How do I calculate margin of error? Enter your numbers above and the result updates instantly. For example: Sample size: 400 · Confidence level: 95 · Observed proportion: 50% → Margin of error: 4.9%.
Is this Margin of Error Calculator free? Yes — completely free with no sign-up or paywall. It runs entirely in your browser and keeps working offline; the numbers you enter never leave your device.
How accurate is it? It uses the standard published formula for margin of error. Results are estimates for planning — not financial, tax, medical, engineering, or legal advice.