Statistics / Tests
T-Test Calculator (One-Sample, Two-Sample, Paired)
Run a one-sample, two-sample (Welch's or Student's), or paired t-test on your data, two-sided or one-sided, and get t, the degrees of freedom, the p-value, and the means.
T-Test Calculator (One-Sample, Two-Sample, Paired): The t statistic is the difference in means divided by its standard error. Welch's test uses each group's own variance and the Welch–Satterthwaite degrees of freedom; Student's pools the variances; the paired test is a one-sample test on the differences. The p-value comes from Student's t distribution, computed in the browser and checked against SciPy: for the example groups, Welch's t = 3.40 with p = 0.0055. Runs 100% locally in your browser with zero server file uploads.
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Significant at the 5% level (p < 0.05): the result would be unlikely if there were no real difference.
A t-test asks whether a mean differs from a value, or two means from each other, more than chance would explain. Welch's test does not assume equal variances and is the safer default for two groups; the paired test compares two measurements on the same subjects. It assumes roughly normal data or reasonably large samples. A small p-value says the difference is unlikely to be chance, not that it is large or important.
Reading the result
p is the probability of a difference at least this large if the true means were equal. Below 0.05 is the usual threshold for significance, but it is a convention, not a law; report the exact p.
With more than two groups, use the one-way ANOVA calculator rather than many t-tests, which inflates false positives.
Assumptions
The t-test assumes independent observations and roughly normal data, or samples large enough (about 30 or more each) for the means to be close to normal. Strong outliers can mislead it.
To see how unusual a single value is instead, use the z-score calculator.
How to use it
- Choose the test: one sample, two samples, or paired.
- Paste the values for each group, and the value to compare with for a one-sample test.
- Choose two-sided or one-sided, and read t, the degrees of freedom, and the p-value.
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Frequently asked questions
Which two-sample test should I use?
Welch's, unless you have a good reason to assume equal variances: it is about as powerful when variances are equal and much safer when they are not.
When is a paired test right?
When each value in one list belongs with one in the other, such as the same people before and after, or matched pairs.
Does a significant p-value mean a big effect?
No: with large samples, tiny differences become significant. Report the difference in means and an effect size such as Cohen's d alongside p.
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