Estimate survey sample size for a proportion with optional finite population correction.
Sample Size Calculator
Formula
n0=(z^2*p*(1-p))/moe^2; finite correction; ceil
z depends on confidence. Population 0 skips finite correction.
This sample size calculator estimates how many survey completes you need for a proportion question. Set confidence, margin of error, and expected proportion. Optionally enter a finite population.
Defaults: unlimited population, 95% confidence, 5% margin, 50% expected proportion. That setup rounds up to 385 completes for a very large population.
Formula
n0 = (z^2 * p * (1 – p)) / moe^2. z is 1.96 at 95%, 2.576 at 99%, otherwise 1.645. If population N > 0, n = n0 / (1 + (n0 – 1)/N). Then ceil.
Worked example
95% confidence, 5% margin, 50% proportion, population 0: n0 = 384.16, ceil = 385. With N = 1000 the finite correction yields about 278.
