How to use the cohen's d effect size calculator
Compute the standardised difference between two means. Enter mean 1, mean 2, sd 1, sd 2, n₁, n₂ in the fields above; the pooled sd, cohen's d, magnitude are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “how big is the difference”, “effect size”, “practical significance”, “calculate cohen's d effect size”, “find pooled sd”.
Formula
Variables
- m1 — Mean 1
- m2 — Mean 2
- s1 — SD 1
- s2 — SD 2
- n1 — n₁
- n2 — n₂
Worked example
With mean 1 = 105, mean 2 = 100, sd 1 = 15, sd 2 = 15, n₁ = 50, n₂ = 50, the calculator gives pooled sd ≈ 15; cohen's d ≈ 0.3333333; magnitude ≈ Small.
Tips for accurate results
- Cohen's d uses the pooled standard deviation, so enter the standard deviation of each group.
- The benchmarks 0.2 (small), 0.5 (medium) and 0.8 (large) are conventions, not rules; judge the effect in its context.
- Report the effect size with its confidence interval, not only the point estimate.
Common mistakes
- Treating the benchmark labels as proof that an effect matters in practice.
- Using this formula for paired data without checking the correct paired-design version.
- Entering the standard error where the standard deviation is required.
Frequently asked questions
What is the cohen's d effect size calculator used for?
Compute the standardised difference between two means. It helps with questions such as “how big is the difference”, “effect size”, “practical significance”.
What is the formula for the cohen's d effect size calculation?
Pooled SD = √(((n1-1) × s1 × s1+(n2-1) × s2 × s2) / (n1+n2-2)). Cohen's d = (m1-m2) / [Pooled SD]. Magnitude = abs([Cohen's d])<0.2?"Negligible":(abs([Cohen's d])<0.5?"Small":(abs([Cohen's d])<0.8?"Medium":"Large")). Here m1 is mean 1, m2 is mean 2, s1 is sd 1, s2 is sd 2, n1 is n₁, n2 is n₂.
What inputs do I need for the cohen's d effect size calculator?
You need mean 1, mean 2, sd 1, sd 2, n₁, n₂. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use the cohen's d effect size calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the cohen's d effect size result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.