Cohen's D Calculator
Standardised effect size from two group means.
Enter your values
Enter the group 1 mean you are working with.
Enter the group 1 sd you are working with.
Enter the group 1 size you are working with.
Enter the group 2 mean you are working with.
Enter the group 2 sd you are working with.
Enter the group 2 size you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Standardised effect size from two group means. |
|---|---|
| Category | Statistics |
| Inputs needed | Group 1 mean, Group 1 SD, Group 1 size, Group 2 mean, Group 2 SD, Group 2 size |
| Main output | Cohen's d |
| Formula | d = (x̄₁ − x̄₂) ÷ s_pooled |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Group 1 mean
- Enter the group 1 mean you are working with.
- Group 1 SD
- Enter the group 1 sd you are working with.
- Group 1 size
- Enter the group 1 size you are working with.
- Group 2 mean
- Enter the group 2 mean you are working with.
- Group 2 SD
- Enter the group 2 sd you are working with.
- Group 2 size
- Enter the group 2 size you are working with.
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.
What do I need to enter into the Cohen's D Calculator?
Just 6 values: group 1 mean, group 1 sd, group 1 size, group 2 mean, group 2 sd, group 2 size. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Cohen's D Calculator work out the 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. It applies the formula d = (x̄₁ − x̄₂) ÷ s_pooled and shows the working so you can check each step by hand.
Is the Cohen's D Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.