Critical Values Calculator

Find critical values for hypothesis tests. Free online z and t critical value calculator.

The Critical Values Calculator finds the z or t critical values for hypothesis testing at various significance levels. Enter your confidence level or alpha value and degrees of freedom (for t-tests) to find the critical values that define rejection regions. Essential for statistical inference and hypothesis testing.

Common use cases

  • Hypothesis test design
  • Confidence interval construction
  • Statistical significance testing
  • Quality control limits
  • Research methodology

How to use

  1. Select z-test or t-test
  2. Enter significance level (α) or confidence level
  3. Enter degrees of freedom (for t-test)
  4. Select one-tailed or two-tailed test
  5. Click calculate for critical value(s)

FAQ

What is a critical value?

The boundary value that separates rejection and non-rejection regions in hypothesis testing.

What are common critical z-values?

For two-tailed: 90%→1.645, 95%→1.96, 99%→2.576. These define rejection regions.

When do I use z vs t?

Use z when σ is known or n>30; use t when σ is unknown and using sample standard deviation.

How do degrees of freedom affect t?

Lower df → wider t-distribution → larger critical values. As df→∞, t approaches z.