How to use the Effect Size Calculator (Cohen's d)
- Enter each group's mean, standard deviation and size.
- Read Cohen's d and its interpretation.
- Use Hedges' g for small samples.
What does this tool do?
A p-value tells you whether a difference is likely real; an effect size tells you how large it is. Cohen's d expresses the difference between two means in units of their pooled standard deviation.
As a rough guide, d = 0.2 is small, 0.5 medium and 0.8 large. Hedges' g corrects d's slight overestimate in small samples, and Glass's Δ uses only the control group's spread.
Why use it?
- Three effect sizes together.
- Pooled standard deviation shown.
- Interpretation using Cohen's guidelines.
The formula
s_pooled = √(((n₁ − 1)s₁² + (n₂ − 1)s₂²) ÷ (n₁ + n₂ − 2))
d = (M₁ − M₂) ÷ s_pooled
g = d × (1 − 3 ÷ (4(n₁ + n₂) − 9))
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Last reviewed by the M2Toolkit team.