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Cohen’s d Effect Size Calculator
Calculate the standardised difference between two means to determine the practical significance of your results. Select your data type, enter your values, and instantly find your effect size and pooled standard deviation.
How to use this calculator
- Choose your data type: Select whether you are pasting Raw Data for both groups, or entering Summary Statistics (the means, standard deviations, and sample sizes) if you have already calculated them.
- Enter your data: Input the values for Group 1 and Group 2.
- Calculate: The tool will automatically compute the pooled standard deviation and Cohen’s d.
Cohen’s d Calculator
Calculate the standardised effect size between two groups.
Cohen’s d
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Interpretation
Pooled Std Dev (SDp)
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Used to standardize the difference
*Technical note: This calculator uses the standard pooled standard deviation formula for independent samples. Cohen’s d interpretation follows standard conventions (0.2 = Small, 0.5 = Medium, 0.8 = Large).
Understanding Your Results
In statistics, a result can be "statistically significant" (a low p-value) but practically meaningless if the actual difference is microscopic. Cohen's d solves this by measuring the distance between two means in terms of standard deviations.
- Cohen's d ($d$): The standardized effect size. A $d$ of 0.5 means the two group means are exactly half a standard deviation apart.
- Interpretation (Rule of Thumb):
- ~0.2: Small effect
- ~0.5: Medium effect
- ~0.8: Large effect
- Pooled Standard Deviation ($SD_{pooled}$): A weighted average of the standard deviations from both groups. It is used as the denominator in the Cohen's d formula ($d = \frac{M_1 - M_2}{SD_{pooled}}$) to standardize the scale of the difference.