T and z critical value calculator
Select a distribution, confidence level and number of tails. For Student's t, also enter the degrees of freedom.
How do I use this page?
- Select Z or t.
- Enter confidence and the number of tails.
- For t, enter df and read the critical value.
What this page does not do: The calculator returns the positive boundary. A two-sided procedure also has a matching negative boundary. Distribution choice depends on the statistical model and assumptions.
Further reading
Theory — what is a critical value in z and t tests?
A critical value marks the boundary of the rejection region for a baseline hypothesis, or the margin used in a confidence interval. In a two sided test, the significance level alpha splits between both tails of the distribution, while in a one sided test it stays entirely on one side. The t distribution also depends on the degrees of freedom and has heavier tails than the normal distribution, especially in small samples. Alpha is simply one minus the chosen confidence level, expressed as a fraction.
Frequently asked questions
Where does alpha come from?
Does df affect a Z critical value?
See also
A/B test calculator
Compare two proportions in an A/B test. Calculate conversion rates, difference, uplift, z score, p value and c
A/B test planning — sample size, MDE and duration
Plan a two-proportion experiment, estimate detectable effect and traffic duration.
ACF and PACF autocorrelation calculator
Calculate ACF and PACF across time-series lags. Review the chart, reference bounds and coefficient table.