Enter a sample mean, standard deviation, and sample size to calculate a confidence interval.
How to Use the Confidence Interval Calculator
Sample mean, standard deviation, and sample size are required; the confidence level dropdown changes which critical value the interval is built from, so 90% and 99% give visibly different widths on the same data.
"95% confidence" doesn't mean a 95% chance the true population mean sits inside this one interval — that's the most common misreading of the term. It means: repeat this exact sampling process many times, build a new interval each time, and roughly 95% of those intervals would end up containing the true mean. The confidence lives in the method over many repetitions, not in any single interval you calculate once.
margin of error = z × (standard deviation ÷ √n)
confidence interval = mean ± margin of errorz comes from the standard normal distribution — 1.645 for 90% confidence, 1.96 for 95%, 2.326 for 98%, 2.576 for 99%. This Confidence Interval Calculator uses whichever you pick. Demanding higher confidence widens the interval, since covering more of the distribution's possible outcomes takes a wider net. A bigger sample narrows it instead — though the margin of error only shrinks with the square root of sample size, so quadrupling your data merely halves the margin, not quarters it.