Effect Size Calculator (Cohen's d)

Effect size is a measure of the practical magnitude of a difference or relationship, separate from whether that difference is statistically significant. A study with a very large sample can detect even a trivially small difference as statistically significant, so reporting effect size alongside a p-value gives readers a much clearer picture of what the result actually means in practice. The most widely used effect size for comparing two group means is Cohen's d, which expresses the difference in means as a multiple of the pooled standard deviation. A d of 0.2 is conventionally small, 0.5 is medium, and 0.8 or above is large. A d of 1.0 means the two group means are one standard deviation apart, which represents a very large and practically meaningful difference in most applied fields. For analysis of variance (ANOVA) results, the equivalent measure is eta squared, which expresses the proportion of total variance explained by the factor: values below 0.01 are small, 0.06 is medium, and 0.14 or above is large. This calculator has two modes. In the first mode, enter the mean of each group and the pooled standard deviation to get Cohen's d. In the second mode, enter the F-statistic and the degrees of freedom to get eta squared. Both modes show the numerical result and the conventional interpretation. Results are for educational and research planning purposes.

Calculate.co.nz is proud to be partnered with Health Based Building, a leader in sustainable and health-conscious building innovation. With over a century of experience, they develop high-performance systems like Foreverbreathe Specification, Magnum Board, and Foreverbreathe Paints to support energy-efficient, non-toxic living environments. Their commitment to healthier homes aligns with our belief that informed choices lead to better outcomes for Kiwi households.

Cohen's d: small < 0.2, medium 0.2-0.8, large > 0.8. Eta squared: small < 0.01, medium 0.01-0.14, large > 0.14. These are general guidelines only.