Effect Size Calculator
Choose an effect-size measure that matches your design and enter the statistics from your analysis. The calculator shows the formula and substitution; it does not test significance or decide whether an effect matters in practice.
Calculate an effect size
Standardizes a mean difference between two independent groups by their pooled sample standard deviation.
Continue your analysis
What should I do next?
You have an effect size. Now show its uncertainty.
Confidence Interval Calculator
Calculates confidence intervals for means, proportions, their differences and correlations, showing the standard error, critical value and every step.
Power Analysis
Plans the sample size a study needs, or the power it will have, for seven common designs, with every assumption stated.
How to report statistics in APA Style
How to write statistical results in APA Style: formatting t, F, r, χ² and p, decimal places and leading zeros, effect sizes and confidence intervals, example sentences for common tests, and when to use a table.
What this calculator cannot determine
- The calculator does not determine which statistical test or effect-size measure is appropriate for your design.
- It does not calculate p-values, confidence intervals, power or raw-data analyses.
- A numerical effect size does not by itself show statistical significance, causal impact or practical importance.
- For paired data, choose the standardizer that matches the research question and report it explicitly.
Method choices
- Paired standardized mean differences depend on the denominator convention. This tool labels d_z (SD of paired differences) and d_av (average marginal SD) separately.
- Magnitude labels are not universal thresholds. This calculator reports numbers and formulas; discuss practical importance in your field and design.
- Cohen's w magnitude conventions are context-dependent and not shown as universal classifications.
Privacy
Values are calculated in your browser and are not sent or saved.