About the Confidence Interval Calculator
Confidence interval for a mean (from mean, SD and n) or a proportion (from successes and n) at 90, 95 or 99% confidence.
Also known as: 95% confidence interval, margin of error calculator, ci calculator, confidence interval for proportion.
How to use this calculator
- Enter interval for a, confidence, sample size n and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
Mean: x̄ ± z·σ/√n · Proportion (Wilson): (p̂ + z²/2n ± z√(p̂(1−p̂)/n + z²/4n²)) / (1 + z²/n)
Worked example
Using the values pre-filled in the calculator above:
- Interval for a: Proportion / conversion rate
- Confidence: 95%
- Successes / conversions = 120
- Sample size n = 1,000
the calculator returns:
- Interval: 10.1299% to 14.1609%
- Observed proportion: 12%
- Margin of error (±): 2.0155%