One Tailed P Value Calculator

Calculator

Distribution

One tailed direction

Input mode

Test statistic

Degrees of freedom 1

Degrees of freedom 2

Alpha level

Decimal precision

Sample mean

Null mean

Standard deviation

Sample size

Sample mode applies to z and t tests. Other choices use the direct statistic.

Example Data Table

Distribution	Tail	Statistic	DF1	DF2	One tailed p value
Normal z	Right	1.96	-	-	0.024998
Normal z	Left	-1.64	-	-	0.050503
Student t	Right	2.1	18	-	0.025045
Chi square	Right	18.31	10	-	0.049954
F distribution	Right	3.18	5	12	0.046689

Formula Used

For a right tailed test, p = 1 - CDF(test statistic).

For a left tailed test, p = CDF(test statistic).

For sample z or t input, statistic = (sample mean - null mean) / (standard deviation / square root of sample size).

The z option uses the normal distribution. The t option uses Student t with degrees of freedom. Chi square and F options use their matching distribution functions.

How to Use This Calculator

Select the distribution that matches your hypothesis test.
Choose right tail or left tail.
Use direct statistic mode for an existing test statistic.
Use sample mode to build a z or t statistic.
Enter alpha to compare significance.
Press calculate, then review the result above the form.
Use CSV or PDF download for records.

One Tailed P Value Guide

Purpose

A one tailed p value calculator helps you test direction. It is useful when your claim points one way. You may test whether a mean is greater. You may test whether a ratio is lower. The tool turns a statistic into a tail probability.

Distribution Choice

Choose the distribution that matches your test. Use z when the standard error is known. Use t for small samples with estimated spread. Use chi square for variance based tests. Use F for ratio based variance tests. Each option uses the chosen tail. A right tail measures values above the statistic. A left tail measures values below it.

Input Options

The calculator also supports sample based entries. Enter the sample mean, null mean, spread, and size. The script builds a z or t statistic. For a t test, degrees of freedom equal sample size minus one. Direct statistic mode is useful when another tool already produced the test statistic.

Decision Rule

The alpha field gives the decision rule. Common values are 0.10, 0.05, and 0.01. If the p value is less than or equal to alpha, the result is statistically significant. That does not prove practical importance. It only shows the observed statistic is unlikely under the null model.

Exports

Exports help with reports. The CSV file opens in spreadsheet software. The PDF file gives a compact result sheet. Both include the statistic, tail, alpha, and decision. Keep the exported file with your method notes. This makes your work easier to review.

Good Practice

Use the example table before running real data. It shows typical inputs and result styles. Then enter your own values carefully. Match the test to the research question. Check whether your hypothesis is left tailed or right tailed. A wrong tail can change the conclusion.

Meaning

A p value is not the probability that the null is true. It is the chance of seeing a statistic at least as extreme, assuming the null model is correct. Pair it with context, sample quality, and effect size. Sound judgment matters after every calculation.

Workflow

Advanced options reduce work. You can switch distributions without leaving the page. You can change precision for cleaner output.

FAQs

What is a one tailed p value?

It is the probability in one selected tail of a distribution. It measures how extreme the statistic is in the direction named by your alternative hypothesis.

When should I use a right tailed test?

Use a right tailed test when your alternative claim says the true value is greater than the null value or benchmark.

When should I use a left tailed test?

Use a left tailed test when your alternative claim says the true value is smaller than the null value or benchmark.

What does alpha mean?

Alpha is your chosen significance level. It is the cutoff used to compare the p value and make a reject or fail to reject decision.

Can I calculate a t test p value?

Yes. Select Student t, enter the t statistic, and add degrees of freedom. Sample mode can also build a t statistic from summary values.

Can this calculator handle chi square tests?

Yes. Select chi square, enter a nonnegative statistic, add degrees of freedom, and choose the correct tail for your hypothesis.

Why are there two degrees of freedom fields?

The F distribution needs numerator and denominator degrees of freedom. Other distributions use only the first field, or none for z tests.

Does a small p value prove my claim?

No. A small p value supports statistical evidence against the null model. It should be judged with design quality, assumptions, and effect size.