Confidence Interval Calculator

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.

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.

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.