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Effect Size Calculator

Enter summary statistics for two independent groups to calculate their signed standardized mean difference, pooled standard deviation and small-sample corrected Hedges’ g.

Group 1
Group 2

For independent groups, this version uses the conventional pooled-SD definition and assumes a shared population variance. Enter sample (n−1) standard deviations, not standard errors.

Each sample size must be 2–1,000,000; means and SDs are limited to ±1,000,000,000 (SDs cannot be negative). At least one SD must be above zero.

What does an effect size calculator measure?

It expresses the difference between two group means in pooled standard-deviation units. Cohen’s d is the uncorrected standardized difference; Hedges’ g applies a small-sample correction.

Frequently asked questions

What does an effect size calculator measure?

It expresses the difference between two group means in pooled standard-deviation units. Cohen’s d is the uncorrected standardized difference; Hedges’ g applies a small-sample correction.

Are my study statistics uploaded?

No. The calculation runs in this browser. Your summary statistics are not sent to a server or saved by this tool.

What is the difference between Cohen’s d and Hedges’ g?

Both standardize the mean difference by a pooled standard deviation. Hedges’ g multiplies d by a small-sample correction, which reduces bias when samples are small.

Can I enter standard errors instead of standard deviations?

No. This calculator expects each group’s sample standard deviation. A standard error describes uncertainty in a sample estimate and is not interchangeable with the SD here.

Does a larger effect size prove that a result is important?

No. Effect size describes a standardized difference, not statistical significance, causation or practical importance. Context, uncertainty, study design and the measurement scale matter.