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Confidence Interval Calculator
Calculate confidence intervals for means and proportions. Choose from 80% to 99.9% confidence levels with automatic z-score lookup.
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Key Features — Confidence Interval Calculator
Confidence intervals for means and proportions
Confidence levels: 80%, 85%, 90%, 92%, 95%, 96%, 98%, 99%, 99.9%
Automatic Z-score lookup
Margin of error and standard error
Lower and upper bounds
Why Use This Tool — Confidence Interval Calculator
Free Online Confidence interval calculatorCalculate confidence interval — for EveryoneFast & Easy 95 confidence interval calculatorMargin of error confidence interval — 100% FreeFree Online Confidence level calculator
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Frequently Asked Questions — Confidence Interval Calculator
What is a confidence interval?
A confidence interval gives a range of values that is likely to contain the true population parameter. A 95% CI means: if you repeated the study 100 times, about 95 of those intervals would contain the true value. It does NOT mean there is a 95% chance the true value is in this specific interval.
What confidence level should I use?
95% is the most common standard in science, medicine, and social research. 99% is used when the stakes of being wrong are very high (clinical trials, safety standards). 90% is used when you can tolerate more uncertainty and want a smaller margin of error. For exploratory research, 80–85% may be acceptable.
What is the difference between CI for a mean vs. a proportion?
For a mean: CI = x̄ ± z·(σ/√n), where you need the sample mean and standard deviation. For a proportion: CI = p ± z·√(p(1−p)/n), where p is the sample proportion (e.g., 0.42 for 42%). Both use the z-score from the chosen confidence level.