Effect Size Calculator (Cohen's d and Hedges' g)

Cohen's d and Hedges' g from two means and SDs or from a t statistic, with a 95% CI and interpretation.

Cohen's d = (M₁ − M₂) ÷ pooled SD. With means 24.5 and 21.2, SDs 5.1 and 4.8 and 30 per group, pooled SD = 4.95 and d = 3.3 ÷ 4.95 = 0.67, a medium effect; Hedges' g = 0.66.

Free, runs in your browser. Nothing you enter is uploaded.

How to use it

  1. Enter each group's mean, SD and n, or a t statistic with the two sample sizes.
  2. Read Cohen's d, Hedges' g and the 95% confidence interval.
  3. Report the effect size alongside the p-value.

What you enter

What you get

Formula

d = (M₁ − M₂)/s_p | s_p = √[((n₁−1)s₁² + (n₂−1)s₂²)/(n₁+n₂−2)] | g = d × [1 − 3/(4(n₁+n₂) − 9)]

Worked example

Input: 24.5 ± 5.1 (n = 30) vs 21.2 ± 4.8 (n = 30)

Result: d = 0.67, g = 0.66

A medium effect: the average in group 1 sits at about the 75th percentile of group 2.

Questions

How do you calculate Cohen's d?

Subtract one group mean from the other and divide by the pooled standard deviation, √[((n₁−1)s₁² + (n₂−1)s₂²) ÷ (n₁ + n₂ − 2)].

What is a large effect size?

By Cohen's convention d = 0.2 is small, 0.5 medium and 0.8 large, but what matters in practice depends on the field.

When should I use Hedges' g?

With small samples (under about 20 per group), because d slightly overestimates the population effect. Hedges' g applies a correction and is standard in meta-analysis.

Can I get Cohen's d from a t value?

Yes, for independent groups d = t × √(1/n₁ + 1/n₂).

References

Related reading

More Statistics

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.