Calculators
Effect Size Calculator (Cohen's d)
Calculate Cohen's d, Hedges' g, and Glass's delta for two independent groups with means, standard deviations, and sample sizes locally in your browser.
Enter the mean, standard deviation, and sample size for two independent groups. Cohen’s d uses the pooled standard deviation. Hedges’ g applies a small-sample correction. Glass’s Δ uses Group 2’s SD as the reference (control group). All calculations run locally in your browser; your values are not uploaded or stored.
Measure the standardized difference between two groups.
Enter the mean, standard deviation, and sample size for two independent groups. Cohen's d uses the pooled standard deviation: d = (M1 − M2) / SDpooled. Hedges' g applies a small-sample bias correction: g = d × (1 − 3 / (4(df) − 1)). Glass's Δ uses the control group (Group 2) standard deviation as the reference. The calculator also reports the 95% confidence interval for Cohen's d and an interpretation guide (very small < 0.2, small < 0.5, medium < 0.8, large ≥ 0.8). Standard deviations must be positive and sample sizes must be whole numbers ≥ 1. All calculations run locally in your browser; your values are not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is Cohen's d?
Cohen's d is the standardized difference between two means divided by the pooled standard deviation: d = (M1 − M2) / SDpooled. It measures how many standard deviations the group means are apart.
When should I use Hedges' g instead of Cohen's d?
Hedges' g applies a small-sample correction factor J = 1 − 3 / (4(df) − 1), where df = n1 + n2 − 2. Use Hedges' g when sample sizes are small (e.g., n < 20 per group) because Cohen's d slightly overestimates the population effect size in small samples.
What is Glass's Δ and when is it appropriate?
Glass's Δ uses only the control group (Group 2) standard deviation as the reference: Δ = (M1 − M2) / SD2. It is appropriate when the control group SD better represents the population standard deviation, or when the treatment is expected to change the variance.
How do I interpret the effect size magnitude?
Common benchmarks: |d| < 0.2 is very small, 0.2–0.5 is small, 0.5–0.8 is medium, and ≥ 0.8 is large. These are conventions, not strict thresholds; interpret in the context of your field and research question.
Does the calculator provide a confidence interval?
Yes. The calculator reports a 95% confidence interval for Cohen's d using the standard error SE = √((n1 + n2)/(n1×n2) + d²/(2(n1 + n2))). The interval is d ± 1.96 × SE.
Are my data values uploaded?
No. All calculations run locally in your browser. CodeASystem does not upload or store your input values.