Sample Size: p = 0.3, 95% Confidence, ±5% Margin of Error
When the expected proportion is 30% (p = 0.3), 95% confidence, and ±5% margin of error, the required sample is 323.
How to use this tool
- Enter expected proportion (p), z* (confidence level) and margin of error (e) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your required sample size (n) and the full breakdown beneath it.
Calculate the sample size needed when prior data suggests the outcome proportion is approximately 30%.
Frequently asked questions
- Why use p = 0.5 when the proportion is unknown?
- p × (1−p) is maximised at p = 0.5, giving the most conservative (largest) required sample size. If you have a prior estimate of p, use it to get a smaller required n.