Sample Size: 95% Confidence, 5% Margin of Error, p = 0.5
For a 95% confidence level, 5% margin of error, and unknown proportion (p = 0.5), the required sample size is 385.
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.
The most common sample size calculation: find the number of survey respondents needed for 95% confidence and a ±5% margin of error.
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.