Sample Size Calculator
Works out how many responses are needed to estimate a percentage to a chosen precision.
Sample size needed
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Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
Without a population limit —
z used —
Reading this —
The formula
n₀ = z² × p(1 − p) ÷ e² n = n₀ ÷ (1 + (n₀ − 1) ÷ N)
The second line is the finite population correction, used only when you enter a population size. For estimating a mean, use the sample size for a mean calculator.
What to watch out for
Halving the margin takes four times the sample. Population size has little effect until the sample is a sizeable share of the population.
Frequently Asked Questions
How many responses for ±5% at 95% confidence?
385 from a large population, or 278 from a population of 1,000.
How many for ±3%?
1,068 at 95% confidence, from a large population.