Cohen's D Calculator

Calculate Cohen’s d with the small-sample Hedges correction.

About Cohen's D Calculator

Cohen’s d is biased upward in small samples, which the Hedges correction removes. The two converge as sample sizes grow beyond about 50 per group. Reporting d with its standard error allows a confidence interval on the effect, which is far more informative than a point estimate alone.

Formula

Cohen's D

d = (M₁ − M₂) ÷ s_pooled, with s_pooled the sample-size weighted standard deviation

pooled SD: Weighted by degrees of freedom from each group
Hedges correction: Removes small-sample upward bias in d

Worked example

With the default values below, Cohen's D Calculator returns Cohen’s d of 0.55090. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean of group 1: 100
Mean of group 2: 92
Standard deviation of group 1minimum 0: 15
Standard deviation of group 2minimum 0: 14
Sample size of group 1minimum 2: 40
Sample size of group 2minimum 2: 38

Results

Cohen’s d: 0.55090
Pooled Standard Deviation: 14.52176
Hedges’ g With Small-Sample Correction: 0.54544
Magnitude: Medium
Standard Error of d: 0.23078

Step by step

1Values used
2Cohen's D
3Cohen’s d= 0.55090
4Pooled Standard Deviation= 14.52176
5Hedges’ g With Small-Sample Correction= 0.54544
6Magnitude= Medium
7Standard Error of d= 0.23078

Assumptions and limits

The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Cohen's D calculated?

d = (M₁ − M₂) ÷ s_pooled, with s_pooled the sample-size weighted standard deviation Cohen’s d is biased upward in small samples, which the Hedges correction removes. The two converge as sample sizes grow beyond about 50 per group.

Why does Cohen's D matter?

Reporting d with its standard error allows a confidence interval on the effect, which is far more informative than a point estimate alone.

What values do I need to enter?

This calculator takes 6 inputs: Mean of group 1, Mean of group 2, Standard deviation of group 1, Standard deviation of group 2, Sample size of group 1, Sample size of group 2. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.