GlenCalc

Sample size calculator

Plan a survey: choose the margin of error and confidence you want to see how many responses you need. Or enter a sample size to see its margin of error.

50% gives the largest, safest sample size when you don’t know.

Leave blank for a very large population.

Your answer

Sample size needed385
For an unlimited population385
z for the confidence level1.96

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How we worked it out

Updated as you change the numbers.

  1. n₀ = z² × p(1 − p) ÷ e². With a finite population N: n = n₀ ÷ (1 + (n₀ − 1) ÷ N).

Method and assumptions

The formula comes from the normal approximation to a proportion. The finite population correction reduces the sample needed when you are surveying a large share of a small population. Response bias and non-response are not accounted for, so collect more than the minimum.

Questions

How many responses do I need for ±5% at 95% confidence?

385 for a large population.

Sources

Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.

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