Cohen’s d Calculator

Standardized mean difference

Cohen’s d expresses the difference between two means in standard-deviation units. It is useful when outcomes are measured on the same continuous scale but a standardized effect size is desired.

Formula

d = (M₁ − M₂) ÷ SDpooled, where the pooled standard deviation is calculated from the two sample standard deviations and sample sizes.

Interpretation

Rules of thumb such as 0.2, 0.5 and 0.8 are sometimes described as small, medium and large, but practical importance depends strongly on the field, outcome and decision context.

Method reference: Cohen’s standardized mean-difference framework.