How to use the Sample Size Calculator
The sample size calculator shows how many people you need to survey for a chosen confidence level and margin of error.
Open the Sample Size Calculator →
What you need to enter
- Confidence level
- Margin of error (%)
- Expected proportion (%)
- Population size (0 = unlimited)
The formula
n = z² × p(1 − p) ÷ e², with an adjustment for finite populations: n ÷ (1 + (n − 1) ÷ N).
Worked example
Using the default values in the calculator:
- Confidence level: 90%
- Margin of error (%): 5
- Expected proportion (%): 50
- Population size (0 = unlimited): 0
| Required sample size | 271 |
| Without population correction | 271 |
Tips for better results
- Use 50% when you do not know the proportion.
- A smaller margin needs a much bigger sample.
Common mistakes to avoid
- Using a sample that is biased.
- Ignoring non-response.