Calculate a standardised effect size between two group means.
Effect size expresses the difference in standard deviations, making it comparable across studies and independent of sample size. Required sample size scales with the inverse square of the effect size. Statistical significance depends on sample size while effect size does not, which is why large studies can find significant but trivial differences.
Effect Size
Cohen’s d = (mean₁ − mean₂) ÷ pooled standard deviation
Cohen’s d = (mean₁ − mean₂) ÷ pooled standard deviation Effect size expresses the difference in standard deviations, making it comparable across studies and independent of sample size. Required sample size scales with the inverse square of the effect size.
Statistical significance depends on sample size while effect size does not, which is why large studies can find significant but trivial differences.
This calculator takes 4 inputs: Mean of group 1, Mean of group 2, Pooled standard deviation, Sample size per group. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.