Sample Size Calculator
How many responses you need for a reliable survey.
Enter your values
Results update instantly as you type — no submit needed.
Results
- Required sample size
- 385
- Unadjusted (infinite population)
- 385
- Margin of error achieved
- 5%
- Responses needed at 60% response rate
- 641
Working
- 1n₀ = 1.96² × 0.5 × 0.5 ÷ 0.05² = 384.2
Quick answer
Required sample size is z²·p(1−p) ÷ e², adjusted downward when the population is small.
n = z²·p(1−p) ÷ e²
How to use the Sample Size Calculator
- 1Set the margin of error you can live with.
- 2Choose a confidence level.
- 3Enter the population size if it is small and known.
Frequently asked questions
Why use 50% as the proportion?
It maximises variance, giving the most conservative (largest) required sample when you have no prior estimate.
Does population size matter?
Only for small populations. Beyond about 20,000 the required sample barely changes.