StatisticsFree

One-Tailed P Value Calculator

Compute a one-tailed p-value from a t statistic and degrees of freedom.

Enter Values

t Statistic

Degrees of Freedom

Tail

Result

Enter values above and click Calculate to see your result.

AI Assistant

Ask about this calculator

I can help you understand the one-tailed p value calculator formula, interpret your results, and answer follow-up questions.

Try asking

Formula

Upper tail: p = 1 - F(t). Lower tail: p = F(t)

One-tailed p-values use one side of the Student t distribution.

Worked Example

t = 1.8, df = 20, upper tail gives the right-tail probability.

When to Use One-Tailed P-Values

A one-tailed test evaluates evidence for an effect in only one direction. It is more powerful than a two-tailed test for detecting effects in the hypothesized direction, but it completely ignores effects in the opposite direction.

Upper tail (right): tests whether the parameter is greater than the null value. Use when your hypothesis predicts an increase or positive effect
Lower tail (left): tests whether the parameter is less than the null value. Use when your hypothesis predicts a decrease or negative effect
One-tailed p-values are exactly half the two-tailed p-value when the effect is in the hypothesized direction
The directional hypothesis must be specified before data collection. Choosing the tail direction after seeing the data invalidates the test

One-tailed tests are appropriate in specific situations (non-inferiority trials, quality control with known direction), but most research uses two-tailed tests for safety and credibility.

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

Frequently Asked Questions

When is one-tailed testing appropriate?

Only when your alternative hypothesis is directional and specified before data collection. Examples include testing whether a new drug performs better (not just differently) than placebo, or whether a manufacturing defect rate exceeds a threshold.

Can I double a one-tailed p to get two-tailed?

For symmetric distributions like the t distribution, the two-tailed p-value equals exactly 2 times the one-tailed p-value in the observed direction. However, this relationship only holds for symmetric tests.

What is the difference between upper and lower tail?

Upper tail gives P(T greater than t), testing for positive effects. Lower tail gives P(T less than t), testing for negative effects. Choose based on the direction your hypothesis predicts.

Is one-tailed testing ever inappropriate?

Yes. If you choose the tail direction after seeing your data, or if an effect in either direction would be meaningful, one-tailed testing is inappropriate. Most journal reviewers expect two-tailed tests unless a strong justification is provided.

AI Assistant

Ask about this calculator

I can help you understand the one-tailed p value calculator formula, interpret your results, and answer follow-up questions.

Try asking

Related Calculators

P Value Calculator Two-Tailed P Value Calculator Test Statistic to P Value Calculator T-Test Calculator

More Statistics Calculators

View all

Average Calculator

Calculate the arithmetic mean of numbers.

Mean Median Mode Calculator

Find mean, median, and mode of a dataset.

Standard Deviation Calculator

Calculate variance and standard deviation.

Standard Error Calculator

Calculate Standard Error of the Mean (SEM).

Explore Other Categories

Accurate and Reliable

All calculations run locally. Trusted statistical analysis with step-by-step breakdowns.