Sample Size Calculator
Work out how many people you need to survey for a chosen margin of error and confidence level.
About the Sample Size Calculator
The required sample size depends on how precise you need to be (margin of error), how sure (confidence level) and how varied the answers are expected to be. Using 50% for the expected proportion gives the largest, safest size. For small populations a correction lowers it.
The formula
n₀ = z² × p(1 − p) ÷ e²; n = n₀ ÷ (1 + (n₀ − 1) ÷ N)
Worked example
95% confidence, ±5%, p = 50%
- n₀ = 1.96² × 0.25 ÷ 0.05² = 384.1
- Survey 385 people
Questions people ask
Why doesn't a big population need a big sample?
Beyond about 20,000 people the population size barely matters; precision depends on the sample itself.
How do I allow for people who don't respond?
Divide by your expected response rate: for 385 completed surveys at a 20% response rate, invite 385 ÷ 0.2 = 1,925 people.