Effect Size (Cohen D) Calculator

Work out effect size (cohen d) instantly with clear inputs, formula shown and shareable results.

About Effect Size (Cohen D) Calculator

Cohen's d expresses the difference between two means in standard deviation units, making it comparable across studies and independent of sample size. Cohen suggested 0.2, 0.5 and 0.8 as small, medium and large.

Formula

Cohen's d

d = (x̄₁ − x̄₂) / s_pooled, s_pooled = √((s₁² + s₂²)/2)

Worked example

With the default values below, Effect Size (Cohen D) Calculator returns Cohen's d of 0.72558814. Change any input above and every figure here is recalculated in your browser.

Inputs used

First group mean: 78.5
Second group mean: 72.1
First group standard deviationminimum 0: 8.2
Second group standard deviationminimum 0: 9.4

Results

Cohen's d: 0.72558814
Interpretation: Medium effect (Cohen's benchmarks)
Pooled standard deviation: 8.82043083
Overlap of the two distributions: 71.6759 %

Step by step

1Values used
2Cohen's d
3Cohen's d= 0.72558814
4Interpretation= Medium effect (Cohen's benchmarks)
5Pooled standard deviation= 8.82043083
6Overlap of the two distributions= 71.6759 %

Assumptions and limits

The result is derived only from the 4 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 8 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

For means 78.5 and 72.1 with SDs 8.2 and 9.4?

The pooled SD is 8.8204, so d ≈ 0.7256 — a medium to large effect.

Why report d as well as p?

A p value depends heavily on sample size; d measures the practical size of the difference regardless of n.