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Sample Size Calculator
Estimate the sample size needed for a t-test using effect size, power, alpha, and an optional attrition buffer. This free version is built for straightforward t-test planning rather than every design under the statistical sun.
Enter your study assumptions
Choose the t-test design, enter the effect size you want to detect, and set your target power and alpha level.
Use Cohen’s d. Rough conventions are 0.20 small, 0.50 medium, 0.80 large.
0.80 is common. 0.90 is stricter and usually means a larger sample.
0.05 is the standard default in many psychology assignments.
Optional. Add this if you expect some participants to drop out or produce unusable data.
Your estimate
The result below shows the minimum analyzable sample and, if you added attrition, a more realistic recruitment target.
The free version is designed for basic t-test planning. Premium is where it makes more sense to put ANOVA, correlations, broader guidance, and messier real-world designs.
What Premium does better
- Free: straightforward t-test sample size planning
- Premium: broader support for more designs and more detailed guidance
- Premium: better for when the study is more complicated than two neat textbook groups
Sample Size Calculator
Estimate sample size for psychology and social science research with this free sample size calculator. Useful for study planning, dissertations, projects, and anyone trying to make more defensible decisions about power and participant numbers.