Cohen's d Calculator

About this calculator

This calculator finds Cohen's d, a standardized effect size that measures the difference between two group means in units of standard deviation: d = (M₁ − M₂) / SD_pooled, where the pooled standard deviation combines the variability of both groups. Unlike a raw mean difference or a p-value, d expresses how large a difference is in a scale-free way, regardless of the units the original data were measured in.

Researchers in psychology, medicine, and social science report Cohen's d alongside statistical significance testing to convey practical importance, since a statistically significant result can still reflect a trivially small effect. Jacob Cohen's commonly cited (though context-dependent) benchmarks are around 0.2 for a small effect, 0.5 for medium, and 0.8 for large. Enter the two group means, their standard deviations, and sample sizes to calculate d.