Online Tool Store
Sample Size Calculator
Calculate the minimum number of responses your survey or experiment needs given a population size, confidence level, margin of error, and expected response split. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Required sample size
Z-score used
How the sample size calculator works
- Enter your population size if known, or leave it blank for a large/unknown population.
- Choose a confidence level and set your acceptable margin of error and expected response split.
- The tool applies the standard survey sample-size formula, with a finite-population correction if a population was entered.
The formula
n₀ = z² × p × (1-p) / e² n = n₀ / (1 + (n₀-1)/N)
z is the z-score for your chosen confidence level (1.96 for 95%), p is the expected response proportion, and e is the margin of error. The second formula corrects the raw sample size n₀ down for a known finite population N.
FAQ
What is the "expected response split" field for?
It is your best estimate of how responses will divide (e.g. 50/50 for an unknown split, or 80/20 if you expect most people to answer one way). 50% is the safest default since it produces the largest, most conservative sample size.
What if I don't know my population size?
Leave the population field blank. The calculator then treats the population as effectively infinite, which is a safe assumption for any population above a few hundred thousand.
What does confidence level actually mean?
A 95% confidence level means that if you repeated the survey many times, about 95% of the resulting confidence intervals would contain the true population value.
Why does a smaller margin of error need more responses?
Margin of error appears squared in the denominator of the formula, so halving it roughly quadruples the required sample size.
How we compare
| Feature | Online Tool Store | SurveyMonkey calculator | Qualtrics calculator |
|---|---|---|---|
| No sign-up or account | ✓ | ✓ | ✓ |
| Finite-population correction included | ✓ | ✓ | ✓ |
| No linked survey product upsell | ✓ | ✗ | ✗ |
For a quick, standalone sample-size figure without a survey-tool sign-up, Sample Size Calculator gets you there directly.