Cohen's D Calculator
Standardised effect size from two group means.
Enter your values
ⓘ Live results
Results
- Cohen's d
- 0.571
- Interpretation
- Medium
- Hedges' g (small-sample corrected)
- 0.563
- Pooled SD
- 10.512
- Percent of group 2 below group 1's mean
- 71.6%
- Common language effect size
- 65.7%
Quick answer
Cohen's d is the difference between two means divided by their pooled standard deviation, so an effect of d = 1 means the groups differ by one whole SD.
d = (x̄₁ − x̄₂) ÷ s_pooled
How to use the Cohen's D Calculator
- 1Enter the mean, standard deviation and size of each group.
- 2Read d and the plain-English interpretation.
- 3Use Hedges' g when either group has fewer than about 20 people.
Worked example
Using the values the calculator loads with:
Inputs
- Group 1 mean78
- Group 1 SD10
- Group 1 size30
- Group 2 mean72
- Group 2 SD11
- Group 2 size30
Results
- Cohen's d0.571
- InterpretationMedium
- Hedges' g (small-sample corrected)0.563
- Pooled SD10.512
- Percent of group 2 below group 1's mean71.6%
- Common language effect size65.7%
Frequently asked questions
Why report effect size at all?
A p-value only says whether a difference exists; d says how big it is, which is what actually matters for decisions.
What is the common language effect size?
The chance that a randomly picked member of group 1 scores above a random member of group 2.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.