Calculate the p-value from a z-score or a t-statistic with degrees of freedom, for one or two-tailed tests. Enter your values; the result updates as you type, with the method shown below.
About this calculator
A p-value is the probability of getting a result at least as extreme as yours if the null hypothesis is true. This calculator turns a z or t statistic into one- and two-tailed p-values and says whether the result is significant at 5% and 1%.
Choose z or t, enter the statistic and, for t, the degrees of freedom.
When to use it
- Statistics homework.
- Reading research papers.
- A/B tests.
- Lab reports.
How to read the result
z = 1.96 gives a two-tailed p-value of about 0.05.
A small p-value suggests the data is unlikely under the null hypothesis; it does not measure the size or importance of an effect.
Mistakes to avoid
- Reading p as the probability that the null hypothesis is true.
- Using one-tailed values for two-sided questions.
- Ignoring effect size and sample size.
Formula
p = P(|Z| ≥ |z|) or Student t with df degrees of freedom
Worked example
z = 1.96, two-tailed: p = 0.05.
Frequently asked questions
What is a p-value?
The probability of a result at least this extreme if the null hypothesis is true.
z or t?
Use t for small samples with an unknown standard deviation.
What does p < 0.05 mean?
The result would be rare if the null hypothesis were true.
One or two tails?
Two tails unless you predicted the direction in advance.
When do I use t instead of z?
When σ is estimated from a small sample.
Related calculators
Results are estimates for information only. Check important figures with a professional.