Sample Size Calculator
Find the minimum sample size for a survey proportion given a confidence level and margin of error.
Runs 100% in your browser — nothing is uploaded.
50% gives the most conservative (largest) sample.
Result
Formula
n = z*² · p(1 − p) ÷ e²
About this calculator
z* is the critical value for the confidence level, p the expected proportion (0.5 is most conservative), and e the margin of error.
How to use
- 1
Enter confidence level and margin of error.
- 2
Optionally adjust the expected proportion.
- 3
Read the required sample size.
Frequently asked questions
Why use 0.5 if I don't know the proportion?
Using p = 0.5 gives the largest possible sample size because p(1-p) is maximized at 0.5. This ensures your sample is large enough regardless of the true proportion.
What is the margin of error?
The margin of error is the maximum expected difference between the sample estimate and the true population proportion. For example, a 5% margin of error means your estimate will be within ±5 percentage points of the true value.
How does population size affect the sample size?
For very large populations, the required sample size barely changes. For smaller populations, the finite population correction reduces the required sample size. If you provide the population size, the calculator applies this correction.
Please note: Results are estimates for general information only and are not professional or medical/financial advice.