How to use the sample size calculator
Find the sample size needed for a survey proportion. Enter margin of error, confidence level, expected proportion in the fields above; the required sample size is computed instantly and plotted on the chart so you can see how the answer changes.
Formula
Variables
- e — Margin of error (%)
- cl — Confidence level (%)
- p — Expected proportion
Worked example
With margin of error = 5 %, confidence level = 95 %, expected proportion = 0.5, the calculator gives required sample size ≈ 385.
Frequently asked questions
What is the formula for the sample size calculation?
Required sample size = ceil((norminv(0.5+cl / 200) / (e / 100))^2 × p × (1-p)). Here e is margin of error (%), cl is confidence level (%), p is expected proportion.
What inputs do I need for the sample size calculator?
You need margin of error in %, confidence level in %, expected proportion. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use this calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.