Sample Size Calculator
Find the survey sample size for a target margin of error.
Results update as you type.
About this calculator
This calculator finds how many people you need to survey for a chosen margin of error and confidence level. It uses n = z²·p(1 − p) / e², then applies a finite-population correction when you supply a total population size. The default 50% proportion gives the largest, safest sample size; if you have a good estimate of the true proportion you can enter it to reduce the required sample.
Frequently asked questions
How is sample size calculated?
The base formula is n = z²·p(1 − p) / e², where z is the confidence critical value, p the expected proportion and e the margin of error as a decimal. The result is rounded up because you cannot survey a fraction of a person.
What is the finite population correction?
When your population is small, you need fewer responses than the base formula suggests. The correction n = n₀ / (1 + (n₀ − 1)/N) shrinks the sample toward the population size N. It matters most when the sample is a large fraction of the whole.
Results are estimates for general guidance only, not financial, medical or tax advice.