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Effect Size Calculator

Calculate Cohen's d effect size for two independent groups using their means, standard deviations, and sample sizes. Get a clear interpretation of the effect instantly.

Calculate Cohen's d Effect Size

Enter the statistics for both groups to calculate the standardized difference between their means.

Group 1
Enter the mean for Group 1.
Enter the standard deviation.
Number of observations.
Group 2
Enter the mean for Group 2.
Enter the standard deviation.
Number of observations.
Example: For Group 1 with mean 85, standard deviation 10, and sample size 30, and Group 2 with mean 78, standard deviation 12, and sample size 30, the calculator estimates Cohen's d.
Cohen's d Effect Size
0.0000
Effect Size
Mean Difference 0
Pooled SD 0
Group 1 Mean 0
Group 2 Mean 0
Measure Group 1 Group 2 Value / Result
Mean — — —
Standard Deviation — — —
Sample Size — — —
Cohen's d Standardized Difference —
Interpretation —

What Is Effect Size?

Effect size is a statistical measure used to describe the magnitude of a difference or relationship. Unlike a significance test, which focuses on whether an observed result is statistically significant, effect size helps explain how large or meaningful the difference is.

One of the most commonly used standardized effect-size measures for comparing two group means is Cohen's d. It expresses the difference between two means in units of standard deviation.

Cohen's d = (Mean₁ − Mean₂) ÷ Pooled Standard Deviation

How to Calculate Effect Size

This calculator uses Cohen's d for two independent groups. It first calculates the pooled standard deviation and then divides the difference between the two means by that value.

  1. Enter the mean for Group 1.
  2. Enter the standard deviation and sample size for Group 1.
  3. Enter the mean, standard deviation, and sample size for Group 2.
  4. Calculate the pooled standard deviation from both groups.
  5. Divide the difference between the two means by the pooled standard deviation.

Cohen's d Formula

For two independent groups, the pooled standard deviation can be calculated using the sample sizes and standard deviations of both groups.

Pooled SD = √[ ((n₁ − 1)s₁² + (n₂ − 1)s₂²) ÷ (n₁ + n₂ − 2) ]

Cohen's d = (Mean₁ − Mean₂) ÷ Pooled SD

Example

Suppose Group 1 has a mean of 85 and Group 2 has a mean of 78. If their pooled standard deviation is approximately 11.05, the effect size is:

d = (85 − 78) ÷ 11.05
d ≈ 0.63

The absolute magnitude is approximately 0.63, which is commonly interpreted as a medium effect under conventional Cohen's d guidelines.

How to Interpret Cohen's d

Cohen's d is often interpreted using approximate guidelines. These ranges are useful for context, but the practical importance of an effect depends on the research field and the specific situation.

0.00 – 0.19 Very small or negligible effect.
0.20 – 0.49 Small effect.
0.50 – 0.79 Medium effect.
0.80 – 1.19 Large effect.
1.20+ Very large effect.
Negative d Indicates the direction of the difference. Its magnitude can be interpreted using the absolute value.

Why Is Effect Size Important?

Shows the Magnitude of a Difference

Effect size provides information about how large the difference between groups is rather than simply stating whether a statistical test found significance.

Helps Compare Studies

Standardized effect sizes can make it easier to compare findings across studies that use different measurement scales.

Useful in Research

Effect sizes are widely used in scientific research, education, psychology, medicine, social sciences, and other quantitative fields.

Useful for Meta-Analysis

Standardized effect sizes such as Cohen's d can be useful when combining and comparing results from multiple studies.

Frequently Asked Questions

Cohen's d is a standardized effect-size measure that describes the difference between two group means in standard deviation units.
Cohen's d is calculated by dividing the difference between two group means by their pooled standard deviation.
A Cohen's d of approximately 0.5 is commonly described as a medium-sized effect under conventional guidelines. Interpretation should also consider the research context.
Yes. A negative Cohen's d indicates the direction of the difference between the two group means. The absolute value describes the magnitude of the effect.
Under commonly used Cohen's d guidelines, an absolute value around 0.8 or greater is often considered a large effect, although appropriate thresholds vary by field.
No. Effect size measures the magnitude of an effect, while statistical significance is evaluated using statistical tests and related measures such as p-values and confidence intervals.
For the pooled-standard-deviation version of Cohen's d used by this calculator, the sample sizes are needed to calculate the pooled standard deviation.
Yes. You can use this calculator directly in your browser to calculate Cohen's d without installing additional software.