Cohen's d Standardized Effect Size Calculator
Cohen's d quantifies the practical magnitude of an experimental intervention independent of sample size by dividing the difference between two group means (M₁ - M₂) by their pooled standard deviation SD_p.
Standardized mean difference in SD units
Mean of experimental group 1
Mean of baseline control group 2
Root-mean-square standard deviation √((s₁² + s₂²) / 2)
Step-by-Step Calculation
Cohen's d quantifies the practical magnitude of an experimental intervention independent of sample size by dividing the difference between two group means (M₁ - M₂) by their pooled standard deviation SD_p.
Real-World Reference Values
Subtle difference (~58% percentile overlap)
Visibly discernible shift (~69% of Group 1 above Group 2 mean)
Substantial clinical/practical impact (~79% non-overlap)
20-Row Comparison Table
Cohen's d Effect Size (d) · Treatment / Group 1 Mean (M₁)
| # | Treatment / Group 1 Mean (M₁) | Cohen's d Effect Size (d) | Δ % |
|---|---|---|---|
| 01 | -2,171.00 | -149.4 | -25000.00% |
| 02 | -1,907.84 | -131.8561 | -22076.02% |
| 03 | -1,644.68 | -114.3123 | -19152.05% |
| 04 | -1,381.53 | -96.7684 | -16228.07% |
| 05 | -1,118.37 | -79.2246 | -13304.09% |
| 06 | -855.2105 | -61.6807 | -10380.12% |
| 07 | -592.0526 | -44.1368 | -7456.14% |
| 08 | -328.8947 | -26.593 | -4532.16% |
| 09 | -65.7368 | -9.0491 | -1608.19% |
| 10 | 79 Current | 0.6 | 0.00% |
| 11 | 460.5789 | 26.0386 | +4239.77% |
| 12 | 723.7368 | 43.5825 | +7163.74% |
| 13 | 986.8947 | 61.1263 | +10087.72% |
| 14 | 1,250.05 | 78.6702 | +13011.70% |
| 15 | 1,513.21 | 96.214 | +15935.67% |
| 16 | 1,776.37 | 113.7579 | +18859.65% |
| 17 | 2,039.53 | 131.3018 | +21783.63% |
| 18 | 2,302.68 | 148.8456 | +24707.60% |
| 19 | 2,565.84 | 166.3895 | +27631.58% |
| 20 | 2,829.00 | 183.9333 | +30555.56% |
Frequently Asked Questions
Why report Cohen's d alongside a p-value?+
With a huge sample size n, even a trivial difference yields p < 0.001; Cohen's d is independent of n and reveals whether the effect is practically meaningful.
How is Pooled Standard Deviation SD_p calculated for equal group sizes?+
For equal group sizes, SD_p = √((s₁² + s₂²) / 2).
How do I find the treatment score M₁ needed for a medium effect size (d = 0.50)?+
Click SOLVE FOR on M₁, set d = 0.50, and enter your control mean M₂ and pooled SD.