📐 Cohen’s D Calculator
Enter the mean, standard deviation, and sample size for two groups to calculate Cohen’s d, a standardized measure of the difference between them.
What this calculator does
Cohen’s d is a standardized effect size that expresses the difference between two group means in units of standard deviation, making it easy to compare the size of an effect across different studies or measurement scales.
How the calculation works
The calculator finds a pooled standard deviation by weighting each group’s variance by its sample size, then divides the difference between the two means by that pooled standard deviation: d = (mean1 – mean2) / pooled SD. It also reports Hedges’ g, a small-sample correction that reduces the slight upward bias of Cohen’s d when sample sizes are small.
As a rough guide, |d| around 0.2 is considered a small effect, 0.5 a medium effect, and 0.8 or higher a large effect. This tool is for general research and educational planning purposes, not a substitute for a full statistical analysis.