Cohen’s d Effect Size Calculator
Cohen’s d expresses the difference between two group means in units of a pooled within-group standard deviation. It complements a test of statistical significance.
How to use this calculator
- Enter two independent groups’ sample sizes, means and sample standard deviations.
- Check that all inputs use the same units, period and population scope.
- Change one assumption at a time when comparing scenarios.
Formula used by cohens d effect size calculator
d = (mean₁ − mean₂) ÷ pooled SD; pooled SD uses each sample variance weighted by its degrees of freedom.
- d = (mean₁ − mean₂) ÷ pooled SD; pooled SD uses each sample variance weighted by its degrees of freedom.
- Conventions for small or large effects vary by field. Interpret with uncertainty, measurement reliability and practical context.
- The tool validates inputs and rounds displayed figures for readability; it does not validate assumptions against external records.
Worked examples
Worked example
Group means are 78 and 72; sample SDs are 10 and 12 for groups of 30 and 28.
The mean difference is 6 and the pooled SD combines both sample variances.
The signed Cohen’s d shows the standardized mean difference.
Understanding the result
Cohen’s d expresses the difference between two group means in units of a pooled within-group standard deviation. It complements a test of statistical significance.
Conventions for small or large effects vary by field. Interpret with uncertainty, measurement reliability and practical context.
Important notes
Conventions for small or large effects vary by field. Interpret with uncertainty, measurement reliability and practical context.
Treat the result as an estimate from the entered assumptions. For commercial, statistical, engineering or health-related decisions, verify relevant specifications and professional guidance.
Frequently asked questions
Can d be negative?
Yes. The sign indicates which group mean is larger under the group order entered.
Is d the same as a p-value?
No. It is an effect-size estimate rather than a test probability.
How should I interpret this result?
Conventions for small or large effects vary by field. Interpret with uncertainty, measurement reliability and practical context.
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Related calculators
- Pooled Standard Deviation Calculator — Combine two sample standard deviations into a pooled estimate when the equal-variance assumption is appropriate.
- Standard Deviation Calculator — Calculate population or sample standard deviation, variance, mean, sum, and count.
- Sample Size for a Mean Calculator — Estimate the sample size needed to estimate a population mean for a stated standard deviation, margin of error and confidence z-score.
- Mean Absolute Deviation Calculator — Measure the average absolute distance of data values from their arithmetic mean.
- Standard Error of the Mean Calculator — Estimate the standard error of a sample mean from sample standard deviation and sample size.
- Relative Standard Deviation Calculator — Express a standard deviation relative to a positive mean as a percentage.
Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
- How to Calculate a Z-Score — Learn how a z-score shows how far a value is from the mean in standard-deviation units.
- How to Calculate Standard Deviation — Learn what standard deviation measures, the population and sample formulas, and how to interpret the result.
- Standard Deviation — Understand standard deviation as a measure of spread and the difference between population and sample standard deviation.
This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.