Confidence Interval Calculator
Compute 90/95/98/99% or custom confidence intervals for means and proportions with margin of error, t/z critical values and small-sample warnings. Free and client-side.
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What is Confidence Interval Calculator?
A Confidence Interval Calculator quantifies the uncertainty around a sample estimate. For means it builds a Student-t interval using n−1 degrees of freedom; for proportions it uses the normal z approximation with a Wald reliability warning — reporting the critical value, margin of error and interval bounds for 90/95/98/99% or any custom confidence level.
Key Benefits
- Honest uncertainty reporting for surveys, A/B tests and experiments
- Correct small-sample behavior with t-based mean intervals
- Immediate critical values without statistical tables
Common Use Cases
- •Reporting survey results with margins of error
- •Summarizing A/B test or experiment outcomes
- •Teaching interval estimation with live critical values
How to compute a confidence interval
- Choose the interval type: Pick mean for measured data (sample mean, standard deviation and size) or proportion for count data (successes out of n).
- Set the confidence level: Use the 95% default, or pick 90%, 98%, 99% or any custom level between 1% and 99.9%.
- Read the interval: Report the interval bounds together with the margin of error and critical value shown beside it.
Key Features
- Mean intervals via Student t with n−1 degrees of freedom
- Proportion intervals via the z approximation with Wald small-count warnings
- 90/95/98/99% presets plus any custom confidence level
- Critical values and margin of error shown beside the interval
- High-precision incomplete beta solving, entirely client-side