What if I don't know my population size?

Leave it blank — the calculator will compute the sample size assuming an effectively unlimited population, which is standard practice for large or unknown populations.

Why does this assume 50% variability?

50% is the most conservative assumption (it produces the largest required sample size), used when you don't already know the expected proportion of your outcome of interest.

Sample Size Calculator

Calculate the minimum sample size needed for a survey or study to achieve a target confidence level and margin of error.

Confidence levelMargin of error

Population size (optional)

Required Sample Size

385

How It Works

Confidence level and margin of error

A 95% confidence level means that if you repeated the survey many times, 95% of the resulting confidence intervals would contain the true population value. Margin of error sets how precise you need that estimate to be.

Finite population correction

If you're sampling from a small, known population (not a huge or effectively infinite one), the required sample size shrinks — this calculator applies that correction automatically when you provide a population size.

Example

A survey targeting a 95% confidence level with a 5% margin of error needs a sample size of 385 from a very large (or unknown-size) population.

Common Mistakes

Skipping the population size for a genuinely small population — this makes the calculator recommend a larger sample than actually necessary.
Choosing an unrealistically tight margin of error, which can require an impractically large sample size for the available time or budget.

Frequently Asked Questions

What if I don't know my population size?: Leave it blank — the calculator will compute the sample size assuming an effectively unlimited population, which is standard practice for large or unknown populations.
Why does this assume 50% variability?: 50% is the most conservative assumption (it produces the largest required sample size), used when you don't already know the expected proportion of your outcome of interest.