P-Value Calculator – Calculate Statistical Significance
Get the p-value for a z-score, one-tailed or two-tailed.
About the P Value Calculator
What is the P Value Calculator?
A p-value calculator converts a z-score into the probability of observing such extreme results, one- or two-tailed.
Why use this calculator?
Significance testing is the guardrail of research claims. A p-value tells you how surprising the data would be if the null hypothesis were true.
How to use it
- Enter a z-score.
- Choose the test direction: two-tailed, right or left.
- Read the p-value and significance verdict at 5% and 1%.
Formula
p = tail probability under the standard normal curve.
A small p-value is evidence, not proof. Report both the effect size and the p-value.
Frequently Asked Questions
What does p < 0.05 mean?
If the null hypothesis were true, results this extreme would appear less than 5% of the time by chance.
Can a p-value prove an effect?
No single test proves anything. Treat it as one piece of evidence alongside sample size and effect size.
What order of operations does this use?
It follows standard PEMDAS/BODMAS rules: parentheses, exponents, multiplication and division, then addition and subtraction.
How precise are the answers?
The calculator uses double-precision floating-point math (about 15 significant digits) and rounds the display. For exact symbolic results use dedicated algebra software.
Can I enter negative numbers or decimals?
Yes. Negative values, decimals and very large or small numbers are all supported unless a field says otherwise.
What does it mean if I get an error?
Errors usually mean an invalid input, such as division by zero or a negative value where only positives make sense. Check your entries and try again.
What is the difference between population and sample?
A population includes every member of a group; a sample is a subset. Sample statistics use n−1 in the denominator to correct for bias.
When should I use median instead of mean?
Use the median when data contains outliers or is skewed, because it is less affected by extreme values than the mean.
What does standard deviation tell me?
It measures how spread out the values are around the mean. A small standard deviation means the data clusters tightly; a large one means it is more scattered.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.