Two-Tailed P Value Calculator

Two-tailed testing accounts for evidence of an effect in both directions (positive and negative). It sums probability from both tails of the distribution, producing a larger p-value than either single tail alone.

Yes. The two-tailed definition is symmetric, so t = 2.0 and t = -2.0 produce the same two-tailed p-value. Only the magnitude of t matters, not the sign.

Use two-tailed whenever your alternative hypothesis is non-directional (testing for "different from" rather than "greater than" or "less than"). Most research defaults to two-tailed unless there is strong prior justification for a directional test.

For symmetric tests, one-tailed p equals two-tailed p divided by 2, but only if the observed effect is in the hypothesized direction. This conversion should only be done if the one-tailed hypothesis was specified before data collection.

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