Use this free online sample size calculator to determine how many survey or study respondents you need for statistically reliable results. Enter your desired confidence level, margin of error, and estimated proportion, and optionally correct for a finite population size to get an accurate sample size instantly.
How to Use the Sample Size Calculator
Select your desired "Confidence Level" (90%, 95%, or 99%) from the dropdown.
Enter your acceptable "Margin of Error (%)" - smaller values require larger samples.
Enter the "Estimated proportion" - use 0.5 if you don't know it, which gives the most conservative (largest) sample size.
Optionally enter your "Population Size" if you know the total population and want a finite-population-corrected sample size.
Click "Calculate" to see the required sample size.
Frequently Asked Questions
It uses the standard formula for estimating a proportion: n0 = (Z² x p x (1-p)) / e², where Z is the z-score for your confidence level (1.645, 1.96, or 2.576), p is the estimated proportion, and e is the margin of error expressed as a decimal.
If you enter a known population size N, the calculator applies the finite population correction n = n0 / (1 + (n0 - 1)/N), which reduces the required sample size when your population is small relative to n0. Leave it blank for an infinite or very large population.
Use 0.5 (50%). This is the most conservative estimate because p x (1-p) is maximized at p = 0.5, so it produces the largest possible required sample size, guaranteeing your result is not underpowered.
The margin of error must be greater than 0. If you leave it blank or enter a zero or negative value, the calculator shows an error message asking you to enter a valid margin of error instead of a result.