P-Value Calculator

Exact p-values from z, t, chi-square and F statistics, left-, right- or two-tailed, with the critical value for your significance level.

The P-Value Calculator converts a test statistic (z, t with df, χ² with df, or F with df₁ and df₂) into an exact p-value using the corresponding cumulative distribution function, reports whether it is significant at your α, and gives the critical value.

Free, runs in your browser. Nothing you enter is uploaded.

How to use it

Choose the distribution.
Enter the statistic and degrees of freedom.
Choose the tail and α.
Read p and the critical value.

What you enter

Statistic
Degrees of freedom (t, χ², F)
Tail
α

What you get

p-value
Significance verdict
Critical value

Formula

p = P(|T| ≥ |t_obs|) (two-tailed)   |   p = 1 − F_χ²(x; df)   |   p = 1 − F_F(f; df₁, df₂)

The p-value is the probability, under the null hypothesis, of a statistic at least as extreme as the one observed. Continuous CDFs are evaluated with regularized incomplete beta and gamma functions (accuracy checked against SciPy).

Worked example

Input: t = 2.5, df = 20, two-tailed

Result: p = 0.0212 · critical t (α 0.05) = ±2.086

The observed t exceeds the critical value, so the result is significant at the 5 % level.

Questions

One-tailed or two-tailed?

Two-tailed unless your hypothesis specified a direction before the data were collected. A one-tailed p is half the two-tailed p for symmetric distributions.

How do I get a p-value from a t statistic?

Select t, enter the statistic and df = n − 1 (one sample) or n₁ + n₂ − 2 (pooled two-sample), and read the two-tailed p.

Limitations

A p-value is not the probability that the null hypothesis is true and says nothing about effect size.
χ² and F are right-tailed only.

References

Wasserstein RL, Lazar NA (2016). The ASA statement on p-values: context, process, and purpose. Am Stat 70:129-133.

More Statistics

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.