Cohen's D Calculator
Find Cohen's d effect size from two group means and SDs.
Cohen's d standardizes the difference between two means.
The math behind it
d = (mean₁ − mean₂) ÷ pooled SD, where pooled SD = √((s₁² + s₂²)/2). 0.2 small, 0.5 medium, 0.8 large.
Worked example
Means 80 vs 74, SDs 10 and 12 → d ≈ 0.54 (medium).
FAQ
Effect size vs p-value?
Effect size shows how big a difference is; p-value shows whether it's statistically reliable.