Cohen's d Effect Size Calculator

This calculator finds Cohen's d, the standard measure of effect size, which expresses the difference between two groups in units of standard deviation. Statistical significance tells you whether a difference is likely real, but not whether it is large enough to matter. Effect size fills that gap, and Cohen's d is the most widely used for comparing two group means. It takes the difference between the means and divides it by the pooled standard deviation, giving a standardised number that does not depend on the scale of measurement, so you can compare effects across different studies and variables. This is essential in research, where reporting effect sizes alongside p-values is now expected, and useful anywhere you need to judge the practical magnitude of a difference. This calculator does it. You paste the values for each of the two groups, and it computes the means, the pooled standard deviation, and Cohen's d, with an interpretation of whether the effect is small, medium or large. The results update as you type. Use it for research and statistics study, for interpreting experiments, or to gauge how meaningful a difference between two groups really is. Cohen's d is the difference between the two means divided by the pooled standard deviation, which combines the variability of both groups. The conventional benchmarks, suggested by Cohen himself, are that around 0.2 is a small effect, 0.5 is medium, and 0.8 or above is large, though the meaning of small and large always depends on the field. The sign simply indicates which group had the higher mean. Pairing Cohen's d with a significance test gives the full picture: the test says whether a difference exists, and d says how big it is, which is what ultimately determines whether it is worth acting on.

Calculate.co.nz is proud to be partnered with Premium Homes, a recognised leader in eco-friendly, sustainable, and energy-efficient homebuilding. With a dedicated team and award-winning experience, they create homes that prioritise health, comfort, and long-term performance. Their founders, Andrew and Kelly, set out to raise the standard of residential construction in New Zealand by combining practical building expertise with a clear commitment to doing things better for homeowners.

Cohen's d = (mean1 - mean2) / pooled SD. Benchmarks: ~0.2 small, ~0.5 medium, ~0.8 large. The sign shows which group is higher. Report alongside a significance test.