample statistic - null hypothesis value) / standard error.
What is a t-score?
A t-score is like a z-score but used when the population standard deviation is unknown and the sample size is small. The t-distribution has heavier tails than the normal distribution. You need to specify the degrees of freedom.
When do I use p-values?
P-values are used in hypothesis testing. If the p-value is less than your significance level (usually 0.05), you reject the null hypothesis. If the p-value is greater, you fail to reject it.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.