Cohen's d Calculator

Use this calculator to express how far two group means sit apart in pooled-SD units. Psychology, education, and A/B sketches often report Cohen's d so that a 10-point gap on different scales can be compared.

The formula is: d = (m₁ − m₂) / s_pooled, with s_pooled = √(((n₁ − 1) s₁² + (n₂ − 1) s₂²) / (n₁ + n₂ − 2)). Enter each group's mean, sample SD, and size. Means 100 vs 90 with s = 15 and n = 30 each give d ≈ 0.67. Rough sketches are 0.2 small, 0.5 medium, and 0.8 large.

This is not Hedges' g (which applies a small-sample correction) and not Glass's Δ (which uses one group's SD). Sign follows m₁ − m₂. You need more than two observations in total and positive SDs.

Mean 1 / Mean 2: Group means.

s₁ / s₂: Sample standard deviations of each group.

n₁ / n₂: Group sizes.

d: Cohen's d, standardised mean difference.