Sample Size: 99% Confidence, 5% Margin of Error, p = 0.5
Achieving 99% confidence with a ±5% margin of error and p = 0.5 requires a sample size of 664.
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 required for a highly reliable survey result with 99% 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.