Frequently Asked Questions

A t-test determines whether there is a statistically significant difference between means — either a sample mean and a hypothesized value, two independent group means, or paired before-and-after measurements.

A one-sample test compares a sample mean to a fixed value. A two-sample test compares means from two independent groups. A paired test compares the same subjects measured twice, using the differences between pairs.

Welch's t-test doesn't assume the two groups have equal variances, which makes it more robust and reliable than the pooled-variance t-test for most real-world data.

Degrees of freedom (df) describes how many independent values went into the estimate. For one-sample and paired tests, df = n − 1. Welch's two-sample test uses the Welch–Satterthwaite equation, which can produce a non-integer df.

The p-value is the probability of observing a t-statistic this extreme if there were truly no difference between the means. A p-value below your significance level (commonly 0.05) suggests the difference is statistically significant.

This calculator reports the two-tailed p-value, which tests for a difference in either direction. For a one-tailed test, divide the reported p-value by 2 (if the observed direction matches your hypothesis).

The paired test needs the mean and standard deviation of the differences between each pair of measurements, plus the number of pairs.

The calculator requires at least 2 observations per group so a standard deviation and degrees of freedom can be computed, but t-tests are most reliable with larger samples.

This calculator flags results as significant when the two-tailed p-value is below 0.05, the most common threshold, though your context may call for a different significance level.

Yes, the t-test calculator is completely free with no sign-up, and all calculations run instantly in your browser.