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Peak Forms sample size calculator
Choose a confidence level and margin of error, optionally add the population size, expected share and response rate: the calculator shows how many complete responses and invitations your survey needs.
n₀ = z² · p · (1 − p) / e² · with a known population size N: n = n₀ / (1 + (n₀ − 1) / N)
Complete responses needed
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Rounded up. This is how many completed responses you need for the chosen margin of error and confidence level.
z-value used
1,960
Two-sided quantile of the standard normal distribution for the confidence level.
Assumption: The formula applies to proportions (e.g. “How many are satisfied?”) and a random selection from the population. No sample size makes up for bias, for example when only very satisfied people respond.
Method and sources: Normal approximation with finite population correction following W. G. Cochran, Sampling Techniques, 3rd edition, Wiley 1977, chapter 4. See also NIST/SEMATECH e-Handbook of Statistical Methods.
Next stepCreate a survey with Peak FormsLimits
What the calculator doesn't answer.
- The formula applies to proportions from a random sample. Means, group comparisons or A/B tests need different calculations.
- A large enough sample does not protect against bias: if certain groups respond less often, the result stays skewed.
- If you want to analyse subgroups (e.g. by region), you need the calculated number per subgroup, not in total.
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