Convert a test statistic into a p-value. This works for a z-statistic, or a t-statistic when you also supply the degrees of freedom.
How to use it
Choose whether your statistic is a z or a t, enter its value, add the degrees of freedom for a t-statistic, and pick one- or two-tailed. Two-tailed is the usual default.
How to read the result
The p-value is the probability of seeing a result at least as extreme as yours if there were truly no effect. A value below 0.05 is conventionally called statistically significant, but remember that a small p-value does not tell you how large or important the effect is.
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Frequently asked questions
What does the p-value mean?
It is the probability of observing your result, or something more extreme, purely by chance if the null hypothesis were true. Small values cast doubt on the null hypothesis.
Should I use one-tailed or two-tailed?
Two-tailed is the safe default and tests for a difference in either direction. Only use one-tailed if you genuinely predicted the direction in advance and would ignore an effect the other way.
Is p < 0.05 always meaningful?
No. Significance depends heavily on sample size, and a tiny, unimportant effect can be significant in a large study. Always report an effect size alongside the p-value.