Find how many responses you need for a given confidence level and margin of error.
How the sample size calculator works
Planning a survey? This calculator finds the sample size for your chosen confidence level and margin of error, with an optional population size correction.
It assumes the most conservative 50% response distribution unless your population is small.
A worked example
On the values this calculator opens with, the sample size is 385. Underneath, z-score comes out at 1.96 and Unadjusted at 385. Change any field and every figure updates as you type.
Step by step
- Choose a confidence level.
- Enter your margin of error.
- Add a population size if it’s small, then read the sample size.
Formula reference
n₀ = z² × 0.25 ÷ e²
finite population: n = n₀ ÷ (1 + (n₀−1)/N)
Frequently asked questions
How big should my sample be?
It depends on confidence and margin of error. For 95% confidence and ±5%, about 385 responses for a large population.
What is margin of error?
The range your result could differ from the true value — smaller margins need larger samples.
Does population size matter?
Only when it’s small. For large populations the sample size barely changes, which is why the field is optional.