Effect size calculator for Cohen's d
Compare the means of two independent groups in units of their pooled standard deviation.
How do I use this page?
- Paste observations for both groups.
- Check their means and standard deviations.
- Read Cohen's d and Hedges' g.
What this page does not do: This version is for two independent groups and uses a pooled variance. It does not replace a significance test or an assessment of study design.
Further reading
Theory — what does Cohen's d effect size show?
Cohen's d standardizes the difference between two means by dividing it by the pooled standard deviation of both groups. The sign of the result indicates the direction of the difference based on the order of the compared groups, while its absolute value describes the size of that difference in standard deviation units. For small samples, Hedges' g applies a correction for bias. A larger d does not always mean a practically important effect — context, measurement scale and the cost of error all matter.
Frequently asked questions
Does a larger d always mean an important effect?
Why is Hedges' g different from d?
See also
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Compare two proportions in an A/B test. Calculate conversion rates, difference, uplift, z score, p value and c
A/B test planning — sample size, MDE and duration
Plan a two-proportion experiment, estimate detectable effect and traffic duration.
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Calculate ACF and PACF across time-series lags. Review the chart, reference bounds and coefficient table.