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Updated 2026-03-07 UTC
Cohen’s d Calculator — Standardized Mean Difference — GetCalcMaster
Compute Cohen’s d (standardized mean difference) from two group means/SDs (with pooled SD). Includes interpretation tips and a worked example.
Cohen’s d expresses a mean difference in standard deviation units. It’s the go‑to effect size for comparing two groups on a continuous outcome.
Important: Educational. For paired or repeated‑measures designs, use the effect size definition that matches the design.
What this calculator is
The Statistics Calculator is an interactive tool inside GetCalcMaster. It’s designed to help you explore scenarios, understand formulas, and document assumptions.
Key features
- Uses pooled SD for two independent groups
- Works with equal or unequal sample sizes
- Direction preserved (sign tells which group is larger)
- Pairs naturally with t-tests and confidence intervals
Formula
s_pooled = sqrt(((n1-1)s1^2 + (n2-1)s2^2) / (n1+n2-2))
d = (M1 - M2) / s_pooledQuick examples
Example: M1=10, s1=2, n1=30; M2=9, s2=2, n2=30 → s_pooled=2, d=(10−9)/2=0.5.If M2 > M1, d is negative (same magnitude, opposite direction).
Verification tips
- If s1=s2 and n1=n2, then s_pooled=s1 and d simplifies to (M1−M2)/s1.
- d is unitless; changing measurement units shouldn’t change d.
Common mistakes
- Using the wrong SD (e.g., SD of one group only) when pooled SD is intended.
- Confusing Cohen’s d with the raw mean difference (units vs SD units).
- Interpreting sign as “good/bad” without defining direction.
How to use it (quick steps)
- Collect M1, s1, n1 for group 1 and M2, s2, n2 for group 2.
- Compute the pooled SD (s_pooled).
- Compute d = (M1 − M2) / s_pooled.
- Interpret |d| in context and report direction separately if helpful.
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FAQ
Is this calculator official?
No. GetCalcMaster provides educational estimates and learning tools. Always verify against official definitions, documents, or professional advice.
Do you store my inputs on the server?
No. Calculations run locally in your browser. Optional remember/restore features (if enabled) use local browser storage.
Tip: For reproducible work, save your inputs and reasoning in Notebook.