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Test Statistic to P Value Calculator

Turn a t test statistic and degrees of freedom into p-values, including two-tailed and one-tailed reference rows.

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Result

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Formula

Student t distribution with given df

Computes tail probabilities from the same t statistic so you can compare one-sided and two-sided interpretations.

Worked Example

t = 2.5, df = 24 shows two-tailed p plus upper and lower one-tailed references.

Converting Test Statistics to P-Values

This calculator takes a single t statistic and degrees of freedom and returns all three p-value interpretations: two-tailed, upper one-tailed, and lower one-tailed. This lets you see the full picture and choose the correct value for your specific hypothesis test.
  • Two-tailed p: for non-directional hypotheses testing whether a parameter differs from the null in either direction
  • Upper one-tailed p: for hypotheses predicting the parameter is greater than the null value
  • Lower one-tailed p: for hypotheses predicting the parameter is less than the null value
  • All three values come from the same underlying t distribution. They differ only in which tail area is reported

Choose your p-value based on the hypothesis you specified before collecting data. Never select the tail type that gives you the smallest p-value after seeing results.

You can also calculate changes using our P Value Calculator, Two-Tailed P Value Calculator, One-Tailed P Value Calculator or Test Statistic Calculator.

Frequently Asked Questions

Which p-value row should I report?

Use the two-tailed value for a two-sided alternative hypothesis. Use upper or lower one-tailed only if your hypothesis was directional and specified before data collection.

Can I enter a z statistic instead of t?

This tool uses the Student t distribution. For large degrees of freedom (above 30), t and z produce nearly identical p-values. For exact normal z-based p-values, use a z-score calculator.

Why does the same t give different p-values?

The three values represent different areas under the t distribution curve. Two-tailed sums both tails. Upper one-tailed uses only the right tail. Lower one-tailed uses only the left tail. The total probability is the same, just divided differently.

What does a very small p-value mean?

A very small p-value (e.g., less than 0.001) means the observed t statistic would be extremely unlikely under the null hypothesis. This provides strong evidence against the null, though it does not prove the alternative hypothesis is true.

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