Sample Size Calculator
How many survey responses you need for a target margin of error.
Enter values
%
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Step by step
The formula
n₀ = z² · 0.25 / e² (with finite-population correction when N is known)
Required sample size depends on the confidence you want and the margin of error you can tolerate, not on population size — which is why national polls need only about a thousand responses. Halving the margin of error quadruples the sample, since error falls with the square root.
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Example calculations
Common questions this calculator answers — select one to load its values.
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Frequently asked questions
Why does it assume a 50% proportion?+
A proportion of 0.5 maximises variance, giving the largest (safest) sample size. If you know the real proportion is far from 50%, you can survey slightly fewer people.
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